mm: replace GRUB's GDT with our own
parent
66a1f8726e
commit
8129518640
GPG Key ID:
EC22E476DC2D3D84
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/* See the end of this file for copyright and license terms. */
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#pragma once
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#include <gay/config.h>
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/** @brief Kernel code segment GDT selector */
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#define X86_KERN_CS 0x08
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/** @brief Kernel data segment GDT selector */
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#define X86_KERN_DS 0x10
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/** @brief Userland code segment GDT selector */
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#define X86_USER_CS 0x18
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/** @brief Userland data segment GDT selector */
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#define X86_USER_DS 0x20
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#ifndef _ASM_SOURCE
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/*
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* This code is mostly obsolete (until we need to actually manipulate the GDT
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* when there is long mode support and we want to enable 32-bit compatibility
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* mode). I wrote this because i initially wanted to use it for an encoder that
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* would dynamically initialize the GDT entries at runtime, but fuck that.
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* I've lost way more than enough time on the utterly useless GDT, so we'll just
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* leave this here as it is until we actually need it. See arch/mm/segment.S
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* for the actual GDT entries and how the GDT is loaded.
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*/
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#include <arch/page.h>
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#include <gay/cdefs.h>
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#include <gay/types.h>
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/** @brief x86 Global Descriptor Table entry as used by the kernel. */
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struct x86_gdt_entry {
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u32 base;
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unsigned limit:20;
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unsigned flags:4;
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#define X86_GDT_SIZE (1u << 2)
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#define X86_GDT_GRANULARITY (1u << 3)
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u8 access;
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#define X86_GDT_ACCESSED (1u << 0)
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#define X86_GDT_RW (1u << 1)
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#define X86_GDT_DIRECTION (1u << 2) /* for data selectors (executable not set) */
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#define X86_GDT_CONFORMING (1u << 2) /* for code selectors (executable set) */
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#define X86_GDT_EXEC (1u << 3)
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#define X86_GDT_TYPE (1u << 4) /* 1 for code/data, 0 for system (e.g. TSS) */
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#define X86_GDT_PRIVL_SHIFT (5)
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#define X86_GDT_PRIVL_MASK (3u << X86_GDT_PRIVL_SHIFT)
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#define X86_GDT_PRIVL(n) (((n) << X86_GDT_PRIVL_SHIFT) & X86_GDT_PRIVL_MASK)
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#define X86_GDT_PRESENT (1u << 7)
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};
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/** @brief x86 Global Descriptor Table entry as laid out in hardware. */
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struct x86_insane_gdt_entry {
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u16 limit0;
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u16 base0;
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u8 base1;
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u8 access;
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unsigned limit1:4;
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unsigned flags:4;
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u8 base2;
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} __packed;
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/** @brief The main GDT (defined in `arch/mm/segment.S`) */
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extern struct x86_insane_gdt_entry x86_gdt[5];
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struct x86_tss {
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u16 link; u16 _reserved0;
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u32 esp0;
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u16 ss0; u16 _reserved1;
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u32 esp1;
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u16 ss1; u16 _reserved2;
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u32 esp2;
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u16 ss2; u16 _reserved3;
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u32 cr3;
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u32 eip;
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u32 eflags;
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u32 eax;
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u32 ecx;
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u32 edx;
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u32 ebx;
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u32 esp;
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u32 ebp;
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u32 esi;
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u32 edi;
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u16 es; u16 _reserved4;
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u16 cs; u16 _reserved5;
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u16 ss; u16 _reserved6;
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u16 ds; u16 _reserved7;
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u16 fs; u16 _reserved8;
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u16 gs; u16 _reserved9;
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u16 ldtr; u16 _reserved10;
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u16 _reserved11; u16 iopb_offset;
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};
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#endif /* not _ASM_SOURCE */
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/*
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* This file is part of GayBSD.
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* Copyright (c) 2021 fef <owo@fef.moe>.
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*
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* GayBSD is nonviolent software: you may only use, redistribute, and/or
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* modify it under the terms of the Cooperative Nonviolent Public License
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* (CNPL) as found in the LICENSE file in the source code root directory
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* or at <https://git.pixie.town/thufie/npl-builder>; either version 7
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* of the license, or (at your option) any later version.
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*
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* GayBSD comes with ABSOLUTELY NO WARRANTY, to the extent
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* permitted by applicable law. See the CNPL for details.
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*/
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@ -0,0 +1,65 @@
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/* See the end of this file for copyright and license terms. */
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#include <arch/segment.h>
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#include <asm/common.h>
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ASM_ENTRY(x86_replace_gdt)
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push %ebp
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mov %esp, %ebp
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lgdt x86_gdt_desc
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ljmp $(X86_KERN_CS), $1f
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1: movl $(X86_KERN_DS), %eax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %gs
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movw %ax, %ss
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pop %ebp
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ret
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ASM_END(x86_replace_gdt)
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.data
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.align 4
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.word 0 /* padding */
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x86_gdt_desc:
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.word x86_gdt - x86_gdt_end - 1 /* limit */
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.long x86_gdt /* base */
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/*
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* TODO: The GDT entry structure is so fundamentally fucked up that i gave
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* up writing an encoder for it half way through, so we just use these
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* hardcoded values for now. They were generated using the code on
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* <https://wiki.osdev.org/GDT_Tutorial#Some_stuff_to_make_your_life_easy>
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* and assign the entire 4 GiB region for both code and data as well as
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* kernel and user mode. Even the Intel manual says you're not supposed
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* to use segmentation anymore and just rely on paging for memory
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* protection instead, so we gladly accept their advice.
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*/
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.align 8
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x86_gdt:
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.quad 0x0000000000000000 /* 0x00 null descriptor */
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.quad 0x00cf9a000000ffff /* 0x08 kernel code, full 4 GiB */
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.quad 0x00cf92000000ffff /* 0x10 kernel data, full 4 GiB */
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.quad 0x00cffa000000ffff /* 0x18 user code, full 4 GiB */
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.quad 0x00cff2000000ffff /* 0x20 user data, full 4 GiB */
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x86_gdt_end:
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.size x86_gdt, . - x86_gdt
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/*
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* This file is part of GayBSD.
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* Copyright (c) 2021 fef <owo@fef.moe>.
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*
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* GayBSD is nonviolent software: you may only use, redistribute, and/or
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* modify it under the terms of the Cooperative Nonviolent Public License
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* (CNPL) as found in the LICENSE file in the source code root directory
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* or at <https://git.pixie.town/thufie/npl-builder>; either version 7
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* of the license, or (at your option) any later version.
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*
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* GayBSD comes with ABSOLUTELY NO WARRANTY, to the extent
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* permitted by applicable law. See the CNPL for details.
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*/
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