From fd8b5bd57ea9f0b8aa3e69c466a0b8fefccf1c99 Mon Sep 17 00:00:00 2001
From: Eric-Paul Ickhorn <ericp.ickhorn@gmail.com>
Date: Tue, 30 Jul 2024 08:52:24 +0200
Subject: [PATCH] Fix bug in GDT when using KVM acceleration

The bug was due to the DS segment being set to the 0th segment entry,
causing a general protection fault.
---
 i386/loader/src-asm/memory/gdt.asm | 131 +----------------------------
 1 file changed, 4 insertions(+), 127 deletions(-)

diff --git a/i386/loader/src-asm/memory/gdt.asm b/i386/loader/src-asm/memory/gdt.asm
index 220c686..897caa1 100755
--- a/i386/loader/src-asm/memory/gdt.asm
+++ b/i386/loader/src-asm/memory/gdt.asm
@@ -2,6 +2,8 @@
 GDT_MEMORY_AREA equ 0x7e00
 GDT_ENTRY_COUNT equ 3
 
+
+
 ; Arguments (note: all arguments are 4-byte padded):
 ;   [Furthest from EBP]
 ;
@@ -109,114 +111,6 @@ make_regular_data_segment_in_gdt:
 
 
 
-; Arguments (note: all arguments are 4-byte padded):
-;   [Furthest from EBP]
-;
-;    4. uint32       privilege_level
-;       - Only 0 to 3; 0 being kernel and 3 being userland.
-;         Everything above 3 is an error.
-;       
-;    3  uint32      num_pages
-;       - Only 20 bits are usable. Everything above will
-;         be cut off to be written to the segment descriptor.
-;
-;    2. ptr32       base_address
-;
-;    1. uint32      segment_index
-;       Note:
-;       - Zero is NOT usable and will throw an error.
-;         the highest theoretical value for this is 8191,
-;         but generally, one should stick to GDT_ENTRY_COUNT.
-;
-;   [Nearest to EBP]
-; COMMENTED OUT DUE TO BEING BUGGY!
-; make_stack_segment_in_gdt:
-; .prolog:
-;     push esi
-;     add esp, 64
-;     mov esi, esp
-
-;     mov [esi + (64 - 4)], eax
-;     mov [esi + (64 - 8)], ebx
-;     mov [esi + (64 - 12)], ecx
-;     mov [esi + (64 - 16)], edx
-;     mov [esi + (64 - 20)], edi
-
-; .get_arguments:
-;     mov eax, [ebp - 4]  ; segment_index
-;     mov ebx, [ebp - 8]  ; base_address
-;     mov ecx, [ebp - 12] ; num_pages
-;     mov edx, [ebp - 16] ; privilege_level
-
-;     mov [esi + 16], eax ; segment_index
-;     mov [esi + 20], ebx ; base_address
-;     mov [esi + 24], ecx ; num_pages
-;     mov [esi + 28], edx ; privilege_level
-
-; .validate_arguments:
-;     cmp eax, 8191
-;     ja .segment_index_too_tall
-;     cmp eax, 0
-;     je .segment_index_is_zero
-
-;     cmp edx, 3
-;     ja .privilege_level_too_large
-
-; .calculate_work_address:
-;     shl eax, 3 ; Entry offset from table start
-;     add eax, GDT_MEMORY_AREA ; Entry address
-;     mov [esi + 12], eax
-
-; .write_segment_limit:
-;     mov [eax], cx
-;     shr ecx, 16
-;     and ecx, 0x0f
-;     mov [eax + 6], cl
-
-; .write_base_address:
-;     mov [eax + 2], bx
-;     shr bx, 16
-;     mov [eax + 4], bl
-;     mov [eax + 7], bh
-
-; .set_default_flags:
-;     or [eax + 6], word (0b1100 << 4)
-
-; .set_stack_specific_access_byte:
-;     mov al, 0b1001
-;     ; OR the privilege level into the access byte
-;     mov dl, [ebp - 16]
-;     shl dl, 1
-;     and dl, 0b0110
-;     or al, dl
-;     shl al, 4 ; Move the 4 bits of general flags to their place
-
-;     ; Stack specific part
-;     or al, 0b0110 ; DATA | STACK | READ_WRITE | NOT_ACCESSED
-
-;     mov ebx, [esi + 12]
-;     mov [ebx + 5], al ; Write to the final destination
-
-; .epilog:
-;     mov edi, [esi + (64 - 20)]
-;     mov edx, [esi + (64 - 16)]
-;     mov ecx, [esi + (64 - 12)]
-;     mov ebx, [esi + (64 - 8)]
-;     mov eax, [esi + (64 - 4)]
-
-;     sub esp, 64
-;     pop esi
-;     ret
-
-; .segment_index_too_tall:
-; .segment_index_is_zero:
-; .privilege_level_too_large:
-;     ; todo(logs): Write debug message.
-;     cli
-;     hlt
-
-
-
 ; Arguments (note: all arguments are 4-byte padded):
 ;   [Furthest from EBP]
 ;
@@ -323,6 +217,7 @@ make_code_segment_in_gdt:
     hlt
 
 
+
 ; [Furthest to EBP]
 ;   1)  Return Address
 ; [Nearest to EBP]
@@ -352,17 +247,6 @@ load_flat_gdt:
     mov esp, ebp
     pop ebp
 
-    ; COMMENTED OUT DUE TO BEING BUGGY
-    ; push ebp
-    ; mov ebp, esp
-    ; push dword 3                    ; Segment Index
-    ; push dword LOWER_STACK_POINTER  ; Start Offset
-    ; push dword 0xfffff              ; Page Count
-    ; push dword 0                    ; Required privilege level
-    ; call make_stack_segment_in_gdt
-    ; mov esp, ebp
-    ; pop ebp
-
     push ebp
     mov ebp, esp
     push dword 1        ; Segment Index
@@ -382,16 +266,12 @@ load_flat_gdt:
     mov es, bx
     mov fs, bx
     mov gs, bx
-    ; mov bx, (3 << 3)
     mov ss, bx
 
     mov ebx, cr0
     or ebx, 1
     mov cr0, ebx
 
-    xor bx, bx
-    mov ds, bx
-
     jmp dword 0x08:.epilog
 
 bits 32
@@ -402,10 +282,7 @@ bits 32
     mov ebx, [esi + (64 - 8)]
     mov eax, [esi + (64 - 4)]
 
-    add esp, 64
-    pop esi
-    jmp stage2_true_entry
-    ; jmp after_gdt_loaded
+    jmp dword stage2_true_entry
 
 align 16
 .gdtr: