From 4046dd7d6c6cb24c45273220e21c20f39f84d174 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Wed, 13 Jan 2021 17:20:49 +0000
Subject: [PATCH] Improve syscall entrypoint

---
 kernel/syscall.asm | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/kernel/syscall.asm b/kernel/syscall.asm
index d82868d..fdecc26 100644
--- a/kernel/syscall.asm
+++ b/kernel/syscall.asm
@@ -32,7 +32,7 @@ stab:	; syscall table
 	dw err, sret ; return version num
 	dw err, sret ; reset disks
 	dw setdd, sret ; select disk
-	dw err, sretb ; open file
+	dw open, sretb ; open file
 .end:
 
 section .text
@@ -46,6 +46,8 @@ absolute 0
 
 	; saved userdata
 PSPAX:	resw 1
+PSPCX:	resw 1
+PSPBP:	resw 1
 	; SS:SP
 PSPSP:	resw 1
 PSPSS:	resw 1
@@ -69,11 +71,10 @@ pspds:	xor ax, ax
 	mov ds, [curpsp]
 	ret
 
-	; Fix CS:AX pointer for reloc
-fixax:	call .l01
-.l01:	pop bx
-	sub bx, .l01
-	add ax, bx
+	; Get reloc offset into CX
+reloff:	call .l01
+.l01:	pop cx
+	sub cx, .l01
 	ret
 
 int21:	push ds
@@ -82,6 +83,8 @@ int21:	push ds
 	call pspds
 	pop word [PSPAX]
 	pop word [PSPDS]
+	mov [PSPCX], cx
+	mov [PSPBP], bp
 	mov [PSPSS], ss
 	mov [PSPSP], sp
 	mov [PSPDX], dx
@@ -89,27 +92,28 @@ int21:	push ds
 	mov [PSPBX], bx
 
 	mov ss, ax
-	mov sp, ( stack+stacksize )
-	; get ptr to syscall table
-	mov al, [PSPAX+1]
+	mov sp, stack
+	; get offset for PIC
+	call reloff
+	; get ptr into syscall table
+	mov al, [PSPAX+1] ; ah is still 0
 	shl ax, 1
 	shl ax, 1
 	add ax, stab
-	call fixax
+	add ax, cx
 	mov bx, ax
 	; load sysret ptr
-	push bx
 	mov ax, [cs:bx+2]
-	call fixax
-	pop bx
+	add ax, cx
 	push ax
 	; load handler ptr
 	mov ax, [cs:bx]
-	call fixax
+	add ax, cx
 	push ax
 	; restore user data
-	mov bx, [PSPBX]
 	mov ax, [PSPAX]
+	mov cx, [PSPCX]
+	mov bx, [PSPBX]
 	mov ds, [PSPDS]
 	; launch ROP chain
 	ret