1e28ab5a96
Rationale: All the changes to the DocBook handbook so far have been careful to keep whitespace changes to a minimum. This is so the translators have as easy a job as possible in identifying exactly what's changed. This has meant the English version has become more and more 'ugly'. Lines indented by the wrong amount, some lines longer than 130 characters, others shorter than 20, gaps of 3 or 4 lines between paragraphs (and sometimes within paragraphs). This makes it difficult to follow the structure of the document, and needlessly complicates fixing SGML problems. It also makes the source practically useless as a teaching aid; the more baroque the source looks, the less likely people are to dive in and contribute. This commit fixes all that -- and boy was it tedious. The snag is, it's touched almost every line in every file in the Handbook. Technically, the changes were made by running (in Emacs) sgml-indent-or-tab (bound to the TAB key) on almost each line (except those in <programlisting>, <screen>, <literallayout>, and other verbatim sections), and then running sgml-fill-element (bound to C-c C-q) on most paragraphs. FWIW, this is the first, only, and last change of this type contemplated.
590 lines
26 KiB
Text
590 lines
26 KiB
Text
<chapter id="kerneldebug">
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<title>Kernel Debugging</title>
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<para><emphasis>Contributed by &a.paul; and &a.joerg;</emphasis></para>
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<sect1>
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<title>Debugging a Kernel Crash Dump with <command>kgdb</command></title>
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<para>Here are some instructions for getting kernel debugging working on a
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crash dump. They assume that you have enough swap space for a crash
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dump. If you have multiple swap partitions and the first one is too
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small to hold the dump, you can configure your kernel to use an
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alternate dump device (in the <literal>config kernel</literal> line), or
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you can specify an alternate using the
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&man.dumpon.8; command. The best way to use &man.dumpon.8; is to set
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the <literal>dumpdev</literal> variable in
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<filename>/etc/rc.conf</filename>. Typically you want to specify one of
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the swap devices specified in <filename>/etc/fstab</filename>. Dumps to
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non-swap devices, tapes for example, are currently not supported. Config
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your kernel using <command>config -g</command>. See <link
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linkend="kernelconfig">Kernel Configuration</link> for details on
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configuring the FreeBSD kernel.</para>
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<para>Use the &man.dumpon.8; command to tell the kernel where to dump to
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(note that this will have to be done after configuring the partition in
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question as swap space via &man.swapon.8;). This is normally arranged
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via <filename>/etc/rc.conf</filename> and <filename>/etc/rc</filename>.
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Alternatively, you can hard-code the dump device via the
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<literal>dump</literal> clause in the <literal>config</literal> line of
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your kernel config file. This is deprecated and should be used only if
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you want a crash dump from a kernel that crashes during booting.</para>
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<note>
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<para>In the following, the term <command>kgdb</command> refers to
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<command>gdb</command> run in “kernel debug mode”. This
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can be accomplished by either starting the <command>gdb</command> with
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the option <option>-k</option>, or by linking and starting it under
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the name <command>kgdb</command>. This is not being done by default,
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however, and the idea is basically deprecated since the GNU folks do
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not like their tools to behave differently when called by another
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name. This feature may well be discontinued in further
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releases.</para>
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</note>
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<para>When the kernel has been built make a copy of it, say
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<filename>kernel.debug</filename>, and then run <command>strip
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-d</command> on the original. Install the original as normal. You
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may also install the unstripped kernel, but symbol table lookup time for
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some programs will drastically increase, and since the whole kernel is
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loaded entirely at boot time and cannot be swapped out later, several
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megabytes of physical memory will be wasted.</para>
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<para>If you are testing a new kernel, for example by typing the new
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kernel's name at the boot prompt, but need to boot a different one in
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order to get your system up and running again, boot it only into single
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user state using the <option>-s</option> flag at the boot prompt, and
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then perform the following steps:</para>
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<screen>&prompt.root; <userinput>fsck -p</userinput>
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&prompt.root; <userinput>mount -a -t ufs</userinput> # so your file system for /var/crash is writable
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&prompt.root; <userinput>savecore -N /kernel.panicked /var/crash</userinput>
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&prompt.root; <userinput>exit</userinput> # ...to multi-user</screen>
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<para>This instructs &man.savecore.8; to use another kernel for symbol
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name extraction. It would otherwise default to the currently running
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kernel and most likely not do anything at all since the crash dump and
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the kernel symbols differ.</para>
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<para>Now, after a crash dump, go to
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<filename>/sys/compile/WHATEVER</filename> and run
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<command>kgdb</command>. From <command>kgdb</command> do:
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<screen><userinput>symbol-file kernel.debug</userinput>
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<userinput>exec-file /var/crash/kernel.0</userinput>
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<userinput>core-file /var/crash/vmcore.0</userinput></screen>
