From 702f0b85d3a26cfaab412ec3663f9b0ca50c48b4 Mon Sep 17 00:00:00 2001
From: hpa <hpa>
Date: Thu, 6 Jun 2002 02:00:50 +0000
Subject: [PATCH] Fix COMBOOT return; support DOS system calls 00 and 4C (both
 are EXIT.)

---
 NEWS         |  6 ++++++
 comboot.inc  | 51 +++++++++++++++++++++++++++++++++------------------
 isolinux.asm |  3 ++-
 ldlinux.asm  |  3 ++-
 pxelinux.asm |  3 ++-
 5 files changed, 45 insertions(+), 21 deletions(-)

diff --git a/NEWS b/NEWS
index 7675b95..7edf945 100644
--- a/NEWS
+++ b/NEWS
@@ -19,6 +19,12 @@ Changes in 1.75:
 	* ISOLINUX: Add a few more workarounds for various broken El
 	  Torito BIOSes.
 	* Make sure .depend files aren't accidentally packed...
+	* ALL: Fix bugs in the extension-detect code; this caused
+	  files like COMBOOT images and CD boot sectors to be
+	  mis-identified as Linux kernels and rejected.
+	* ALL: Fix the return from COMBOOT.
+	* ALL: Do some of the early groundwork for supporting DOS
+	  system calls in COMBOOT.
 
 Changes in 1.74:
 	* SYSLINUX: fix bug that would cause valid kernel images to be
diff --git a/comboot.inc b/comboot.inc
index fbda097..5d7019e 100644
--- a/comboot.inc
+++ b/comboot.inc
@@ -37,12 +37,15 @@ is_comboot_image:
 		;
 		; Set up the DOS vectors in the IVT (INT 20h-3fh)
 		;
-		mov dword [4*0x20],comboot_return	; INT 20h vector
-		mov eax,comboot_bogus
-		mov di,4*0x21
-		mov cx,31		; All remaining DOS vectors
+		mov di,4*0x20		; DOS interrupt vectors
+		mov eax,comboot_return
+		stosd
+		mov ax,comboot_int21
+		stosd
+		mov ax,comboot_bogus
+		mov cx,30		; All remaining DOS vectors
 		rep stosd
-	
+
 		mov cx,comboot_seg
 		mov es,cx
 
@@ -79,8 +82,7 @@ comboot_end_cmd: mov al,0Dh		; CR after last character
 		mov [es:80h], al	; Store command line length
 
 		mov [SavedSSSP],sp
-		mov ax,ss		; Save away SS:SP
-		mov [SavedSSSP+2],ax
+		mov [SavedSSSP+2],ss	; Save away SS:SP
 
 		call vgaclearmode	; Reset video
 
@@ -94,23 +96,36 @@ comboot_end_cmd: mov al,0Dh		; CR after last character
 
 ; Proper return vector
 comboot_return:	cli			; Don't trust anyone
-		lss sp,[cs:SavedSSSP]
-		mov ds,ax
-		mov es,ax
-		sti
-		cld
-		jmp cb_enter
+		xor ax,ax
+		jmp comboot_exit
+
+; INT 21h: generic DOS system call
+comboot_int21:	and ah,ah		; 00 = return
+		je comboot_return
+		cmp ah,4Ch		; 4C = return with status
+		je comboot_return
 
-; Attempted to execute DOS system call
+		; Otherwise fall through to comboot_bogus
+
+; Attempted to execute non-21h DOS system call
 comboot_bogus:	cli			; Don't trust anyone
+		mov ax,err_notdos
+
+;
+; Generic COMBOOT return to command line code
+;
+comboot_exit:
 		lss sp,[cs:SavedSSSP]
-		mov ds,ax
-		mov es,ax
+		xor dx,dx
+		mov ds,dx
+		mov es,dx
 		sti
 		cld
+		and ax,ax
+		je .nomsg
 		mov si,KernelCName
 		call cwritestr
-		mov si,err_notdos
+		xchg si,ax
 		call cwritestr
-		jmp cb_enter
+.nomsg:		jmp cb_enter
 
diff --git a/isolinux.asm b/isolinux.asm
index bfa6b91..3373f59 100644
--- a/isolinux.asm
+++ b/isolinux.asm
@@ -646,7 +646,8 @@ checkerr_msg:	db 'Image checksum error, sorry...', CR, LF, 0
 
 err_bootfailed	db CR, LF, 'Boot failed: press a key to retry...'
 bailmsg		equ err_bootfailed
-crlf_msg	db CR, LF, 0
+crlf_msg	db CR, LF
+null_msg	db 0
 
 ;
 ; El Torito spec packet
diff --git a/ldlinux.asm b/ldlinux.asm
index 008a82f..955665a 100644
--- a/ldlinux.asm
+++ b/ldlinux.asm
@@ -1685,7 +1685,8 @@ loading_msg     db 'Loading ', 0
 dotdot_msg      db '.'
 dot_msg         db '.', 0
 aborted_msg	db ' aborted.'			; Fall through to crlf_msg!
-crlf_msg	db CR, LF, 0
+crlf_msg	db CR, LF
+null_msg	db 0
 crff_msg	db CR, FF, 0
 syslinux_cfg	db 'SYSLINUXCFG'
 ;
diff --git a/pxelinux.asm b/pxelinux.asm
index a549432..9615b45 100644
--- a/pxelinux.asm
+++ b/pxelinux.asm
@@ -2297,7 +2297,8 @@ dotdot_msg      db '.'
 dot_msg         db '.', 0
 fourbs_msg	db BS, BS, BS, BS, 0
 aborted_msg	db ' aborted.'			; Fall through to crlf_msg!
-crlf_msg	db CR, LF, 0
+crlf_msg	db CR, LF
+null_msg	db 0
 crff_msg	db CR, FF, 0
 default_str	db 'default', 0
 default_len	equ ($-default_str)
-- 
2.7.4