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Eitan Adler a2c917de8e linux-emulation: modernize a bit 
- remove references to CVS, p4, etc.
- remove indirect reference to FreeBSD.org
- stop talking about old defaults
- update links

This page can be cleaned up some more, but has some valuable information
so should not be deleted without moving the information elsewhere first.
2018-05-29 00:14:05 +00:00

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<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
"http://www.FreeBSD.org/XML/share/xml/freebsd50.dtd">
<!-- $FreeBSD$ -->
<!-- The FreeBSD Documentation Project -->
<article xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
<info><title>&linux; emulation in &os;</title>
<author><personname><firstname>Roman</firstname><surname>Divacky</surname></personname><affiliation>
<address><email>rdivacky@FreeBSD.org</email></address>
</affiliation></author>
<legalnotice xml:id="trademarks" role="trademarks">
&tm-attrib.adobe;
&tm-attrib.ibm;
&tm-attrib.freebsd;
&tm-attrib.linux;
&tm-attrib.netbsd;
&tm-attrib.realnetworks;
&tm-attrib.oracle;
&tm-attrib.sun;
&tm-attrib.general;
</legalnotice>
<pubdate>$FreeBSD$</pubdate>
<releaseinfo>$FreeBSD$</releaseinfo>
<abstract>
<para>This masters thesis deals with updating the &linux; emulation layer
(the so called <firstterm>Linuxulator</firstterm>). The task was to update the layer to match
the functionality of &linux; 2.6. As a reference implementation, the
&linux; 2.6.16 kernel was chosen. The concept is loosely based on
the NetBSD implementation. Most of the work was done in the summer
of 2006 as a part of the Google Summer of Code students program.
The focus was on bringing the <firstterm>NPTL</firstterm> (new &posix;
thread library) support into the emulation layer, including
<firstterm>TLS</firstterm> (thread local storage),
<firstterm>futexes</firstterm> (fast user space mutexes),
<firstterm>PID mangling</firstterm>, and some other minor
things. Many small problems were identified and fixed in the
process. My work was integrated into the main &os; source
repository and will be shipped in the upcoming 7.0R release. We,
the emulation development team, are working on making the
&linux; 2.6 emulation the default emulation layer in &os;.</para>
</abstract>
</info>
<sect1 xml:id="intro">
<title>Introduction</title>
<para>In the last few years the open source &unix; based operating systems
started to be widely deployed on server and client machines. Among
these operating systems I would like to point out two: &os;, for its BSD
heritage, time proven code base and many interesting features and
&linux; for its wide user base, enthusiastic open developer community
and support from large companies. &os; tends to be used on server
class machines serving heavy duty networking tasks with less usage on
desktop class machines for ordinary users. While &linux; has the same
usage on servers, but it is used much more by home based users. This
leads to a situation where there are many binary only programs available
for &linux; that lack support for &os;.</para>
<para>Naturally, a need for the ability to run &linux; binaries on a &os;
system arises and this is what this thesis deals with: the emulation of
the &linux; kernel in the &os; operating system.</para>
<para>During the Summer of 2006 Google Inc. sponsored a project which
focused on extending the &linux; emulation layer (the so called Linuxulator)
in &os; to include &linux; 2.6 facilities. This thesis is written as a
part of this project.</para>
</sect1>
<sect1 xml:id="inside">
<title>A look inside&hellip;</title>
<para>In this section we are going to describe every operating system in
question. How they deal with syscalls, trapframes etc., all the low-level
stuff. We also describe the way they understand common &unix;
primitives like what a PID is, what a thread is, etc. In the third
subsection we talk about how &unix; on &unix; emulation could be done
in general.</para>
<sect2 xml:id="what-is-unix">
<title>What is &unix;</title>
<para>&unix; is an operating system with a long history that has
influenced almost every other operating system currently in use.
Starting in the 1960s, its development continues to this day (although
in different projects). &unix; development soon forked into two main
ways: the BSDs and System III/V families. They mutually influenced
themselves by growing a common &unix; standard. Among the
contributions originated in BSD we can name virtual memory, TCP/IP
networking, FFS, and many others. The System V branch contributed to
SysV interprocess communication primitives, copy-on-write, etc. &unix;
itself does not exist any more but its ideas have been used by many
other operating systems world wide thus forming the so called &unix;-like
operating systems. These days the most influential ones are &linux;,
Solaris, and possibly (to some extent) &os;. There are in-company
&unix; derivatives (AIX, HP-UX etc.), but these have been more and
more migrated to the aforementioned systems. Let us summarize typical
&unix; characteristics.</para>
</sect2>
<sect2 xml:id="tech-details">
<title>Technical details</title>
<para>Every running program constitutes a process that represents a state
of the computation. Running process is divided between kernel-space
and user-space. Some operations can be done only from kernel space
(dealing with hardware etc.), but the process should spend most of its
lifetime in the user space. The kernel is where the management of the
processes, hardware, and low-level details take place. The kernel
provides a standard unified &unix; API to the user space. The most
important ones are covered below.</para>
<sect3 xml:id="kern-proc-comm">
<title>Communication between kernel and user space process</title>
<para>Common &unix; API defines a syscall as a way to issue commands
from a user space process to the kernel. The most common
implementation is either by using an interrupt or specialized
instruction (think of
<literal>SYSENTER</literal>/<literal>SYSCALL</literal> instructions
for ia32). Syscalls are defined by a number. For example in &os;,
the syscall number&nbsp;85 is the &man.swapon.2; syscall and the
syscall number&nbsp;132 is &man.mkfifo.2;. Some syscalls need
parameters, which are passed from the user-space to the kernel-space
in various ways (implementation dependant). Syscalls are
synchronous.</para>
<para>Another possible way to communicate is by using a
<firstterm>trap</firstterm>. Traps occur asynchronously after
some event occurs (division by zero, page fault etc.). A trap
can be transparent for a process (page fault) or can result in
a reaction like sending a <firstterm>signal</firstterm>
(division by zero).</para>
</sect3>
<sect3 xml:id="proc-proc-comm">
<title>Communication between processes</title>
<para>There are other APIs (System V IPC, shared memory etc.) but the
single most important API is signal. Signals are sent by processes
or by the kernel and received by processes. Some signals
can be ignored or handled by a user supplied routine, some result
in a predefined action that cannot be altered or ignored.</para>
</sect3>
<sect3 xml:id="proc-mgmt">
<title>Process management</title>
<para>Kernel instances are processed first in the system (so called
init). Every running process can create its identical copy using
the &man.fork.2; syscall. Some slightly modified versions of this
syscall were introduced but the basic semantic is the same. Every
running process can morph into some other process using the
&man.exec.3; syscall. Some modifications of this syscall were
introduced but all serve the same basic purpose. Processes end
their lives by calling the &man.exit.2; syscall. Every process is
identified by a unique number called PID. Every process has a
defined parent (identified by its PID).</para>
</sect3>
<sect3 xml:id="thread-mgmt">
<title>Thread management</title>
<para>Traditional &unix; does not define any API nor implementation
for threading, while &posix; defines its threading API but the
implementation is undefined. Traditionally there were two ways of
implementing threads. Handling them as separate processes (1:1
threading) or envelope the whole thread group in one process and
managing the threading in userspace (1:N threading). Comparing
main features of each approach:</para>
<para>1:1 threading</para>
<itemizedlist>
<listitem>
<para>- heavyweight threads</para>
</listitem>
<listitem>
<para>- the scheduling cannot be altered by the user
(slightly mitigated by the &posix; API)</para>
</listitem>
<listitem>
<para>+ no syscall wrapping necessary</para>
</listitem>
<listitem>
<para>+ can utilize multiple CPUs</para>
</listitem>
</itemizedlist>
<para>1:N threading</para>
<itemizedlist>
<listitem>
<para>+ lightweight threads</para>
</listitem>
<listitem>
<para>+ scheduling can be easily altered by the user</para>
</listitem>
<listitem>
<para>- syscalls must be wrapped </para>
</listitem>
<listitem>
<para>- cannot utilize more than one CPU</para>
</listitem>
</itemizedlist>
</sect3>
</sect2>
<sect2 xml:id="what-is-freebsd">
<title>What is &os;?</title>
<para>The &os; project is one of the oldest open source operating
systems currently available for daily use. It is a direct descendant
of the genuine &unix; so it could be claimed that it is a true &unix;
although licensing issues do not permit that. The start of the project
dates back to the early 1990's when a crew of fellow BSD users patched
the 386BSD operating system. Based on this patchkit a new operating
system arose named &os; for its liberal license. Another group created
the NetBSD operating system with different goals in mind. We will
focus on &os;.</para>
<para>&os; is a modern &unix;-based operating system with all the
features of &unix;. Preemptive multitasking, multiuser facilities,
TCP/IP networking, memory protection, symmetric multiprocessing
support, virtual memory with merged VM and buffer cache, they are all
there. One of the interesting and extremely useful features is the
ability to emulate other &unix;-like operating systems. As of
December&nbsp;2006 and 7-CURRENT development, the following
emulation functionalities are supported:</para>
<itemizedlist>
<listitem>
<para>&os;/i386 emulation on &os;/amd64</para>
</listitem>
<listitem>
<para>&os;/i386 emulation on &os;/ia64</para>
</listitem>
<listitem>
<para>&linux;-emulation of &linux; operating system on &os;</para>
</listitem>
<listitem>
<para>NDIS-emulation of Windows networking drivers interface</para>
</listitem>
<listitem>
<para>NetBSD-emulation of NetBSD operating system</para>
</listitem>
<listitem>
<para>PECoff-support for PECoff &os; executables</para>
</listitem>
<listitem>
<para>SVR4-emulation of System V revision 4 &unix;</para>
</listitem>
</itemizedlist>
<para>Actively developed emulations are the &linux; layer and various
&os;-on-&os; layers. Others are not supposed to work properly nor
be usable these days.</para>
<sect3 xml:id="freebsd-tech-details">
<title>Technical details</title>
<para>&os; is traditional flavor of &unix; in the sense of dividing the
run of processes into two halves: kernel space and user space run.
There are two types of process entry to the kernel: a syscall and a
trap. There is only one way to return. In the subsequent sections
we will describe the three gates to/from the kernel. The whole
description applies to the i386 architecture as the Linuxulator
only exists there but the concept is similar on other architectures.
