Avoid accessing .data or .bss from real mode, since they may not be within

the current segment. Load and store the necessary BIOS registers from
protected mode, preserving the register values across the real mode and
protected mode switches by directly patching instructions. This allows for
boot(8) to be larger than 64KB.

1 files changed, 44 insertions, 13 deletions

diff --git a/sys/arch/i386/stand/libsa/gidt.S b/sys/arch/i386/stand/libsa/gidt.S
index 9fe2d7ac0cb..6ea7e8323d4 100644
--- a/sys/arch/i386/stand/libsa/gidt.S
+++ b/sys/arch/i386/stand/libsa/gidt.S
@@ -1,4 +1,4 @@
-/*	$OpenBSD: gidt.S,v 1.32 2006/12/26 19:30:44 tom Exp $	*/
+/*	$OpenBSD: gidt.S,v 1.33 2012/10/08 12:46:37 jsing Exp $	*/
 
 /*
  * Copyright (c) 1997 Michael Shalayeff
@@ -314,6 +314,10 @@ IEMUENT(44); IEMUENT(45); IEMUENT(46); IEMUENT(47)
  * entry point for BIOS real-mode interface
  * all the magic for real-prot mode switching is here
  *
+ * Note: Once in real mode access to .data or .bss should be avoided since it
+ * may not be reachable within the current segment. The following code also
+ * assumes that .text is writeable.
+ *
  * Call:	%eax, %ecx, %edx, %ebx, %ebp, %esi, %edi, %es, %ds
  * Return:	%eax, %edx, %ecx, %eflags (as returned from BIOS)
  *
@@ -322,7 +326,7 @@ IEMUENT(44); IEMUENT(45); IEMUENT(46); IEMUENT(47)
 	.align	8, 0x90
 EMUh:
 	/* save %eax */
-	mov	%eax, 3f
+	mov	%eax, 5f
 	pop	%eax
 
 	pusha
@@ -334,18 +338,29 @@ EMUh:
 	/* save BIOS int vector */
 	mov	%al, intno
 
+	/* Load BIOS registers prior to switching to real mode. */
+	movl	_C_LABEL(BIOS_regs)+BIOSR_ES, %eax
+	mov	%eax, 7f
+	movl	_C_LABEL(BIOS_regs)+BIOSR_DS, %eax
+	mov	%eax, 6f
+
 	prot2real
 
 	push	%ds
 
-	addr32  movw (_C_LABEL(BIOS_regs)+(BIOSR_ES) - LINKADDR), %ax
-	movw	%ax, %es
-	addr32  movw (_C_LABEL(BIOS_regs)+(BIOSR_DS) - LINKADDR), %ax
-	movw	%ax, %ds
+	# data32 movl $Leax, %eax
+	.byte	0x66, 0xb8
+7:	.long	0x90909090
+	mov	%ax, %es
+
+	# data32 movl $Leax, %eax
+	.byte	0x66, 0xb8
+6:	.long	0x90909090
+	mov	%ax, %ds
 
 	# data32 movl $Leax, %eax
 	.byte	0x66, 0xb8
-3:	.long	0x90909090
+5:	.long	0x90909090
 
 	;sti
 	int	$0
@@ -354,20 +369,36 @@ intno	= . - 1
 
 	pop	%ds
 
-	addr32 movl %ebx, (_C_LABEL(BIOS_regs)+(BIOSR_BX) - LINKADDR)
-	movw	%es, %bx
-	addr32 movw %bx, (_C_LABEL(BIOS_regs)+(BIOSR_ES) - LINKADDR)
+	/* Preserve BX and ES for protected mode. */
+	addr32 movl %eax, (2f - LINKADDR)
+	movl	%ebx, %eax
+	addr32 movl %eax, (4f - LINKADDR)
+	movl	%es, %eax
+	addr32 movl %eax, (3f - LINKADDR)
+	addr32 movl (2f - LINKADDR), %eax
+
 	movb	%ah, %bh
 	lahf
 	xchgb	%ah, %bh
 
+	/* Preserve AX for protected mode. */
 	addr32 movl %eax, (2f - LINKADDR)
 
 	real2prot
 
 	# movl $Leax, %eax
-	.byte 0xb8
-2:	.long 0x90909090
+	.byte	0xb8
+4:	.long	0x90909090
+	movl	%eax, _C_LABEL(BIOS_regs)+BIOSR_BX
+
+	# movl $Leax, %eax
+	.byte	0xb8
+3:	.long	0x90909090
+	movl	%eax, _C_LABEL(BIOS_regs)+BIOSR_ES
+
+	# movl $Leax, %eax
+	.byte	0xb8
+2:	.long	0x90909090
 
 	/* pass BIOS return values back to caller */
 	movl	%eax, 0xb*4(%esp)
@@ -413,6 +444,6 @@ ENTRY(bootbuf)
 	sti
 
 	/* Jump to buffer */
-	ljmp $0x0, $0x7c00
+	ljmp	$0x0, $0x7c00
 
 	.end