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and voila, you can debug the crash dump using the kernel sources just
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like you can for any other program.</para>
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<para>Here is a script log of a <command>kgdb</command> session
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illustrating the procedure. Long lines have been folded to improve
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readability, and the lines are numbered for reference. Despite this, it
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is a real-world error trace taken during the development of the pcvt
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console driver.</para>
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<screen> 1:Script started on Fri Dec 30 23:15:22 1994
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2:&prompt.root; <userinput>cd /sys/compile/URIAH</userinput>
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3:&prompt.root; <userinput>kgdb kernel /var/crash/vmcore.1</userinput>
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4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel...done.
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5:IdlePTD 1f3000
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6:panic: because you said to!
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7:current pcb at 1e3f70
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8:Reading in symbols for ../../i386/i386/machdep.c...done.
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9:<prompt>(kgdb)</prompt> <userinput>where</userinput>
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10:#0 boot (arghowto=256) (../../i386/i386/machdep.c line 767)
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11:#1 0xf0115159 in panic ()
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12:#2 0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
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13:#3 0xf010185e in db_fncall ()
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14:#4 0xf0101586 in db_command (-266509132, -266509516, -267381073)
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15:#5 0xf0101711 in db_command_loop ()
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16:#6 0xf01040a0 in db_trap ()
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17:#7 0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
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18:#8 0xf019d2eb in trap_fatal (...)
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19:#9 0xf019ce60 in trap_pfault (...)
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20:#10 0xf019cb2f in trap (...)
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21:#11 0xf01932a1 in exception:calltrap ()
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22:#12 0xf0191503 in cnopen (...)
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23:#13 0xf0132c34 in spec_open ()
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24:#14 0xf012d014 in vn_open ()
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25:#15 0xf012a183 in open ()
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26:#16 0xf019d4eb in syscall (...)
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27:<prompt>(kgdb)</prompt> <userinput>up 10</userinput>
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28:Reading in symbols for ../../i386/i386/trap.c...done.
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29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
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30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
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31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
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32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
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33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
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34:ss = -266427884}) (../../i386/i386/trap.c line 283)
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35:283 (void) trap_pfault(&frame, FALSE);
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36:<prompt>(kgdb)</prompt> <userinput>frame frame->tf_ebp frame->tf_eip</userinput>
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37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
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38:#0 0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
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39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
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40:403 return ((*linesw[tp->t_line].l_open)(dev, tp));
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41:<prompt>(kgdb)</prompt> <userinput>list</userinput>
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42:398
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43:399 tp->t_state |= TS_CARR_ON;
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44:400 tp->t_cflag |= CLOCAL; /* cannot be a modem (:-) */
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45:401
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46:402 #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
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47:403 return ((*linesw[tp->t_line].l_open)(dev, tp));
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48:404 #else
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49:405 return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
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50:406 #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
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51:407 }
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52:<prompt>(kgdb)</prompt> <userinput>print tp</userinput>
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53:Reading in symbols for ../../i386/i386/cons.c...done.
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54:$1 = (struct tty *) 0x1bae
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55:<prompt>(kgdb)</prompt> <userinput>print tp->t_line</userinput>
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56:$2 = 1767990816
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57:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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58:#1 0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
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59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
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60: return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
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61:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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62:#2 0xf0132c34 in spec_open ()
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63:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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64:#3 0xf012d014 in vn_open ()
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65:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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66:#4 0xf012a183 in open ()
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67:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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68:#5 0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
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69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
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70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
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71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
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72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
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73:673 error = (*callp->sy_call)(p, args, rval);
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74:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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75:Initial frame selected; you cannot go up.