The information was taken from [1] and the source code.</para>
<sect4 xml:id="freebsd-sys-entries">
<title>System entries</title>
<para>&os; has an abstraction called an execution class loader,
which is a wedge into the &man.execve.2; syscall. This employs a
structure <literal>sysentvec</literal>, which describes an
executable ABI. It contains things like errno translation table,
signal translation table, various functions to serve syscall needs
(stack fixup, coredumping, etc.). Every ABI the &os; kernel wants
to support must define this structure, as it is used later in the
syscall processing code and at some other places. System entries
are handled by trap handlers, where we can access both the
kernel-space and the user-space at once.</para>
</sect4>
<sect4 xml:id="freebsd-syscalls">
<title>Syscalls</title>
<para>Syscalls on &os; are issued by executing interrupt
<literal>0x80</literal> with register <varname>%eax</varname> set
to a desired syscall number with arguments passed on the stack.</para>
<para>When a process issues an interrupt <literal>0x80</literal>, the
<literal>int0x80</literal> syscall trap handler is issued (defined
in <filename>sys/i386/i386/exception.s</filename>), which prepares
arguments (i.e. copies them on to the stack) for a
call to a C function &man.syscall.2; (defined in
<filename>sys/i386/i386/trap.c</filename>), which processes the
passed in trapframe. The processing consists of preparing the
syscall (depending on the <literal>sysvec</literal> entry),
determining if the syscall is 32-bit or 64-bit one (changes size
of the parameters), then the parameters are copied, including the
syscall. Next, the actual syscall function is executed with
processing of the return code (special cases for
<literal>ERESTART</literal> and <literal>EJUSTRETURN</literal>
errors). Finally an <literal>userret()</literal> is scheduled,
switching the process back to the users-pace. The parameters to
the actual syscall handler are passed in the form of
<literal>struct thread *td</literal>,
<literal>struct syscall args *</literal> arguments where the second
parameter is a pointer to the copied in structure of
parameters.</para>
</sect4>
<sect4 xml:id="freebsd-traps">
<title>Traps</title>
<para>Handling of traps in &os; is similar to the handling of
syscalls. Whenever a trap occurs, an assembler handler is called.
It is chosen between alltraps, alltraps with regs pushed or
calltrap depending on the type of the trap. This handler prepares
arguments for a call to a C function <literal>trap()</literal>
(defined in <filename>sys/i386/i386/trap.c</filename>), which then
processes the occurred trap. After the processing it might send a
signal to the process and/or exit to userland using
<literal>userret()</literal>.</para>
</sect4>
<sect4 xml:id="freebsd-exits">
<title>Exits</title>
<para>Exits from kernel to userspace happen using the assembler
routine <literal>doreti</literal> regardless of whether the kernel
was entered via a trap or via a syscall. This restores the program
status from the stack and returns to the userspace.</para>
</sect4>
<sect4 xml:id="freebsd-unix-primitives">
<title>&unix; primitives</title>
<para>&os; operating system adheres to the traditional &unix; scheme,
where every process has a unique identification number, the so
called <firstterm>PID</firstterm> (Process ID). PID numbers are
allocated either linearly or randomly ranging from
<literal>0</literal> to <literal>PID_MAX</literal>. The allocation
of PID numbers is done using linear searching of PID space. Every
thread in a process receives the same PID number as result of the
&man.getpid.2; call.</para>
<para>There are currently two ways to implement threading in &os;.
The first way is M:N threading followed by the 1:1 threading model.
The default library used is M:N threading
(<literal>libpthread</literal>) and you can switch at runtime to
1:1 threading (<literal>libthr</literal>). The plan is to switch
to 1:1 library by default soon. Although those two libraries use
the same kernel primitives, they are accessed through different
API(es). The M:N library uses the <literal>kse_*</literal> family
of syscalls while the 1:1 library uses the <literal>thr_*</literal>
family of syscalls. Because of this, there is no general concept
of thread ID shared between kernel and userspace. Of course, both
threading libraries implement the pthread thread ID API. Every
kernel thread (as described by <literal>struct thread</literal>)
has td tid identifier but this is not directly accessible
from userland and solely serves the kernel's needs. It is also
used for 1:1 threading library as pthread's thread ID but handling
of this is internal to the library and cannot be relied on.</para>
<para>As stated previously there are two implementations of threading
in &os;. The M:N library divides the work between kernel space and
userspace. Thread is an entity that gets scheduled in the kernel
but it can represent various number of userspace threads.
M userspace threads get mapped to N kernel threads thus saving
resources while keeping the ability to exploit multiprocessor
parallelism. Further information about the implementation can be
obtained from the man page or [1]. The 1:1 library directly maps a
userland thread to a kernel thread thus greatly simplifying the
scheme. None of these designs implement a fairness mechanism (such
a mechanism was implemented but it was removed recently because it
caused serious slowdown and made the code more difficult to deal
with).</para>
</sect4>
</sect3>
</sect2>
<sect2 xml:id="what-is-linux">
<title>What is &linux;</title>
<para>&linux; is a &unix;-like kernel originally developed by Linus
Torvalds, and now being contributed to by a massive crowd of
programmers all around the world. From its mere beginnings to today,
with wide support from companies such as IBM or Google, &linux; is
being associated with its fast development pace, full hardware support
and benevolent dictator model of organization.</para>
<para>&linux; development started in 1991 as a hobbyist project at
University of Helsinki in Finland. Since then it has obtained all the
features of a modern &unix;-like OS: multiprocessing, multiuser
support, virtual memory, networking, basically everything is there.
There are also highly advanced features like virtualization etc.</para>
<para>As of 2006 &linux; seems to be the most widely used open source
operating system with support from independent software vendors like
Oracle, RealNetworks, Adobe, etc. Most of the commercial software
distributed for &linux; can only be obtained in a binary form so
recompilation for other operating systems is impossible.</para>
<para>Most of the &linux; development happens in a
<application>Git</application> version control system.
<application>Git</application> is a distributed system so there is
no central source of the &linux; code, but some branches are considered
prominent and official. The version number scheme implemented by
&linux; consists of four numbers A.B.C.D. Currently development
happens in 2.6.C.D, where C represents major version, where new
features are added or changed while D is a minor version for bugfixes
only.</para>
<para>More information can be obtained from [3].</para>
<sect3 xml:id="linux-tech-details">
<title>Technical details</title>
<para>&linux; follows the traditional &unix; scheme of dividing the run
of a process in two halves: the kernel and user space. The kernel can
be entered in two ways: via a trap or via a syscall. The return is
handled only in one way. The further description applies to
&linux;&nbsp;2.6 on the &i386; architecture. This information was
taken from [2].</para>
<sect4 xml:id="linux-syscalls">
<title>Syscalls</title>
<para>Syscalls in &linux; are performed (in userspace) using
<literal>syscallX</literal> macros where X substitutes a number
representing the number of parameters of the given syscall. This
macro translates to a code that loads <varname>%eax</varname>
register with a number of the syscall and executes interrupt
<literal>0x80</literal>. After this syscall return is called,
which translates negative return values to positive
<literal>errno</literal> values and sets <literal>res</literal> to
<literal>-1</literal> in case of an error. Whenever the interrupt
<literal>0x80</literal> is called the process enters the kernel in
system call trap handler. This routine saves all registers on the
stack and calls the selected syscall entry. Note that the &linux;
calling convention expects parameters to the syscall to be passed
via registers as shown here:</para>
<orderedlist>
<listitem>
<para>parameter -&gt; <varname>%ebx</varname></para>
</listitem>
<listitem>
<para>parameter -&gt; <varname>%ecx</varname></para>
</listitem>
<listitem>
<para>parameter -&gt; <varname>%edx</varname></para>
</listitem>
<listitem>
<para>parameter -&gt; <varname>%esi</varname></para>
</listitem>
<listitem>
<para>parameter -&gt; <varname>%edi</varname></para>
</listitem>
<listitem>
<para>parameter -&gt; <varname>%ebp</varname></para>
</listitem>
</orderedlist>
<para>There are some exceptions to this, where &linux; uses different
calling convention (most notably the <literal>clone</literal>
syscall).</para>
</sect4>
<sect4 xml:id="linux-traps">
<title>Traps</title>
<para>The trap handlers are introduced in
<filename>arch/i386/kernel/traps.c</filename> and most of these
handlers live in <filename>arch/i386/kernel/entry.S</filename>,
where handling of the traps happens.</para>
</sect4>
<sect4 xml:id="linux-exits">
<title>Exits</title>
<para>Return from the syscall is managed by syscall &man.exit.3;,
which checks for the process having unfinished work, then checks
whether we used user-supplied selectors. If this happens stack
fixing is applied and finally the registers are restored from the
stack and the process returns to the userspace.</para>
</sect4>
<sect4 xml:id="linux-unix-primitives">
<title>&unix; primitives</title>
<para>In the 2.6 version, the &linux; operating system redefined some
of the traditional &unix; primitives, notably PID, TID and thread.
PID is defined not to be unique for every process, so for some
processes (threads) &man.getppid.2; returns the same value. Unique
identification of process is provided by TID. This is because
<firstterm>NPTL</firstterm> (New &posix; Thread Library) defines
threads to be normal processes (so called 1:1 threading). Spawning
a new process in &linux;&nbsp;2.6 happens using the
<literal>clone</literal> syscall (fork variants are reimplemented using
it). This clone syscall defines a set of flags that affect
behavior of the cloning process regarding thread implementation.
The semantic is a bit fuzzy as there is no single flag telling the
syscall to create a thread.</para>
<para>Implemented clone flags are:</para>
<itemizedlist>
<listitem>
<para><literal>CLONE_VM</literal> - processes share their memory
space</para>
</listitem>
<listitem>
<para><literal>CLONE_FS</literal> - share umask, cwd and
namespace</para>
</listitem>
<listitem>
<para><literal>CLONE_FILES</literal> - share open
files</para>
</listitem>
<listitem>
<para><literal>CLONE_SIGHAND</literal> - share signal handlers
and blocked signals</para>
</listitem>
<listitem>
<para><literal>CLONE_PARENT</literal> - share parent</para>
</listitem>
<listitem>
<para><literal>CLONE_THREAD</literal> - be thread (further
explanation below)</para>
</listitem>
<listitem>
<para><literal>CLONE_NEWNS</literal> - new namespace</para>
</listitem>
<listitem>
<para><literal>CLONE_SYSVSEM</literal> - share SysV undo
structures</para>
</listitem>
<listitem>
<para><literal>CLONE_SETTLS</literal> - setup TLS at supplied
address</para>
</listitem>
<listitem>
<para><literal>CLONE_PARENT_SETTID</literal> - set TID in the
parent</para>
</listitem>
<listitem>
<para><literal>CLONE_CHILD_CLEARTID</literal> - clear TID in the
child</para>
</listitem>
<listitem>
<para><literal>CLONE_CHILD_SETTID</literal> - set TID in the
child</para>
</listitem>
</itemizedlist>
<para><literal>CLONE_PARENT</literal> sets the real parent to the
parent of the caller. This is useful for threads because if thread
A creates thread B we want thread B to be parented to the parent of
the whole thread group. <literal>CLONE_THREAD</literal> does
exactly the same thing as <literal>CLONE_PARENT</literal>,
<literal>CLONE_VM</literal> and <literal>CLONE_SIGHAND</literal>,
rewrites PID to be the same as PID of the caller, sets exit signal
to be none and enters the thread group.
