[syslinux:lwip] core: pxe: don't switch to the PXE stack if we're on it...

[syslinux-bot for H. Peter Anvin]

Commit-ID:  0c1dff8db43c78e8538b04abadb80e27c0d008f7
Gitweb:     http://syslinux.zytor.com/commit/0c1dff8db43c78e8538b04abadb80e27c0d008f7
Author:     H. Peter Anvin <hpa at zytor.com>
AuthorDate: Fri, 11 Sep 2009 16:03:30 -0700
Committer:  Eric W. Biederman <ebiederm at xmission.com>
CommitDate: Tue, 12 Apr 2011 14:40:52 -0700

core: pxe: don't switch to the PXE stack if we're on it...

Don't switch to the PXE stack if we are already on it.  This can
happen if we take an interrupt inside the stack switch code.

Signed-off-by: H. Peter Anvin <hpa at zytor.com>


---
 core/pxelinux.asm |   22 ++++++++++++++++++++++
 1 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/core/pxelinux.asm b/core/pxelinux.asm
index ab8526a..be304be 100644
--- a/core/pxelinux.asm
+++ b/core/pxelinux.asm
@@ -375,9 +375,15 @@ pxenv:
 		call timer_cleanup
 
 .store_stack:
+		pushf
+		cli
+		inc word [cs:PXEStackLock]
+		jnz .skip1
 		mov [cs:PXEStack],sp
 		mov [cs:PXEStack+2],ss
 		lss sp,[cs:InitStack]
+.skip1:
+		popf
 
 		; Pre-clear the Status field
 		mov word [es:di],cs
@@ -392,7 +398,13 @@ pxenv:
 		add sp,6
 		mov [cs:PXEStatus],ax
 
+		pushf
+		cli
+		dec word [cs:PXEStackLock]
+		jns .skip2
 		lss sp,[cs:PXEStack]
+.skip2:
+		popf
 
 		mov bp,sp
 		and ax,ax
@@ -419,6 +431,16 @@ pxenv:
                 global PXEEntry
 PXEEntry	equ pxenv.jump+1
 
+;
+; The PXEStackLock keeps us from switching stacks if we take an interrupt
+; (which ends up calling pxenv) while we are already on the PXE stack.
+; It will be -1 normally, 0 inside a PXE call, and a positive value
+; inside a *nested* PXE call.
+;
+		section .data16
+		alignb 2
+PXEStackLock	dw -1
+
 		section .bss16
 		alignb 2
 PXEStatus	resb 2