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76:<prompt>(kgdb)</prompt> <userinput>quit</userinput>
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77:&prompt.root; <userinput>exit</userinput>
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78:exit
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79:
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80:Script done on Fri Dec 30 23:18:04 1994</screen>
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<para>Comments to the above script:</para>
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<variablelist>
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<varlistentry>
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<term>line 6:</term>
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<listitem>
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<para>This is a dump taken from within DDB (see below), hence the
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panic comment “because you said to!”, and a rather
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long stack trace; the initial reason for going into DDB has been a
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page fault trap though.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>line 20:</term>
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<listitem>
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<para>This is the location of function <function>trap()</function>
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in the stack trace.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>line 36:</term>
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<listitem>
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<para>Force usage of a new stack frame; this is no longer necessary
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now. The stack frames are supposed to point to the right
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locations now, even in case of a trap. (I do not have a new core
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dump handy <g>, my kernel has not panicked for a rather long
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time.) From looking at the code in source line 403, there is a
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high probability that either the pointer access for
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“tp” was messed up, or the array access was out of
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bounds.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>line 52:</term>
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<listitem>
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<para>The pointer looks suspicious, but happens to be a valid
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address.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>line 56:</term>
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<listitem>
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<para>However, it obviously points to garbage, so we have found our
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error! (For those unfamiliar with that particular piece of code:
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<literal>tp->t_line</literal> refers to the line discipline of
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the console device here, which must be a rather small integer
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number.)</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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<sect1>
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<title>Debugging a crash dump with DDD</title>
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<para>Examining a kernel crash dump with a graphical debugger like
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<command>ddd</command> is also possible. Add the <option>-k</option>
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option to the <command>ddd</command> command line you would use
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normally. For example;</para>
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<screen>&prompt.root; <userinput>ddd -k /var/crash/kernel.0 /var/crash/vmcore.0</userinput></screen>
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<para>You should then be able to go about looking at the crash dump using
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<command>ddd</command>'d graphical interface.</para>
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</sect1>
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<sect1>
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<title>Post-mortem Analysis of a Dump</title>
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<para>What do you do if a kernel dumped core but you did not expect it,
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and it is therefore not compiled using <command>config -g</command>? Not
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everything is lost here. Do not panic!</para>
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<para>Of course, you still need to enable crash dumps. See above on the
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options you have to specify in order to do this.</para>
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<para>Go to your kernel compile directory, and edit the line containing
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<literal>COPTFLAGS?=-O</literal>. Add the <option>-g</option> option
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there (but <emphasis>do not</emphasis> change anything on the level of
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optimization). If you do already know roughly the probable location of
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the failing piece of code (e.g., the <devicename>pcvt</devicename>
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driver in the example above), remove all the object files for this code.
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Rebuild the kernel. Due to the time stamp change on the Makefile, there
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will be some other object files rebuild, for example
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<filename>trap.o</filename>. With a bit of luck, the added
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<option>-g</option> option will not change anything for the generated
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code, so you will finally get a new kernel with similar code to the
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faulting one but some debugging symbols. You should at least verify the
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old and new sizes with the
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&man.size.1; command. If there is a mismatch, you probably need to
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give up here.</para>
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<para>Go and examine the dump as described above. The debugging symbols
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might be incomplete for some places, as can be seen in the stack trace
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in the example above where some functions are displayed without line
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numbers and argument lists. If you need more debugging symbols, remove
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the appropriate object files and repeat the <command>kgdb</command>
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session until you know enough.</para>
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<para>All this is not guaranteed to work, but it will do it fine in most
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cases.</para>
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</sect1>
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<sect1>
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<title>On-line Kernel Debugging Using DDB</title>
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<para>While <command>kgdb</command> as an offline debugger provides a very
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high level of user interface, there are some things it cannot do. The
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most important ones being breakpointing and single-stepping kernel
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code.</para>
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<para>If you need to do low-level debugging on your kernel, there is an
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on-line debugger available called DDB. It allows to setting
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breakpoints, single-steping kernel functions, examining and changing
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kernel variables, etc. However, it cannot access kernel source files,