<literal>CLONE_SETTLS</literal> sets up GDT entries for TLS
handling. The <literal>CLONE_*_*TID</literal> set of flags
sets/clears user supplied address to TID or 0.</para>
<para>As you can see the <literal>CLONE_THREAD</literal> does most
of the work and does not seem to fit the scheme very well. The
original intention is unclear (even for authors, according to
comments in the code) but I think originally there was one
threading flag, which was then parcelled among many other flags
but this separation was never fully finished. It is also unclear
what this partition is good for as glibc does not use that so only
hand-written use of the clone permits a programmer to access this
features.</para>
<para>For non-threaded programs the PID and TID are the same. For
threaded programs the first thread PID and TID are the same and
every created thread shares the same PID and gets assigned a
unique TID (because <literal>CLONE_THREAD</literal> is passed in)
also parent is shared for all processes forming this threaded
program.</para>
<para>The code that implements &man.pthread.create.3; in NPTL defines
the clone flags like this:</para>
<programlisting>int clone_flags = (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGNAL
| CLONE_SETTLS | CLONE_PARENT_SETTID
| CLONE_CHILD_CLEARTID | CLONE_SYSVSEM
#if __ASSUME_NO_CLONE_DETACHED == 0
| CLONE_DETACHED
#endif
| 0);</programlisting>
<para>The <literal>CLONE_SIGNAL</literal> is defined like</para>
<programlisting>#define CLONE_SIGNAL (CLONE_SIGHAND | CLONE_THREAD)</programlisting>
<para>the last 0 means no signal is sent when any of the threads
exits.</para>
</sect4>
</sect3>
</sect2>
<sect2 xml:id="what-is-emu">
<title>What is emulation</title>
<para>According to a dictionary definition, emulation is the ability of
a program or device to imitate another program or device. This is
achieved by providing the same reaction to a given stimulus as the
emulated object. In practice, the software world mostly sees three
types of emulation - a program used to emulate a machine (QEMU, various
game console emulators etc.), software emulation of a hardware facility
(OpenGL emulators, floating point units emulation etc.) and operating
system emulation (either in kernel of the operating system or as a
userspace program).</para>
<para>Emulation is usually used in a place, where using the original
component is not feasible nor possible at all. For example someone
might want to use a program developed for a different operating
system than they use. Then emulation comes in handy. Sometimes
there is no other way but to use emulation - e.g. when the hardware
device you try to use does not exist (yet/anymore) then there is no
other way but emulation. This happens often when porting an operating
system to a new (non-existent) platform. Sometimes it is just
cheaper to emulate.</para>
<para>Looking from an implementation point of view, there are two main
approaches to the implementation of emulation. You can either emulate
the whole thing - accepting possible inputs of the original object,
maintaining inner state and emitting correct output based on the state
and/or input. This kind of emulation does not require any special
conditions and basically can be implemented anywhere for any
device/program. The drawback is that implementing such emulation is
quite difficult, time-consuming and error-prone. In some cases we can
use a simpler approach. Imagine you want to emulate a printer that
prints from left to right on a printer that prints from right to left.
It is obvious that there is no need for a complex emulation layer but
simply reversing of the printed text is sufficient. Sometimes the
emulating environment is very similar to the emulated one so just a
thin layer of some translation is necessary to provide fully working
emulation! As you can see this is much less demanding to implement,
so less time-consuming and error-prone than the previous approach. But
the necessary condition is that the two environments must be similar
enough. The third approach combines the two previous. Most of the
time the objects do not provide the same capabilities so in a case of
emulating the more powerful one on the less powerful we have to emulate
the missing features with full emulation described above.</para>
<para>This master thesis deals with emulation of &unix; on &unix;, which
is exactly the case, where only a thin layer of translation is
sufficient to provide full emulation. The &unix; API consists of a set
of syscalls, which are usually self contained and do not affect some
global kernel state.</para>
<para>There are a few syscalls that affect inner state but this can be
dealt with by providing some structures that maintain the extra
state.</para>
<para>No emulation is perfect and emulations tend to lack some parts but
this usually does not cause any serious drawbacks. Imagine a game
console emulator that emulates everything but music output. No doubt
that the games are playable and one can use the emulator. It might
not be that comfortable as the original game console but its an
acceptable compromise between price and comfort.</para>
<para>The same goes with the &unix; API. Most programs can live with a
very limited set of syscalls working. Those syscalls tend to be the
oldest ones (&man.read.2;/&man.write.2;, &man.fork.2; family,
&man.signal.3; handling, &man.exit.3;, &man.socket.2; API) hence it is
easy to emulate because their semantics is shared among all
&unix;es, which exist todays.</para>
</sect2>
</sect1>
<sect1 xml:id="freebsd-emulation">
<title>Emulation</title>
<sect2>
<title>How emulation works in &os;</title>
<para>As stated earlier, &os; supports running binaries from several
other &unix;es. This works because &os; has an abstraction called the
execution class loader. This wedges into the &man.execve.2; syscall,
so when &man.execve.2; is about to execute a binary it examines its
type.</para>
<para>There are basically two types of binaries in &os;. Shell-like text
scripts which are identified by <literal>#!</literal> as their first
two characters and normal (typically <firstterm>ELF</firstterm>)
binaries, which are a representation of a compiled executable object.
The vast majority (one could say all of them) of binaries in &os; are
from type ELF. ELF files contain a header, which specifies the OS ABI
for this ELF file. By reading this information, the operating system
can accurately determine what type of binary the given file is.</para>
<para>Every OS ABI must be registered in the &os; kernel. This applies
to the &os; native OS ABI, as well. So when &man.execve.2; executes a
binary it iterates through the list of registered APIs and when it
finds the right one it starts to use the information contained in the
OS ABI description (its syscall table, <literal>errno</literal>
translation table, etc.). So every time the process calls a syscall,
it uses its own set of syscalls instead of some global one. This
effectively provides a very elegant and easy way of supporting
execution of various binary formats.</para>
<para>The nature of emulation of different OSes (and also some other
subsystems) led developers to invite a handler event mechanism. There
are various places in the kernel, where a list of event handlers are
called. Every subsystem can register an event handler and they are
called accordingly. For example, when a process exits there is a
handler called that possibly cleans up whatever the subsystem needs
to be cleaned.</para>
<para>Those simple facilities provide basically everything that is needed
for the emulation infrastructure and in fact these are basically the
only things necessary to implement the &linux; emulation layer.</para>
</sect2>
<sect2 xml:id="freebsd-common-primitives">
<title>Common primitives in the &os; kernel</title>
<para>Emulation layers need some support from the operating system. I am
going to describe some of the supported primitives in the &os;
operating system.</para>
<sect3 xml:id="freebsd-locking-primitives">
<title>Locking primitives</title>
<para>Contributed by: &a.attilio.email;</para>
<para>The &os; synchronization primitive set is based on the idea to
supply a rather huge number of different primitives in a way that
the better one can be used for every particular, appropriate
situation.</para>
<para>To a high level point of view you can consider three kinds of
synchronization primitives in the &os; kernel:</para>
<itemizedlist>
<listitem>
<para>atomic operations and memory barriers</para>
</listitem>
<listitem>
<para>locks</para>
</listitem>
<listitem>
<para>scheduling barriers</para>
</listitem>
</itemizedlist>
<para>Below there are descriptions for the 3 families. For every lock,
you should really check the linked manpage (where possible) for
more detailed explanations.</para>
<sect4 xml:id="freebsd-atomic-op">
<title>Atomic operations and memory barriers</title>
<para>Atomic operations are implemented through a set of functions
performing simple arithmetics on memory operands in an atomic way
with respect to external events (interrupts, preemption, etc.).
Atomic operations can guarantee atomicity just on small data types
(in the magnitude order of the <literal>.long.</literal>
architecture C data type), so should be rarely used directly in the
end-level code, if not only for very simple operations (like flag
setting in a bitmap, for example). In fact, it is rather simple
and common to write down a wrong semantic based on just atomic
operations (usually referred as lock-less). The &os; kernel offers
a way to perform atomic operations in conjunction with a memory
barrier. The memory barriers will guarantee that an atomic
operation will happen following some specified ordering with
respect to other memory accesses. For example, if we need that an
atomic operation happen just after all other pending writes (in
terms of instructions reordering buffers activities) are completed,
we need to explicitly use a memory barrier in conjunction to this
atomic operation. So it is simple to understand why memory
barriers play a key role for higher-level locks building (just
as refcounts, mutexes, etc.). For a detailed explanatory on atomic
operations, please refer to &man.atomic.9;. It is far, however,
noting that atomic operations (and memory barriers as well) should
ideally only be used for building front-ending locks (as
mutexes).</para>
</sect4>
<sect4 xml:id="freebsd-refcounts">
<title>Refcounts</title>
<para>Refcounts are interfaces for handling reference counters.
