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