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and only has access to the global and static symbols, not to the full
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debug information like <command>kgdb</command>.</para>
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<para>To configure your kernel to include DDB, add the option line
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<programlisting>
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options DDB</programlisting>
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to your config file, and rebuild. (See <link
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linkend="kernelconfig">Kernel Configuration</link> for details on
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configuring the FreeBSD kernel.</para>
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<note>
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<para>Note that if you have an older version of the boot blocks, your
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debugger symbols might not be loaded at all. Update the boot blocks;
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the recent ones load the DDB symbols automagically.)</para>
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</note>
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<para>Once your DDB kernel is running, there are several ways to enter
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DDB. The first, and earliest way is to type the boot flag
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<option>-d</option> right at the boot prompt. The kernel will start up
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in debug mode and enter DDB prior to any device probing. Hence you can
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even debug the device probe/attach functions.</para>
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<para>The second scenario is a hot-key on the keyboard, usually
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Ctrl-Alt-ESC. For syscons, this can be remapped; some of the
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distributed maps do this, so watch out. There is an option available
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for serial consoles that allows the use of a serial line BREAK on the
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console line to enter DDB (<literal>options BREAK_TO_DEBUGGER</literal>
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in the kernel config file). It is not the default since there are a lot
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of crappy serial adapters around that gratuitously generate a BREAK
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condition, for example when pulling the cable.</para>
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<para>The third way is that any panic condition will branch to DDB if the
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kernel is configured to use it. For this reason, it is not wise to
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configure a kernel with DDB for a machine running unattended.</para>
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<para>The DDB commands roughly resemble some <command>gdb</command>
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commands. The first thing you probably need to do is to set a
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breakpoint:</para>
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<screen><userinput>b function-name</userinput>
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<userinput>b address</userinput></screen>
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<para>Numbers are taken hexadecimal by default, but to make them distinct
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from symbol names; hexadecimal numbers starting with the letters
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<literal>a-f</literal> need to be preceded with <literal>0x</literal>
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(this is optional for other numbers). Simple expressions are allowed,
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for example: <literal>function-name + 0x103</literal>.</para>
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<para>To continue the operation of an interrupted kernel, simply
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type:</para>
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<screen><userinput>c</userinput></screen>
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<para>To get a stack trace, use:</para>
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<screen><userinput>trace</userinput></screen>
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<note>
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<para>Note that when entering DDB via a hot-key, the kernel is currently
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servicing an interrupt, so the stack trace might be not of much use
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for you.</para>
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</note>
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<para>If you want to remove a breakpoint, use</para>
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<screen><userinput>del</userinput>
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<userinput>del address-expression</userinput></screen>
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<para>The first form will be accepted immediately after a breakpoint hit,
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and deletes the current breakpoint. The second form can remove any
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breakpoint, but you need to specify the exact address; this can be
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obtained from:</para>
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<screen><userinput>show b</userinput></screen>
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<para>To single-step the kernel, try:</para>
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<screen><userinput>s</userinput></screen>
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<para>This will step into functions, but you can make DDB trace them until
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the matching return statement is reached by:</para>
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<screen><userinput>n</userinput></screen>
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<note>
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<para>This is different from <command>gdb</command>'s
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<command>next</command> statement; it is like <command>gdb</command>'s
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<command>finish</command>.</para>
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</note>
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<para>To examine data from memory, use (for example):
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<screen><userinput>x/wx 0xf0133fe0,40</userinput>
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<userinput>x/hd db_symtab_space</userinput>
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<userinput>x/bc termbuf,10</userinput>
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<userinput>x/s stringbuf</userinput></screen>
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for word/halfword/byte access, and hexadecimal/decimal/character/ string
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display. The number after the comma is the object count. To display
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the next 0x10 items, simply use:</para>
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<screen><userinput>x ,10</userinput></screen>
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<para>Similarly, use
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<screen><userinput>x/ia foofunc,10</userinput></screen>
|
|
|
|
to disassemble the first 0x10 instructions of
|
|
<function>foofunc</function>, and display them along with their offset
|
|
from the beginning of <function>foofunc</function>.</para>
|
|
|
|
<para>To modify memory, use the write command:</para>
|
|
|
|
<screen><userinput>w/b termbuf 0xa 0xb 0</userinput>
|
|
<userinput>w/w 0xf0010030 0 0</userinput></screen>
|
|
|
|
<para>The command modifier
|
|
(<literal>b</literal>/<literal>h</literal>/<literal>w</literal>)
|
|
specifies the size of the data to be written, the first following
|
|
expression is the address to write to and the remainder is interpreted
|
|
as data to write to successive memory locations.</para>
|
|
|
|
<para>If you need to know the current registers, use:</para>
|
|
|
|
<screen><userinput>show reg</userinput></screen>
|
|
|
|
<para>Alternatively, you can display a single register value by e.g.