They are implemented through atomic operations and are intended to
be used just for cases, where the reference counter is the only
one thing to be protected, so even something like a spin-mutex is
deprecated. Using the refcount interface for structures, where
a mutex is already used is often wrong since we should probably
close the reference counter in some already protected paths. A
manpage discussing refcount does not exist currently, just check
<filename>sys/refcount.h</filename> for an overview of the
existing API.</para>
</sect4>
<sect4 xml:id="freebsd-locks">
<title>Locks</title>
<para>&os; kernel has huge classes of locks. Every lock is defined
by some peculiar properties, but probably the most important is the
event linked to contesting holders (or in other terms, the
behavior of threads unable to acquire the lock). &os;'s locking
scheme presents three different behaviors for contenders:</para>
<orderedlist>
<listitem>
<para>spinning</para>
</listitem>
<listitem>
<para>blocking</para>
</listitem>
<listitem>
<para>sleeping</para>
</listitem>
</orderedlist>
<note>
<para>numbers are not casual</para>
</note>
</sect4>
<sect4 xml:id="freebsd-spinlocks">
<title>Spinning locks</title>
<para>Spin locks let waiters to spin until they cannot acquire the
lock. An important matter do deal with is when a thread contests
on a spin lock if it is not descheduled. Since the &os; kernel
is preemptive, this exposes spin lock at the risk of deadlocks
that can be solved just disabling interrupts while they are
acquired. For this and other reasons (like lack of priority
propagation support, poorness in load balancing schemes between
CPUs, etc.), spin locks are intended to protect very small paths
of code, or ideally not to be used at all if not explicitly
requested (explained later).</para>
</sect4>
<sect4 xml:id="freebsd-blocking">
<title>Blocking</title>
<para>Block locks let waiters to be descheduled and blocked until
the lock owner does not drop it and wakes up one or more
contenders. In order to avoid starvation issues, blocking locks
do priority propagation from the waiters to the owner. Block
locks must be implemented through the turnstile interface and are
intended to be the most used kind of locks in the kernel, if no
particular conditions are met.</para>
</sect4>
<sect4 xml:id="freebsd-sleeping">
<title>Sleeping</title>
<para>Sleep locks let waiters to be descheduled and fall asleep
until the lock holder does not drop it and wakes up one or more
waiters. Since sleep locks are intended to protect large paths
of code and to cater asynchronous events, they do not do any form
of priority propagation. They must be implemented through the
&man.sleepqueue.9; interface.</para>
<para>The order used to acquire locks is very important, not only for
the possibility to deadlock due at lock order reversals, but even
because lock acquisition should follow specific rules linked to
locks natures. If you give a look at the table above, the
practical rule is that if a thread holds a lock of level n (where
the level is the number listed close to the kind of lock) it is not
allowed to acquire a lock of superior levels, since this would
break the specified semantic for a path. For example, if a thread
holds a block lock (level 2), it is allowed to acquire a spin lock
(level 1) but not a sleep lock (level 3), since block locks are
intended to protect smaller paths than sleep lock (these rules are
not about atomic operations or scheduling barriers,
however).</para>
<para>This is a list of lock with their respective behaviors:</para>
<itemizedlist>
<listitem>
<para>spin mutex - spinning - &man.mutex.9;</para>
</listitem>
<listitem>
<para>sleep mutex - blocking - &man.mutex.9;</para>
</listitem>
<listitem>
<para>pool mutex - blocking - &man.mtx.pool.9;</para>
</listitem>
<listitem>
<para>sleep family - sleeping - &man.sleep.9; pause tsleep
msleep msleep spin msleep rw msleep sx</para>
</listitem>
<listitem>
<para>condvar - sleeping - &man.condvar.9;</para>
</listitem>
<listitem>
<para>rwlock - blocking - &man.rwlock.9;</para>
</listitem>
<listitem>
<para>sxlock - sleeping - &man.sx.9;</para>
</listitem>
<listitem>
<para>lockmgr - sleeping - &man.lockmgr.9;</para>
</listitem>
<listitem>
<para>semaphores - sleeping - &man.sema.9;</para>
</listitem>
</itemizedlist>
<para>Among these locks only mutexes, sxlocks, rwlocks and lockmgrs
are intended to handle recursion, but currently recursion is only
supported by mutexes and lockmgrs.</para>
</sect4>
<sect4 xml:id="freebsd-scheduling">
<title>Scheduling barriers</title>
<para>Scheduling barriers are intended to be used in order to drive
scheduling of threading. They consist mainly of three
different stubs:</para>
<itemizedlist>
<listitem>
<para>critical sections (and preemption)</para>
</listitem>
<listitem>
<para>sched_bind</para>
</listitem>
<listitem>
<para>sched_pin</para>
</listitem>
</itemizedlist>
<para>Generally, these should be used only in a particular context
and even if they can often replace locks, they should be avoided
because they do not let the diagnose of simple eventual problems
with locking debugging tools (as &man.witness.4;).</para>
</sect4>
<sect4 xml:id="freebsd-critical">
<title>Critical sections</title>
<para>The &os; kernel has been made preemptive basically to deal with
interrupt threads. In fact, in order to avoid high interrupt
latency, time-sharing priority threads can be preempted by
interrupt threads (in this way, they do not need to wait to be
scheduled as the normal path previews). Preemption, however,
introduces new racing points that need to be handled, as well.
Often, in order to deal with preemption, the simplest thing to do
is to completely disable it. A critical section defines a piece of
code (borderlined by the pair of functions &man.critical.enter.9;
and &man.critical.exit.9;, where preemption is guaranteed to not
happen (until the protected code is fully executed). This can
often replace a lock effectively but should be used carefully in
order to not lose the whole advantage that preemption
brings.</para>
</sect4>
<sect4 xml:id="freebsd-schedpin">
<title>sched_pin/sched_unpin</title>
<para>Another way to deal with preemption is the
<function>sched_pin()</function> interface. If a piece of code
is closed in the <function>sched_pin()</function> and
<function>sched_unpin()</function> pair of functions it is
guaranteed that the respective thread, even if it can be preempted,
it will always be executed on the same CPU. Pinning is very
effective in the particular case when we have to access at
per-cpu datas and we assume other threads will not change those
data. The latter condition will determine a critical section
as a too strong condition for our code.</para>
</sect4>
<sect4 xml:id="freebsd-schedbind">
<title>sched_bind/sched_unbind</title>
<para><function>sched_bind</function> is an API used in order to bind
a thread to a particular CPU for all the time it executes the code,
until a <function>sched_unbind</function> function call does not
unbind it. This feature has a key role in situations where you
cannot trust the current state of CPUs (for example, at very early
stages of boot), as you want to avoid your thread to migrate on
inactive CPUs. Since <function>sched_bind</function> and
<function>sched_unbind</function> manipulate internal scheduler
structures, they need to be enclosed in
<function>sched_lock</function> acquisition/releasing when
used.</para>
</sect4>
</sect3>
<sect3 xml:id="freebsd-proc">
<title>Proc structure</title>
<para>Various emulation layers sometimes require some additional
per-process data. It can manage separate structures (a list, a tree
etc.) containing these data for every process but this tends to be
slow and memory consuming. To solve this problem the &os;
<literal>proc</literal> structure contains
<literal>p_emuldata</literal>, which is a void pointer to some
emulation layer specific data. This <literal>proc</literal> entry
is protected by the proc mutex.</para>
<para>The &os; <literal>proc</literal> structure contains a
<literal>p_sysent</literal> entry that identifies, which ABI this
process is running. In fact, it is a pointer to the
<literal>sysentvec</literal> described above. So by comparing this
pointer to the address where the <literal>sysentvec</literal>
structure for the given ABI is stored we can effectively determine
whether the process belongs to our emulation layer. The code
typically looks like:</para>
<programlisting>if (__predict_true(p-&gt;p_sysent != &amp;elf_&linux;_sysvec))
return;</programlisting>
<para>As you can see, we effectively use the
<literal>__predict_true</literal> modifier to collapse the most
common case (&os; process) to a simple return operation thus
preserving high performance. This code should be turned into a
macro because currently it is not very flexible, i.e. we do not
support &linux;64 emulation nor A.OUT &linux; processes
on i386.</para>
</sect3>
<sect3 xml:id="freebsd-vfs">
<title>VFS</title>
<para>The &os; VFS subsystem is very complex but the &linux; emulation
layer uses just a small subset via a well defined API. It can either
operate on vnodes or file handlers. Vnode represents a virtual
vnode, i.e. representation of a node in VFS. Another representation
is a file handler, which represents an opened file from the
perspective of a process. A file handler can represent a socket or
an ordinary file. A file handler contains a pointer to its vnode.
More then one file handler can point to the same vnode.</para>
<sect4 xml:id="freebsd-namei">
<title>namei</title>
<para>The &man.namei.9; routine is a central entry point to pathname
lookup and translation. It traverses the path point by point from
the starting point to the end point using lookup function, which is
internal to VFS. The &man.namei.9; syscall can cope with symlinks,
absolute and relative paths. When a path is looked up using
&man.namei.9; it is inputed to the name cache. This behavior can
be suppressed. This routine is used all over the kernel and its
performance is very critical.</para>
</sect4>
<sect4 xml:id="freebsd-vn">
<title>vn_fullpath</title>
<para>The &man.vn.fullpath.9; function takes the best effort to
traverse VFS name cache and returns a path for a given (locked)
vnode. This process is unreliable but works just fine for the most
common cases. The unreliability is because it relies on VFS cache
(it does not traverse the on medium structures), it does not work
with hardlinks, etc. This routine is used in several places in the
Linuxulator.</para>
</sect4>
<sect4 xml:id="freebsd-vnode">
<title>Vnode operations</title>
<itemizedlist>
<listitem>
<para><function>fgetvp</function> - given a thread and a file
descriptor number it returns the associated vnode</para>
</listitem>
<listitem>
<para>&man.vn.lock.9; - locks a vnode</para>
</listitem>
<listitem>
<para><function>vn_unlock</function> - unlocks a vnode</para>
</listitem>
<listitem>
<para>&man.VOP.READDIR.9; - reads a directory referenced by
a vnode</para>
</listitem>
<listitem>
<para>&man.VOP.GETATTR.9; - gets attributes of a file or a
directory referenced by a vnode</para>
</listitem>
<listitem>
<para>&man.VOP.LOOKUP.9; - looks up a path to a given
directory</para>
</listitem>
<listitem>
<para>&man.VOP.OPEN.9; - opens a file referenced by a
vnode</para>
</listitem>
<listitem>
<para>&man.VOP.CLOSE.9; - closes a file referenced by a
vnode</para>
</listitem>
<listitem>
<para>&man.vput.9; - decrements the use count for a vnode and
unlocks it</para>
</listitem>
<listitem>
<para>&man.vrele.9; - decrements the use count for a vnode</para>
</listitem>
<listitem>
<para>&man.vref.9; - increments the use count for a vnode</para>
</listitem>
</itemizedlist>
</sect4>
<sect4 xml:id="freebsd-file-handler">
<title>File handler operations</title>
<itemizedlist>
<listitem>
<para><function>fget</function> - given a thread and a file
descriptor number it returns associated file handler and
references it</para>
</listitem>
<listitem>
<para><function>fdrop</function> - drops a reference to a file
handler</para>
</listitem>
<listitem>
<para><function>fhold</function> - references a file
handler</para>
</listitem>
</itemizedlist>
</sect4>
</sect3>
</sect2>
</sect1>
<sect1 xml:id="md">
<title>&linux; emulation layer -MD part</title>
<para>This section deals with implementation of &linux; emulation layer in
&os; operating system. It first describes the machine dependent part
talking about how and where interaction between userland and kernel is
implemented. It talks about syscalls, signals, ptrace, traps, stack
fixup. This part discusses i386 but it is written generally so other
architectures should not differ very much. The next part is the machine
independent part of the Linuxulator. This section only covers i386 and ELF
handling. A.OUT is obsolete and untested.</para>
<sect2 xml:id="syscall-handling">
<title>Syscall handling</title>
<para>Syscall handling is mostly written in
<filename>linux_sysvec.c</filename>, which covers most of the routines
pointed out in the <literal>sysentvec</literal> structure. When a
&linux; process running on &os; issues a syscall, the general syscall
routine calls linux prepsyscall routine for the &linux; ABI.</para>
<sect3 xml:id="linux-prepsyscall">
<title>&linux; prepsyscall</title>
<para>&linux; passes arguments to syscalls via registers (that is why
it is limited to 6 parameters on i386) while &os; uses the stack.
The &linux; prepsyscall routine must copy parameters from registers
to the stack. The order of the registers is:
<varname>%ebx</varname>, <varname>%ecx</varname>,
<varname>%edx</varname>, <varname>%esi</varname>,
<varname>%edi</varname>, <varname>%ebp</varname>. The catch is that
this is true for only <emphasis>most</emphasis> of the syscalls.
Some (most notably <function>clone</function>) uses a different
order but it is luckily easy to fix by inserting a dummy parameter
in the <function>linux_clone</function> prototype.</para>
</sect3>
<sect3 xml:id="syscall-writing">
<title>Syscall writing</title>
<para>Every syscall implemented in the Linuxulator must have its
prototype with various flags in <filename>syscalls.master</filename>.
