Karina
70 lines
2.3 KiB
C
70 lines
2.3 KiB
C
// SPDX-License-Identifier: GPL-3.0-or-later
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// Copyright (c) 2025 0xKarinyash
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// just fucking kill me already
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#include "gdt.h"
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GDTDescriptor gdt[5];
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GDTPtr gdt_ptr;
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TSS tss;
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u8 double_fault_stack[4096] = {0};
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void gdt_set_gate(int num, u64 base, u64 limit, u8 access, u8 gran) {
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gdt[num].base_low = (base & 0xFFFF); // left only low 16bit
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gdt[num].base_middle = (base >> 16) & 0xFF; // 3rd address byte
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gdt[num].base_high = (base >> 24) & 0xFF; // 4th address byte
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gdt[num].limit_low = (limit & 0xFFFF);
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gdt[num].granularity = (limit >> 16) & 0x0F; // leave only first 4 bits (limit's left 16-19)
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gdt[num].granularity |= gran & 0xF0; // get flags and concate two pieces so FFFFLLLL (L -- Limit; F -- flags)
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gdt[num].access = access;
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// fuck intel
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}
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void write_tss(int num) {
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u64 base = (u64)&tss;
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u64 limit = sizeof(tss);
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u64 access = 0x89; // TSS_PRESENT | TSS_EXECUTABLE | TSS_ACCESSED;
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u64 gran = 0;
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gdt_set_gate(num, base, limit, access, gran);
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*((u64*)&gdt[num+1]) = (base >> 32); // really scary but not really hard
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// just direct write 8bytes to num+1
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// basically a hack but at this point i dont care
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}
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extern void gdt_flush(u64 gdt_ptr_addr); // entry.asm
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void gdt_init() {
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gdt_ptr.limit = (sizeof(GDTDescriptor) * 5) - 1;
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gdt_ptr.base = (u64)&gdt;
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// 0: Null
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gdt_set_gate(0, 0, 0, 0, 0);
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// 1: Kernel Code (Ring 0)
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// Access: 0x9A (Present, Ring0, Exec/Read)
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// Granularity: 0x20 (Long Mode bit) access ring 0 code exec code readable
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gdt_set_gate(1, 0, 0, 0x9A, 0x20); // 0x9A = 1 00 1 1 0 1
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// 0x20 = 0010 0000 bit 5 is "Long mode"
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// 2: Kernel Data (Ring 0)
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// Access: 0x92 (Present, Ring0, Read/Write)
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gdt_set_gate(2, 0, 0, 0x92, 0); // 0x92 can't exec
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for (u64 i = 0; i < sizeof(TSS); i++) ((u8*)&tss)[i] = 0; // hack. zeroifying tss struct
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tss.iomap_base = sizeof(TSS);
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tss.ist1 = (u64)double_fault_stack + sizeof(double_fault_stack);
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write_tss(3);
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gdt_flush((u64)&gdt_ptr);
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// telling cpu that TSS info located at gdt[3]; offset = 3 * 8 = 24 = 0x18
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__asm__ volatile ("ltr %%ax" :: "a" (0x18));
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}
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