|
|
|
|
<screen><userinput>p $eax</userinput></screen>
|
|
|
|
and modify it by:</para>
|
|
|
|
<screen><userinput>set $eax new-value</userinput></screen>
|
|
|
|
<para>Should you need to call some kernel functions from DDB, simply
|
|
say:</para>
|
|
|
|
<screen><userinput>call func(arg1, arg2, ...)</userinput></screen>
|
|
|
|
<para>The return value will be printed.</para>
|
|
|
|
<para>For a &man.ps.1; style summary of all running processes, use:</para>
|
|
|
|
<screen><userinput>ps</userinput></screen>
|
|
|
|
<para>Now you have now examined why your kernel failed, and you wish to
|
|
reboot. Remember that, depending on the severity of previous
|
|
malfunctioning, not all parts of the kernel might still be working as
|
|
expected. Perform one of the following actions to shut down and reboot
|
|
your system:</para>
|
|
|
|
<screen><userinput>call diediedie()</userinput></screen>
|
|
|
|
<para>This will cause your kernel to dump core and reboot, so you can
|
|
later analyze the core on a higher level with kgdb. This command
|
|
usually must be followed by another <command>continue</command>
|
|
statement. There is now an alias for this:
|
|
<command>panic</command>.</para>
|
|
|
|
<screen><userinput>call boot(0)</userinput></screen>
|
|
|
|
<para>Which might be a good way to cleanly shut down the running system,
|
|
<function>sync()</function> all disks, and finally reboot. As long as
|
|
the disk and file system interfaces of the kernel are not damaged, this
|
|
might be a good way for an almost clean shutdown.</para>
|
|
|
|
<screen><userinput>call cpu_reset()</userinput></screen>
|
|
|
|
<para>is the final way out of disaster and almost the same as hitting the
|
|
Big Red Button.</para>
|
|
|
|
<para>If you need a short command summary, simply type:</para>
|
|
|
|
<screen><userinput>help</userinput></screen>
|
|
|
|
<para>However, it is highly recommended to have a printed copy of the
|
|
&man.ddb.4; manual page ready for a debugging
|
|
session. Remember that it is hard to read the on-line manual while
|
|
single-stepping the kernel.</para>
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>On-line Kernel Debugging Using Remote GDB</title>
|
|
|
|
<para>This feature has been supported since FreeBSD 2.2, and it's actually
|
|
a very neat one.</para>
|
|
|
|
<para>GDB has already supported <emphasis>remote debugging</emphasis> for
|
|
a long time. This is done using a very simple protocol along a serial
|
|
line. Unlike the other methods described above, you will need two
|
|
machines for doing this. One is the host providing the debugging
|
|
environment, including all the sources, and a copy of the kernel binary
|
|
with all the symbols in it, and the other one is the target machine that
|
|
simply runs a similar copy of the very same kernel (but stripped of the
|
|
debugging information).</para>
|
|
|
|
<para>You should configure the kernel in question with <command>config
|
|
-g</command>, include <option>DDB</option> into the configuration, and
|
|
compile it as usual. This gives a large blurb of a binary, due to the
|
|
debugging information. Copy this kernel to the target machine, strip
|
|
the debugging symbols off with <command>strip -x</command>, and boot it
|
|
using the <option>-d</option> boot option. Connect the first serial
|
|
line of the target machine to any serial line of the debugging host.