The form of the file is:</para>
<programlisting>...
AUE_FORK STD { int linux_fork(void); }
...
AUE_CLOSE NOPROTO { int close(int fd); }
...</programlisting>
<para>The first column represents the syscall number. The second
column is for auditing support. The third column represents the
syscall type. It is either <literal>STD</literal>,
<literal>OBSOL</literal>, <literal>NOPROTO</literal> and
<literal>UNIMPL</literal>. <literal>STD</literal> is a standard
syscall with full prototype and implementation.
<literal>OBSOL</literal> is obsolete and defines just the prototype.
<literal>NOPROTO</literal> means that the syscall is implemented
elsewhere so do not prepend ABI prefix, etc.
<literal>UNIMPL</literal> means that the syscall will be
substituted with the <function>nosys</function> syscall
(a syscall just printing out a message about the syscall not being
implemented and returning <literal>ENOSYS</literal>).</para>
<para>From <filename>syscalls.master</filename> a script generates
three files: <filename>linux_syscall.h</filename>,
<filename>linux_proto.h</filename> and
<filename>linux_sysent.c</filename>. The
<filename>linux_syscall.h</filename> contains definitions of syscall
names and their numerical value, e.g.:</para>
<programlisting>...
#define LINUX_SYS_linux_fork 2
...
#define LINUX_SYS_close 6
...</programlisting>
<para>The <filename>linux_proto.h</filename> contains structure
definitions of arguments to every syscall, e.g.:</para>
<programlisting>struct linux_fork_args {
register_t dummy;
};</programlisting>
<para>And finally, <filename>linux_sysent.c</filename> contains
structure describing the system entry table, used to actually
dispatch a syscall, e.g.:</para>
<programlisting>{ 0, (sy_call_t *)linux_fork, AUE_FORK, NULL, 0, 0 }, /* 2 = linux_fork */
{ AS(close_args), (sy_call_t *)close, AUE_CLOSE, NULL, 0, 0 }, /* 6 = close */</programlisting>
<para>As you can see <function>linux_fork</function> is implemented
in Linuxulator itself so the definition is of <literal>STD</literal>
type and has no argument, which is exhibited by the dummy argument
structure. On the other hand <function>close</function> is just an
alias for real &os; &man.close.2; so it has no linux arguments
structure associated and in the system entry table it is not prefixed
with linux as it calls the real &man.close.2; in the kernel.</para>
</sect3>
<sect3 xml:id="dummy-syscalls">
<title>Dummy syscalls</title>
<para>The &linux; emulation layer is not complete, as some syscalls are
not implemented properly and some are not implemented at all. The
emulation layer employs a facility to mark unimplemented syscalls
with the <literal>DUMMY</literal> macro. These dummy definitions
reside in <filename>linux_dummy.c</filename> in a form of
<literal>DUMMY(syscall);</literal>, which is then translated to
various syscall auxiliary files and the implementation consists
of printing a message saying that this syscall is not implemented.
The <literal>UNIMPL</literal> prototype is not used because we want
to be able to identify the name of the syscall that was called in
order to know what syscalls are more important to implement.</para>
</sect3>
</sect2>
<sect2 xml:id="signal-handling">
<title>Signal handling</title>
<para>Signal handling is done generally in the &os; kernel for all
binary compatibilities with a call to a compat-dependent layer.
&linux; compatibility layer defines
<function>linux_sendsig</function> routine for this purpose.</para>
<sect3 xml:id="linux-sendsig">
<title>&linux; sendsig</title>
<para>This routine first checks whether the signal has been installed
with a <literal>SA_SIGINFO</literal> in which case it calls
<function>linux_rt_sendsig</function> routine instead. Furthermore,
it allocates (or reuses an already existing) signal handle context,
then it builds a list of arguments for the signal handler. It
translates the signal number based on the signal translation table,
assigns a handler, translates sigset. Then it saves context for the
<function>sigreturn</function> routine (various registers, translated
trap number and signal mask). Finally, it copies out the signal
context to the userspace and prepares context for the actual
signal handler to run.</para>
</sect3>
<sect3 xml:id="linux-rt-sendsig">
<title>linux_rt_sendsig</title>
<para>This routine is similar to <function>linux_sendsig</function>
just the signal context preparation is different. It adds
<literal>siginfo</literal>, <literal>ucontext</literal>, and some
&posix; parts. It might be worth considering whether those two
functions could not be merged with a benefit of less code duplication
and possibly even faster execution.</para>
</sect3>
<sect3 xml:id="linux-sigreturn">
<title>linux_sigreturn</title>
<para>This syscall is used for return from the signal handler. It does
some security checks and restores the original process context. It
also unmasks the signal in process signal mask.</para>
</sect3>
</sect2>
<sect2 xml:id="ptrace">
<title>Ptrace</title>
<para>Many &unix; derivates implement the &man.ptrace.2; syscall in order
to allow various tracking and debugging features. This facility
enables the tracing process to obtain various information about the
traced process, like register dumps, any memory from the process
address space, etc. and also to trace the process like in stepping an
instruction or between system entries (syscalls and traps).
&man.ptrace.2; also lets you set various information in the traced
process (registers etc.). &man.ptrace.2; is a &unix;-wide standard
implemented in most &unix;es around the world.</para>
<para>&linux; emulation in &os; implements the &man.ptrace.2; facility
in <filename>linux_ptrace.c</filename>. The routines for converting
registers between &linux; and &os; and the actual &man.ptrace.2;
syscall emulation syscall. The syscall is a long switch block that
implements its counterpart in &os; for every &man.ptrace.2; command.
The &man.ptrace.2; commands are mostly equal between &linux; and &os;
so usually just a small modification is needed. For example,
<literal>PT_GETREGS</literal> in &linux; operates on direct data while
&os; uses a pointer to the data so after performing a (native)
&man.ptrace.2; syscall, a copyout must be done to preserve &linux;
semantics.</para>
<para>The &man.ptrace.2; implementation in Linuxulator has some known
weaknesses. There have been panics seen when using
<command>strace</command> (which is a &man.ptrace.2; consumer) in the
Linuxulator environment. Also <literal>PT_SYSCALL</literal> is not
implemented.</para>
</sect2>
<sect2 xml:id="traps">
<title>Traps</title>
<para>Whenever a &linux; process running in the emulation layer traps
the trap itself is handled transparently with the only exception of
the trap translation. &linux; and &os; differs in opinion on what a
trap is so this is dealt with here. The code is actually very
short:</para>
<programlisting>static int
translate_traps(int signal, int trap_code)
{
if (signal != SIGBUS)
return signal;
switch (trap_code) {
case T_PROTFLT:
case T_TSSFLT:
case T_DOUBLEFLT:
case T_PAGEFLT:
return SIGSEGV;
default:
return signal;
}
}</programlisting>
</sect2>
<sect2 xml:id="stack-fixup">
<title>Stack fixup</title>
<para>The RTLD run-time link-editor expects so called AUX tags on stack
during an <function>execve</function> so a fixup must be done to ensure
this. Of course, every RTLD system is different so the emulation layer
must provide its own stack fixup routine to do this. So does
Linuxulator. The <function>elf_linux_fixup</function> simply copies
out AUX tags to the stack and adjusts the stack of the user space
process to point right after those tags. So RTLD works in a
smart way.</para>
</sect2>
<sect2 xml:id="aout-support">
<title>A.OUT support</title>
<para>The &linux; emulation layer on i386 also supports &linux; A.OUT
binaries. Pretty much everything described in the previous sections
must be implemented for A.OUT support (beside traps translation and
signals sending). The support for A.OUT binaries is no longer
maintained, especially the 2.6 emulation does not work with it but
this does not cause any problem, as the linux-base in ports probably
do not support A.OUT binaries at all. This support will probably be
removed in future. Most of the stuff necessary for loading &linux;
A.OUT binaries is in <filename>imgact_linux.c</filename> file.</para>
</sect2>
</sect1>
<sect1 xml:id="mi">
<title>&linux; emulation layer -MI part</title>
<para>This section talks about machine independent part of the
Linuxulator. It covers the emulation infrastructure needed for &linux;
2.6 emulation, the thread local storage (TLS) implementation (on i386)
and futexes. Then we talk briefly about some syscalls.</para>
<sect2 xml:id="nptl-desc">
<title>Description of NPTL</title>
<para>One of the major areas of progress in development of &linux; 2.6
was threading. Prior to 2.6, the &linux; threading support was
implemented in the <application>linuxthreads</application> library.
The library was a partial implementation of &posix; threading. The
threading was implemented using separate processes for each thread
using the <function>clone</function> syscall to let them share the
address space (and other things). The main weaknesses of this
approach was that every thread had a different PID, signal handling
was broken (from the pthreads perspective), etc. Also the performance
was not very good (use of <literal>SIGUSR</literal> signals for
threads synchronization, kernel resource consumption, etc.) so to
overcome these problems a new threading system was developed and
named NPTL.</para>
<para>The NPTL library focused on two things but a third thing came
along so it is usually considered a part of NPTL. Those two things
were embedding of threads into a process structure and futexes. The
additional third thing was TLS, which is not directly required by NPTL
but the whole NPTL userland library depends on it. Those improvements
yielded in much improved performance and standards conformance. NPTL
is a standard threading library in &linux; systems these days.</para>
<para>The &os; Linuxulator implementation approaches the NPTL in three
main areas. The TLS, futexes and PID mangling, which is meant to
simulate the &linux; threads. Further sections describe each of these
areas.</para>
</sect2>
<sect2 xml:id="linux26-emu">
<title>&linux; 2.6 emulation infrastructure</title>
<para>These sections deal with the way &linux; threads are managed and
how we simulate that in &os;.</para>
<sect3 xml:id="linux26-runtime">
<title>Runtime determining of 2.6 emulation</title>
<para>The &linux; emulation layer in &os; supports runtime setting of
the emulated version. This is done via &man.sysctl.8;, namely
<literal>compat.linux.osrelease</literal>.
Setting this &man.sysctl.8; affects runtime
behavior of the emulation layer. When set to 2.6.x it sets the
value of <literal>linux_use_linux26</literal> while setting to
something else keeps it unset. This variable (plus per-prison
variables of the very same kind) determines whether 2.6
infrastructure (mainly PID mangling) is used in the code or not.
