From 5ba30e5f8b1540fce7fb50c23f2937bac7e1f38d Mon Sep 17 00:00:00 2001
From: Eric-Paul Ickhorn <ericp.ickhorn@gmail.com>
Date: Tue, 6 Aug 2024 13:20:40 +0200
Subject: [PATCH] Fix GDT length being insufficient

The value of the GDTR register was set to be 8 bytes too small, with
the effect that an accurate emulation software would error out because
of trying to read over the buffer size, as was noticed when using KVM.

This was due to the perception that the 0th index wouldn't be taken
into account; for that reason, the length of the GDTR was one index
short of what it should've been.
---
 i386/loader/src-asm/memory/gdt.asm | 38 +++++++++++++++++-------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/i386/loader/src-asm/memory/gdt.asm b/i386/loader/src-asm/memory/gdt.asm
index 897caa1..7d1cfcb 100755
--- a/i386/loader/src-asm/memory/gdt.asm
+++ b/i386/loader/src-asm/memory/gdt.asm
@@ -239,28 +239,40 @@ load_flat_gdt:
 .set_segments:
     push ebp
     mov ebp, esp
-    push dword 2        ; Segment Index
-    push dword 0        ; Start Offset
-    push dword 0xfffff  ; Page Count
-    push dword 0        ; Required privilege level
+    push dword 2            ; Segment Index
+    push dword 0            ; Start Offset
+    push dword 0x000fffff   ; Page Count
+    push dword 0            ; Required privilege level
     call make_regular_data_segment_in_gdt
     mov esp, ebp
     pop ebp
 
     push ebp
     mov ebp, esp
-    push dword 1        ; Segment Index
-    push dword 0x0000   ; Start Offset
-    push dword 0xfffff  ; Page Count
-    push dword 0        ; Required privilege level
+    push dword 1            ; Segment Index
+    push dword 0x0000       ; Start Offset
+    push dword 0x000fffff   ; Page Count
+    push dword 0            ; Required privilege level
     call make_code_segment_in_gdt
     mov esp, ebp
     pop ebp
 
     mov bx, 0
     mov ds, bx
+    mov es, bx
+    mov fs, bx
+    mov gs, bx
+    mov ss, bx
     lgdt [.gdtr]
 
+    mov ebx, cr0
+    or ebx, 1
+    mov cr0, ebx
+
+    jmp dword (1 << 3):.epilog
+
+bits 32
+.epilog:
     mov bx, (2 << 3)
     mov ds, bx
     mov es, bx
@@ -268,14 +280,6 @@ load_flat_gdt:
     mov gs, bx
     mov ss, bx
 
-    mov ebx, cr0
-    or ebx, 1
-    mov cr0, ebx
-
-    jmp dword 0x08:.epilog
-
-bits 32
-.epilog:
     mov edi, [esi + (64 - 20)]
     mov edx, [esi + (64 - 16)]
     mov ecx, [esi + (64 - 12)]
@@ -287,7 +291,7 @@ bits 32
 align 16
 .gdtr:
 .gdtr.length:
-    dw 15
+    dw 23
 .gdtr.pointer:
     dd GDT_MEMORY_AREA