|
|
Now, on the debugging machine, go to the compile directory of the target
|
|
kernel, and start gdb:</para>
|
|
|
|
<screen>&prompt.user; <userinput>gdb -k kernel</userinput>
|
|
GDB is free software and you are welcome to distribute copies of it
|
|
under certain conditions; type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB; type "show warranty" for details.
|
|
GDB 4.16 (i386-unknown-freebsd),
|
|
Copyright 1996 Free Software Foundation, Inc...
|
|
<prompt>(kgdb)</prompt> </screen>
|
|
|
|
<para>Initialize the remote debugging session (assuming the first serial
|
|
port is being used) by:</para>
|
|
|
|
<screen><prompt>(kgdb)</prompt> <userinput>target remote /dev/cuaa0</userinput></screen>
|
|
|
|
<para>Now, on the target host (the one that entered DDB right before even
|
|
starting the device probe), type:</para>
|
|
|
|
<screen>Debugger("Boot flags requested debugger")
|
|
Stopped at Debugger+0x35: movb $0, edata+0x51bc
|
|
<prompt>db></prompt> <userinput>gdb</userinput></screen>
|
|
|
|
<para>DDB will respond with:</para>
|
|
|
|
<screen>Next trap will enter GDB remote protocol mode</screen>
|
|
|
|
<para>Every time you type <command>gdb</command>, the mode will be toggled
|
|
between remote GDB and local DDB. In order to force a next trap
|
|
immediately, simply type <command>s</command> (step). Your hosting GDB
|
|
will now gain control over the target kernel:</para>
|
|
|
|
<screen>Remote debugging using /dev/cuaa0
|
|
Debugger (msg=0xf01b0383 "Boot flags requested debugger")
|
|
at ../../i386/i386/db_interface.c:257
|
|
<prompt>(kgdb)</prompt></screen>
|
|
|
|
<para>You can use this session almost as any other GDB session, including
|
|
full access to the source, running it in gud-mode inside an Emacs window
|
|
(which gives you an automatic source code display in another Emacs
|
|
window) etc.</para>
|
|
|
|
<para>Remote GDB can also be used to debug LKMs. First build the LKM with
|
|
debugging symbols:</para>
|
|
|
|
<screen>&prompt.root; <userinput>cd /usr/src/lkm/linux</userinput>
|
|
&prompt.root; <userinput>make clean; make COPTS=-g</userinput></screen>
|
|
|
|
<para>Then install this version of the module on the target machine, load
|
|
it and use <command>modstat</command> to find out where it was
|
|
loaded:</para>
|
|
|
|
<screen>&prompt.root; <userinput>linux</userinput>
|
|
&prompt.root; <userinput>modstat</userinput>
|
|
Type Id Off Loadaddr Size Info Rev Module Name
|
|
EXEC 0 4 f5109000 001c f510f010 1 linux_mod</screen>
|
|
|
|
<para>Take the load address of the module and add 0x20 (probably to
|
|
account for the a.out header). This is the address that the module code
|
|
was relocated to. Use the <command>add-symbol-file</command> command in
|
|
GDB to tell the debugger about the module:</para>
|
|
|
|
<screen><prompt>(kgdb)</prompt> <userinput>add-symbol-file /usr/src/lkm/linux/linux_mod.o 0xf5109020</userinput>
|
|
add symbol table from file "/usr/src/lkm/linux/linux_mod.o" at
|
|
text_addr = 0xf5109020? (y or n) <userinput>y</userinput>
|
|
<prompt>(kgdb)</prompt></screen>
|
|
|
|
<para>You now have access to all the symbols in the LKM.</para>
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>Debugging a Console Driver</title>
|
|
|
|
<para>Since you need a console driver to run DDB on, things are more
|
|
complicated if the console driver itself is failing. You might remember
|
|
the use of a serial console (either with modified boot blocks, or by
|
|
specifying <option>-h</option> at the <prompt>Boot:</prompt> prompt),
|
|
and hook up a standard terminal onto your first serial port. DDB works
|
|
on any configured console driver, of course also on a serial
|
|
console.</para>
|
|
</sect1>
|
|
</chapter>
|
|
|
|
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