The version setting is done system-wide and this affects all &linux;
processes. The &man.sysctl.8; should not be changed when running any
&linux; binary as it might harm things.</para>
</sect3>
<sect3 xml:id="linux-proc-thread">
<title>&linux; processes and thread identifiers</title>
<para>The semantics of &linux; threading are a little confusing and
uses entirely different nomenclature to &os;. A process in
&linux; consists of a <literal>struct task</literal> embedding two
identifier fields - PID and TGID. PID is <emphasis>not</emphasis>
a process ID but it is a thread ID. The TGID identifies a thread
group in other words a process. For single-threaded process the
PID equals the TGID.</para>
<para>The thread in NPTL is just an ordinary process that happens to
have TGID not equal to PID and have a group leader not equal to
itself (and shared VM etc. of course). Everything else happens in
the same way as to an ordinary process. There is no separation of
a shared status to some external structure like in &os;. This
creates some duplication of information and possible data
inconsistency. The &linux; kernel seems to use task -&gt; group
information in some places and task information elsewhere and it is
really not very consistent and looks error-prone.</para>
<para>Every NPTL thread is created by a call to the
<function>clone</function> syscall with a specific set of flags
(more in the next subsection). The NPTL implements strict
1:1 threading.</para>
<para>In &os; we emulate NPTL threads with ordinary &os; processes that
share VM space, etc. and the PID gymnastic is just mimicked in the
emulation specific structure attached to the process. The
structure attached to the process looks like:</para>
<programlisting>struct linux_emuldata {
pid_t pid;
int *child_set_tid; /* in clone(): Child.s TID to set on clone */
int *child_clear_tid;/* in clone(): Child.s TID to clear on exit */
struct linux_emuldata_shared *shared;
int pdeath_signal; /* parent death signal */
LIST_ENTRY(linux_emuldata) threads; /* list of linux threads */
};</programlisting>
<para>The PID is used to identify the &os; process that attaches this
structure. The <function>child_se_tid</function> and
<function>child_clear_tid</function> are used for TID address
copyout when a process exits and is created. The
<varname>shared</varname> pointer points to a structure shared
among threads. The <varname>pdeath_signal</varname> variable
identifies the parent death signal and the
<varname>threads</varname> pointer is used to link this structure
to the list of threads. The <literal>linux_emuldata_shared</literal>
structure looks like:</para>
<programlisting>struct linux_emuldata_shared {
int refs;
pid_t group_pid;
LIST_HEAD(, linux_emuldata) threads; /* head of list of linux threads */
};</programlisting>
<para>The <varname>refs</varname> is a reference counter being used
to determine when we can free the structure to avoid memory leaks.
The <varname>group_pid</varname> is to identify PID ( = TGID) of the
whole process ( = thread group). The <varname>threads</varname>
pointer is the head of the list of threads in the process.</para>
<para>The <literal>linux_emuldata</literal> structure can be obtained
from the process using <function>em_find</function>. The prototype
of the function is:</para>
<programlisting>struct linux_emuldata *em_find(struct proc *, int locked);</programlisting>
<para>Here, <varname>proc</varname> is the process we want the emuldata
structure from and the locked parameter determines whether we want to
lock or not. The accepted values are <literal>EMUL_DOLOCK</literal>
and <literal>EMUL_DOUNLOCK</literal>. More about locking
later.</para>
</sect3>
<sect3 xml:id="pid-mangling">
<title>PID mangling</title>
<para>Because of the described different view knowing what a process
ID and thread ID is between &os; and &linux; we have to translate
the view somehow. We do it by PID mangling. This means that we
fake what a PID (=TGID) and TID (=PID) is between kernel and
userland. The rule of thumb is that in kernel (in Linuxulator)
PID = PID and TGID = shared -&gt; group pid and to userland we
present <literal>PID = shared -&gt; group_pid</literal> and
<literal>TID = proc -&gt; p_pid</literal>.
The PID member of <literal>linux_emuldata structure</literal> is
a &os; PID.</para>
<para>The above affects mainly getpid, getppid, gettid syscalls. Where
we use PID/TGID respectively. In copyout of TIDs in
<function>child_clear_tid</function> and
<function>child_set_tid</function> we copy out &os; PID.</para>
</sect3>
<sect3 xml:id="clone-syscall">
<title>Clone syscall</title>
<para>The <function>clone</function> syscall is the way threads are
created in &linux;. The syscall prototype looks like this:</para>
<programlisting>int linux_clone(l_int flags, void *stack, void *parent_tidptr, int dummy,
void * child_tidptr);</programlisting>
<para>The <varname>flags</varname> parameter tells the syscall how
exactly the processes should be cloned. As described above, &linux;
can create processes sharing various things independently, for
example two processes can share file descriptors but not VM, etc.
Last byte of the <varname>flags</varname> parameter is the exit
signal of the newly created process. The <varname>stack</varname>
parameter if non-<literal>NULL</literal> tells, where the thread
stack is and if it is <literal>NULL</literal> we are supposed to
copy-on-write the calling process stack (i.e. do what normal
&man.fork.2; routine does). The <varname>parent_tidptr</varname>
parameter is used as an address for copying out process PID (i.e.
thread id) once the process is sufficiently instantiated but is
not runnable yet. The <varname>dummy</varname> parameter is here
because of the very strange calling convention of this syscall on
i386. It uses the registers directly and does not let the compiler
do it what results in the need of a dummy syscall. The
<varname>child_tidptr</varname> parameter is used as an address
for copying out PID once the process has finished forking and when
the process exits.</para>
<para>The syscall itself proceeds by setting corresponding flags
depending on the flags passed in. For example,
<literal>CLONE_VM</literal> maps to RFMEM (sharing of VM), etc.
The only nit here is <literal>CLONE_FS</literal> and
<literal>CLONE_FILES</literal> because &os; does not allow setting
this separately so we fake it by not setting RFFDG (copying of fd
table and other fs information) if either of these is defined. This
does not cause any problems, because those flags are always set
together. After setting the flags the process is forked using
the internal <function>fork1</function> routine, the process is
instrumented not to be put on a run queue, i.e. not to be set
runnable. After the forking is done we possibly reparent the newly
created process to emulate <literal>CLONE_PARENT</literal> semantics.
Next part is creating the emulation data. Threads in &linux; does
not signal their parents so we set exit signal to be 0 to disable
this. After that setting of <varname>child_set_tid</varname> and
<varname>child_clear_tid</varname> is performed enabling the
functionality later in the code. At this point we copy out the PID
to the address specified by <varname>parent_tidptr</varname>. The
setting of process stack is done by simply rewriting thread frame
<varname>%esp</varname> register (<varname>%rsp</varname> on amd64).
Next part is setting up TLS for the newly created process. After
this &man.vfork.2; semantics might be emulated and finally the newly
created process is put on a run queue and copying out its PID to the
parent process via <function>clone</function> return value is
done.</para>
<para>The <function>clone</function> syscall is able and in fact is
used for emulating classic &man.fork.2; and &man.vfork.2; syscalls.
Newer glibc in a case of 2.6 kernel uses <function>clone</function>
to implement &man.fork.2; and &man.vfork.2; syscalls.</para>
</sect3>
<sect3 xml:id="locking">
<title>Locking</title>
<para>The locking is implemented to be per-subsystem because we do not
expect a lot of contention on these. There are two locks:
<literal>emul_lock</literal> used to protect manipulating of
<literal>linux_emuldata</literal> and
<literal>emul_shared_lock</literal> used to manipulate
<literal>linux_emuldata_shared</literal>. The
<literal>emul_lock</literal> is a nonsleepable blocking mutex while
<literal>emul_shared_lock</literal> is a sleepable blocking
<literal>sx_lock</literal>. Because of the per-subsystem locking we
can coalesce some locks and that is why the em find offers the
non-locking access.</para>
</sect3>
</sect2>
<sect2 xml:id="tls">
<title>TLS</title>
<para>This section deals with TLS also known as thread local
storage.</para>
<sect3 xml:id="trheading-intro">
<title>Introduction to threading</title>
<para>Threads in computer science are entities within a process that
can be scheduled independently from each other. The threads in the
process share process wide data (file descriptors, etc.) but also
have their own stack for their own data. Sometimes there is a need
for process-wide data specific to a given thread. Imagine a name of
the thread in execution or something like that. The traditional
&unix; threading API, <application>pthreads</application> provides
a way to do it via &man.pthread.key.create.3;,
&man.pthread.setspecific.3; and &man.pthread.getspecific.3; where a
thread can create a key to the thread local data and using
&man.pthread.getspecific.3; or &man.pthread.getspecific.3; to
manipulate those data. You can easily see that this is not the most
comfortable way this could be accomplished. So various producers of
C/C++ compilers introduced a better way. They defined a new modifier
keyword thread that specifies that a variable is thread specific. A
new method of accessing such variables was developed as well (at
least on i386). The <application>pthreads</application> method tends
to be implemented in userspace as a trivial lookup table. The
performance of such a solution is not very good. So the new method
uses (on i386) segment registers to address a segment, where TLS area
is stored so the actual accessing of a thread variable is just
appending the segment register to the address thus addressing via it.
The segment registers are usually <varname>%gs</varname> and
<varname>%fs</varname> acting like segment selectors. Every thread
has its own area where the thread local data are stored and the
segment must be loaded on every context switch. This method is very
fast and used almost exclusively in the whole i386 &unix; world.
Both &os; and &linux; implement this approach and it yields very good
results. The only drawback is the need to reload the segment on
every context switch which can slowdown context switches. &os; tries
to avoid this overhead by using only 1 segment descriptor for this
while &linux; uses 3. Interesting thing is that almost nothing uses
more than 1 descriptor (only <application>Wine</application> seems to
use 2) so &linux; pays this unnecessary price for context
switches.</para>
</sect3>
<sect3 xml:id="i386-segs">
<title>Segments on i386</title>
<para>The i386 architecture implements the so called segments. A
segment is a description of an area of memory. The base address
(bottom) of the memory area, the end of it (ceiling), type,
protection, etc. The memory described by a segment can be accessed
using segment selector registers (<varname>%cs</varname>,
<varname>%ds</varname>, <varname>%ss</varname>,
<varname>%es</varname>, <varname>%fs</varname>,
<varname>%gs</varname>). For example let us suppose we have a
segment which base address is 0x1234 and length and this code:</para>
<programlisting>mov %edx,%gs:0x10</programlisting>
<para>This will load the content of the <varname>%edx</varname>
register into memory location 0x1244. Some segment registers have
a special use, for example <varname>%cs</varname> is used for code
segment and <varname>%ss</varname> is used for stack segment but
<varname>%fs</varname> and <varname>%gs</varname> are generally
unused. Segments are either stored in a global GDT table or in a
local LDT table. LDT is accessed via an entry in the GDT. The
LDT can store more types of segments. LDT can be per process.
Both tables define up to 8191 entries.</para>
</sect3>
<sect3 xml:id="linux-i386">
<title>Implementation on &linux; i386</title>
<para>There are two main ways of setting up TLS in &linux;. It can be
set when cloning a process using the <function>clone</function>
syscall or it can call <function>set_thread_area</function>. When a
process passes <literal>CLONE_SETTLS</literal> flag to
<function>clone</function>, the kernel expects the memory pointed to
by the <varname>%esi</varname> register a &linux; user space
representation of a segment, which gets translated to the machine
representation of a segment and loaded into a GDT slot. The
GDT slot can be specified with a number or -1 can be used meaning
that the system itself should choose the first free slot. In
practice, the vast majority of programs use only one TLS entry and
does not care about the number of the entry. We exploit this in the
emulation and in fact depend on it.</para>
</sect3>
<sect3 xml:id="tls-emu">
<title>Emulation of &linux; TLS</title>
<sect4 xml:id="tls-i386">
<title>i386</title>
<para>Loading of TLS for the current thread happens by calling
<function>set_thread_area</function> while loading TLS for a
second process in <function>clone</function> is done in the
separate block in <function>clone</function>. Those two functions
are very similar. The only difference being the actual loading of
the GDT segment, which happens on the next context switch for the
newly created process while <function>set_thread_area</function>
must load this directly. The code basically does this. It copies
the &linux; form segment descriptor from the userland. The code
checks for the number of the descriptor but because this differs
between &os; and &linux; we fake it a little. We only support
indexes of 6, 3 and -1. The 6 is genuine &linux; number, 3 is
genuine &os; one and -1 means autoselection. Then we set the
descriptor number to constant 3 and copy out this to the
userspace. We rely on the userspace process using the number from
the descriptor but this works most of the time (have never seen a
case where this did not work) as the userspace process typically
passes in 1. Then we convert the descriptor from the &linux; form
to a machine dependant form (i.e. operating system independent
form) and copy this to the &os; defined segment descriptor.
Finally we can load it. We assign the descriptor to threads PCB
(process control block) and load the <varname>%gs</varname>
segment using <function>load_gs</function>. This loading must be
done in a critical section so that nothing can interrupt us.
The <literal>CLONE_SETTLS</literal> case works exactly like this
just the loading using <function>load_gs</function> is not
performed. The segment used for this (segment number 3) is
shared for this use between &os; processes and &linux; processes
so the &linux; emulation layer does not add any overhead over
plain &os;.</para>
</sect4>
<sect4 xml:id="tls-amd64">
<title>amd64</title>
<para>The amd64 implementation is similar to the i386 one but there
was initially no 32bit segment descriptor used for this purpose
(hence not even native 32bit TLS users worked) so we had to add
such a segment and implement its loading on every context switch
(when a flag signaling use of 32bit is set). Apart from this the
TLS loading is exactly the same just the segment numbers are
different and the descriptor format and the loading differs
slightly.</para>
</sect4>
</sect3>
</sect2>
<sect2 xml:id="futexes">
<title>Futexes</title>
<sect3 xml:id="sync-intro">
<title>Introduction to synchronization</title>
<para>Threads need some kind of synchronization and &posix; provides
some of them: mutexes for mutual exclusion, read-write locks for
mutual exclusion with biased ratio of reads and writes and condition
variables for signaling a status change. It is interesting to note
that &posix; threading API lacks support for semaphores. Those
synchronization routines implementations are heavily dependant on
the type threading support we have. In pure 1:M (userspace) model
the implementation can be solely done in userspace and thus be very
fast (the condition variables will probably end up being implemented
using signals, i.e. not fast) and simple. In 1:1 model, the
situation is also quite clear - the threads must be synchronized
using kernel facilities (which is very slow because a syscall must be
performed). The mixed M:N scenario just combines the first and
second approach or rely solely on kernel. Threads synchronization is
a vital part of thread-enabled programming and its performance can
affect resulting program a lot. Recent benchmarks on &os; operating
system showed that an improved sx_lock implementation yielded 40%
speedup in <firstterm>ZFS</firstterm> (a heavy sx user), this
is in-kernel stuff but it shows clearly how important the performance
of synchronization primitives is.</para>
<para>Threaded programs should be written with as little contention on
locks as possible. Otherwise, instead of doing useful work the
thread just waits on a lock. Because of this, the most well written
threaded programs show little locks contention.</para>
</sect3>
<sect3 xml:id="futex-intro">
<title>Futexes introduction</title>
<para>&linux; implements 1:1 threading, i.e. it has to use in-kernel
synchronization primitives. As stated earlier, well written threaded
programs have little lock contention. So a typical sequence
could be performed as two atomic increase/decrease mutex reference
counter, which is very fast, as presented by the following
example:</para>
<programlisting>pthread_mutex_lock(&amp;mutex);
....
pthread_mutex_unlock(&amp;mutex);</programlisting>
<para>1:1 threading forces us to perform two syscalls for those mutex
calls, which is very slow.</para>
<para>The solution &linux;&nbsp;2.6 implements is called futexes.
Futexes implement the check for contention in userspace and call
kernel primitives only in a case of contention. Thus the typical
case takes place without any kernel intervention. This yields
reasonably fast and flexible synchronization primitives
implementation.</para>
</sect3>
<sect3 xml:id="futex-api">
<title>Futex API</title>
<para>The futex syscall looks like this:</para>
<programlisting>int futex(void *uaddr, int op, int val, struct timespec *timeout, void *uaddr2, int val3);</programlisting>
<para>In this example <varname>uaddr</varname> is an address of the
mutex in userspace, <varname>op</varname> is an operation we are
about to perform and the other parameters have per-operation
meaning.</para>
<para>Futexes implement the following operations:</para>
<itemizedlist>
<listitem>
<para><literal>FUTEX_WAIT</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_WAKE</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_FD</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_REQUEUE</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_CMP_REQUEUE</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_WAKE_OP</literal></para>
</listitem>
</itemizedlist>
<sect4 xml:id="futex-wait">
<title>FUTEX_WAIT</title>
<para>This operation verifies that on address
<varname>uaddr</varname> the value <varname>val</varname>
is written. If not, <literal>EWOULDBLOCK</literal> is
returned, otherwise the thread is queued on the futex and gets
suspended. If the argument <varname>timeout</varname> is
non-zero it specifies the maximum time for the sleeping,
otherwise the sleeping is infinite.</para>
</sect4>
<sect4 xml:id="futex-wake">
<title>FUTEX_WAKE</title>
<para>This operation takes a futex at <varname>uaddr</varname>
and wakes up <varname>val</varname> first futexes queued
on this futex.</para>
</sect4>
<sect4 xml:id="futex-fd">
<title>FUTEX_FD</title>
<para>This operations associates a file descriptor with a given
futex.</para>
</sect4>
<sect4 xml:id="futex-requeue">
<title>FUTEX_REQUEUE</title>
<para>This operation takes <varname>val</varname> threads
queued on futex at <varname>uaddr</varname>, wakes them up,
and takes <varname>val2</varname> next threads and requeues them
on futex at <varname>uaddr2</varname>.</para>
</sect4>
<sect4 xml:id="futex-cmp-requeue">
<title>FUTEX_CMP_REQUEUE</title>
<para>This operation does the same as
<literal>FUTEX_REQUEUE</literal> but it checks that
<varname>val3</varname> equals to <varname>val</varname>
first.</para>
</sect4>
<sect4 xml:id="futex-wake-op">
<title>FUTEX_WAKE_OP</title>
<para>This operation performs an atomic operation on
<varname>val3</varname> (which contains coded some other value)
and <varname>uaddr</varname>. Then it wakes up
<varname>val</varname> threads on futex at
<varname>uaddr</varname> and if the atomic operation returned a
positive number it wakes up <varname>val2</varname> threads on
futex at <varname>uaddr2</varname>.</para>
<para>The operations implemented in
<literal>FUTEX_WAKE_OP</literal>:</para>
<itemizedlist>
<listitem>
<para><literal>FUTEX_OP_SET</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_OP_ADD</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_OP_OR</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_OP_AND</literal></para>
</listitem>
<listitem>
<para><literal>FUTEX_OP_XOR</literal></para>
</listitem>
</itemizedlist>
<note>
<para>There is no <varname>val2</varname> parameter in the
futex prototype. The <varname>val2</varname> is taken from the
<varname>struct timespec *timeout</varname> parameter
for operations <literal>FUTEX_REQUEUE</literal>,
<literal>FUTEX_CMP_REQUEUE</literal> and
<literal>FUTEX_WAKE_OP</literal>.</para>
</note>
</sect4>
</sect3>
<sect3 xml:id="futex-emu">
<title>Futex emulation in &os;</title>
<para>The futex emulation in &os; is taken from NetBSD and further
extended by us. It is placed in <filename>linux_futex.c</filename>
and <filename>linux_futex.h</filename> files. The
<literal>futex</literal> structure looks like:</para>
<programlisting>struct futex {
void *f_uaddr;
int f_refcount;
LIST_ENTRY(futex) f_list;
TAILQ_HEAD(lf_waiting_paroc, waiting_proc) f_waiting_proc;
};</programlisting>
<para>And the structure <literal>waiting_proc</literal> is:</para>
<programlisting>struct waiting_proc {
struct thread *wp_t;
struct futex *wp_new_futex;
TAILQ_ENTRY(waiting_proc) wp_list;
};</programlisting>
<sect4 xml:id="futex-get">
<title>futex_get / futex_put</title>
<para>A futex is obtained using the <function>futex_get</function>
function, which searches a linear list of futexes and returns the
found one or creates a new futex. When releasing a futex from the
use we call the <function>futex_put</function> function, which
decreases a reference counter of the futex and if the refcount
reaches zero it is released.</para>
</sect4>
<sect4 xml:id="futex-sleep">
<title>futex_sleep</title>
<para>When a futex queues a thread for sleeping it creates a
<literal>working_proc</literal> structure and puts this structure
to the list inside the futex structure then it just performs a
&man.tsleep.9; to suspend the thread. The sleep can be timed out.
After &man.tsleep.9; returns (the thread was woken up or it timed
out) the <literal>working_proc</literal> structure is removed
from the list and is destroyed. All this is done in the
<function>futex_sleep</function> function. If we got woken up
from <function>futex_wake</function> we have
<varname>wp_new_futex</varname> set so we sleep on it. This way
the actual requeueing is done in this function.</para>
</sect4>
<sect4 xml:id="futex-wake-2">
<title>futex_wake</title>
<para>Waking up a thread sleeping on a futex is performed in the
<function>futex_wake</function> function. First in this function
we mimic the strange &linux; behavior, where it wakes up N threads
for all operations, the only exception is that the REQUEUE
operations are performed on N+1 threads. But this usually does not
make any difference as we are waking up all threads. Next in the
function in the loop we wake up n threads, after this we check if
there is a new futex for requeueing. If so, we requeue up to n2
threads on the new futex. This cooperates with
<function>futex_sleep</function>.</para>
</sect4>
<sect4 xml:id="futex-wake-op-2">
<title>futex_wake_op</title>
<para>The <literal>FUTEX_WAKE_OP</literal> operation is quite
complicated. First we obtain two futexes at addresses
<varname>uaddr</varname> and <varname>uaddr2</varname> then we
perform the atomic operation using <varname>val3</varname> and
<varname>uaddr2</varname>. Then <varname>val</varname> waiters
on the first futex is woken up and if the atomic operation
condition holds we wake up <varname>val2</varname> (i.e.
<varname>timeout</varname>) waiter on the second futex.</para>
</sect4>
<sect4 xml:id="futex-atomic-op">
<title>futex atomic operation</title>
<para>The atomic operation takes two parameters
<varname>encoded_op</varname> and <varname>uaddr</varname>.
The encoded operation encodes the operation itself,
comparing value, operation argument, and comparing argument.
The pseudocode for the operation is like this one:</para>
<programlisting>oldval = *uaddr2
*uaddr2 = oldval OP oparg</programlisting>
<para>And this is done atomically. First a copying in of the number
at <varname>uaddr</varname> is performed and the operation is
done. The code handles page faults and if no page fault occurs
<varname>oldval</varname> is compared to
<varname>cmparg</varname> argument with cmp comparator.</para>
</sect4>
<sect4 xml:id="futex-locking">
<title>Futex locking</title>
<para>Futex implementation uses two lock lists protecting
<function>sx_lock</function> and global locks (either Giant
or another <function>sx_lock</function>). Every operation is
performed locked from the start to the very end.</para>
</sect4>
</sect3>
</sect2>
<sect2 xml:id="syscall-impl">
<title>Various syscalls implementation</title>
<para>In this section I am going to describe some smaller syscalls that
are worth mentioning because their implementation is not obvious or
those syscalls are interesting from other point of view.</para>
<sect3 xml:id="syscall-at">
<title>*at family of syscalls</title>
<para>During development of &linux; 2.6.16 kernel, the *at syscalls
were added. Those syscalls (<function>openat</function> for example)
work exactly like their at-less counterparts with the slight
exception of the <varname>dirfd</varname> parameter. This
parameter changes where the given file, on which the syscall is to be
performed, is. When the <varname>filename</varname> parameter is
absolute <varname>dirfd</varname> is ignored but when the path to
the file is relative, it comes to the play. The
<varname>dirfd</varname> parameter is a directory relative to which
the relative pathname is checked. The <varname>dirfd</varname>
parameter is a file descriptor of some directory or
<literal>AT_FDCWD</literal>. So for example the
<function>openat</function> syscall can be like this:</para>
<programlisting>file descriptor 123 = /tmp/foo/, current working directory = /tmp/
openat(123, /tmp/bah\, flags, mode) /* opens /tmp/bah */
openat(123, bah\, flags, mode) /* opens /tmp/foo/bah */
openat(AT_FDWCWD, bah\, flags, mode) /* opens /tmp/bah */
openat(stdio, bah\, flags, mode) /* returns error because stdio is not a directory */</programlisting>
<para>This infrastructure is necessary to avoid races when opening
files outside the working directory. Imagine that a process consists
of two threads, thread&nbsp;A and thread&nbsp;B. Thread&nbsp;A
issues <literal>open(./tmp/foo/bah., flags, mode)</literal> and
before returning it gets preempted and thread&nbsp;B runs.
Thread&nbsp;B does not care about the needs of thread&nbsp;A and
renames or removes <filename>/tmp/foo/</filename>. We got a race.
To avoid this we can open <filename>/tmp/foo</filename> and use it
as <varname>dirfd</varname> for <function>openat</function>
syscall. This also enables user to implement per-thread
working directories.</para>
<para>&linux; family of *at syscalls contains:
<function>linux_openat</function>,
<function>linux_mkdirat</function>,
<function>linux_mknodat</function>,
<function>linux_fchownat</function>,
<function>linux_futimesat</function>,
<function>linux_fstatat64</function>,
<function>linux_unlinkat</function>,
<function>linux_renameat</function>,
<function>linux_linkat</function>,
<function>linux_symlinkat</function>,
<function>linux_readlinkat</function>,
<function>linux_fchmodat</function> and
<function>linux_faccessat</function>. All these are implemented
using the modified &man.namei.9; routine and simple
wrapping layer.</para>
<sect4 xml:id="implementation">
<title>Implementation</title>
<para>The implementation is done by altering the
&man.namei.9; routine (described above) to take
additional parameter <varname>dirfd</varname> in its
<literal>nameidata</literal> structure, which specifies the
starting point of the pathname lookup instead of using the
current working directory every time. The resolution of
<varname>dirfd</varname> from file descriptor number to a
vnode is done in native *at syscalls. When
<varname>dirfd</varname> is <literal>AT_FDCWD</literal> the
<varname>dvp</varname> entry in <literal>nameidata</literal>
structure is <literal>NULL</literal> but when
<varname>dirfd</varname> is a different number we obtain a
file for this file descriptor, check whether this file
is valid and if there is vnode attached to it then we get a vnode.
Then we check this vnode for being a directory. In the actual
&man.namei.9; routine we simply substitute the
<varname>dvp</varname> vnode for <varname>dp</varname> variable
in the &man.namei.9; function, which determines the
starting point. The &man.namei.9; is not used
directly but via a trace of different functions on various
levels. For example the <function>openat</function> goes like
this:</para>
<programlisting>openat() --&gt; kern_openat() --&gt; vn_open() -&gt; namei()</programlisting>
<para>For this reason <function>kern_open</function> and
<function>vn_open</function> must be altered to incorporate
the additional <varname>dirfd</varname> parameter. No compat
layer is created for those because there are not many users of
this and the users can be easily converted. This general
implementation enables &os; to implement their own *at syscalls.
This is being discussed right now.</para>
</sect4>
</sect3>
<sect3 xml:id="ioctl">
<title>Ioctl</title>
<para>The ioctl interface is quite fragile due to its generality.
We have to bear in mind that devices differ between &linux; and &os;
so some care must be applied to do ioctl emulation work right. The
ioctl handling is implemented in <filename>linux_ioctl.c</filename>,
where <function>linux_ioctl</function> function is defined. This
function simply iterates over sets of ioctl handlers to find a
handler that implements a given command. The ioctl syscall has three
parameters, the file descriptor, command and an argument. The
command is a 16-bit number, which in theory is divided into high
8&nbsp;bits determining class of the ioctl command and low
8&nbsp;bits, which are the actual command within the given set.
The emulation takes advantage of this division. We implement
handlers for each set, like <function>sound_handler</function>
or <function>disk_handler</function>. Each handler has a maximum
command and a minimum command defined, which is used for determining
what handler is used. There are slight problems with this approach
because &linux; does not use the set division consistently so
sometimes ioctls for a different set are inside a set they should
not belong to (SCSI generic ioctls inside cdrom set, etc.). &os;
currently does not implement many &linux; ioctls (compared to
NetBSD, for example) but the plan is to port those from NetBSD.
The trend is to use &linux; ioctls even in the native &os; drivers
because of the easy porting of applications.</para>
</sect3>
<sect3 xml:id="debugging">
<title>Debugging</title>
<para>Every syscall should be debuggable. For this purpose we
introduce a small infrastructure. We have the ldebug facility, which
tells whether a given syscall should be debugged (settable via a
sysctl). For printing we have LMSG and ARGS macros. Those are used
for altering a printable string for uniform debugging messages.</para>
</sect3>
</sect2>
</sect1>
<sect1 xml:id="conclusion">
<title>Conclusion</title>
<sect2 xml:id="results">
<title>Results</title>
<para>As of April 2007 the &linux; emulation layer is capable of
emulating the &linux;&nbsp;2.6.16 kernel quite well. The remaining
problems concern futexes, unfinished *at family of syscalls,
problematic signals delivery, missing <function>epoll</function> and
<function>inotify</function> and probably some bugs we have not
discovered yet. Despite this we are capable of running basically all
the &linux; programs included in &os; Ports&nbsp;Collection with
Fedora&nbsp;Core&nbsp;4 at 2.6.16 and there are some rudimentary
reports of success with Fedora&nbsp;Core&nbsp;6 at 2.6.16. The
Fedora&nbsp;Core&nbsp;6 linux_base was recently committed enabling
some further testing of the emulation layer and giving us some more
hints where we should put our effort in implementing missing
stuff.</para>
<para>We are able to run the most used applications like
<package>www/linux-firefox</package>,
<package>www/linux-opera</package>,
<package>net-im/skype</package> and some games from
the Ports&nbsp;Collection. Some of the programs exhibit bad behavior
under 2.6 emulation but this is currently under investigation and
hopefully will be fixed soon. The only big application that is
known not to work is the &linux; &java; Development Kit and this is
because of the requirement of <function>epoll</function>
facility which is not directly related to the &linux;
kernel 2.6.</para>
<para>We hope to enable 2.6.16 emulation by default some time after
&os; 7.0 is released at least to expose the 2.6 emulation parts for
some wider testing. Once this is done we can switch to
Fedora&nbsp;Core&nbsp;6 linux_base, which is the ultimate plan.</para>
</sect2>
<sect2 xml:id="future-work">
<title>Future work</title>
<para>Future work should focus on fixing the remaining issues with
futexes, implement the rest of the *at family of syscalls, fix the
signal delivery and possibly implement the <function>epoll</function>
and <function>inotify</function> facilities.</para>
<para>We hope to be able to run the most important programs flawlessly
soon, so we will be able to switch to the 2.6 emulation by default and
make the Fedora&nbsp;Core&nbsp;6 the default linux_base because our
currently used Fedora&nbsp;Core&nbsp;4 is not supported any
more.</para>
<para>The other possible goal is to share our code with NetBSD and
DragonflyBSD. NetBSD has some support for 2.6 emulation but its far
from finished and not really tested. DragonflyBSD has expressed some
interest in porting the 2.6 improvements.</para>
<para>Generally, as &linux; develops we would like to keep up with their
development, implementing newly added syscalls. Splice comes to mind
first. Some already implemented syscalls are also heavily crippled,
for example <function>mremap</function> and others. Some performance
improvements can also be made, finer grained locking and others.</para>
</sect2>
<sect2 xml:id="team">
<title>Team</title>
<para>I cooperated on this project with (in alphabetical order):</para>
<itemizedlist>
<listitem>
<para>&a.jhb.email;</para>
</listitem>
<listitem>
<para>&a.kib.email;</para>
</listitem>
<listitem>
<para>Emmanuel Dreyfus</para>
</listitem>
<listitem>
<para>Scot Hetzel</para>
</listitem>
<listitem>
<para>&a.jkim.email;</para>
</listitem>
<listitem>
<para>&a.netchild.email;</para>
</listitem>
<listitem>
<para>&a.ssouhlal.email;</para>
</listitem>
<listitem>
<para>Li Xiao</para>
</listitem>
<listitem>
<para>&a.davidxu.email;</para>
</listitem>
</itemizedlist>
<para>I would like to thank all those people for their advice, code
reviews and general support.</para>
</sect2>
</sect1>
<sect1 xml:id="literatures">
<title>Literatures</title>
<orderedlist>
<listitem>
<para>Marshall Kirk McKusick - George V. Nevile-Neil. Design
and Implementation of the &os; operating system. Addison-Wesley,
2005.</para>
</listitem>
<listitem>
<para><uri xlink:href="https://tldp.org">https://tldp.org</uri></para>
</listitem>
<listitem>
<para><uri xlink:href="https://www.kernel.org">https://www.kernel.org</uri></para>
</listitem>
</orderedlist>
</sect1>
</article>
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