[coreboot-gerrit] Change in coreboot[master]: smm: Push a null stack base pointer

[Raul Rangel (Code Review)]

Raul Rangel has uploaded this change for review. ( https://review.coreboot.org/27226

Change subject: smm: Push a null stack base pointer
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smm: Push a null stack base pointer

When generating a backtrace we need an indicator when we have hit the
beginning of the stack. The i386 ABI states that %ebp points to the next
stack frame. NULL can be used to indicate the end of the stack.

We could add a NULL return pointer at %ebp+4, but I decided to omit it
since a NULL stack pointer can be used as an indicator that there is no
return pointer.

BUG=b:80539294
TEST=built and tested on grunt

Change-Id: I8a48114d31a5c716335d264fa4fe4da41dc5bf11
Signed-off-by: Raul E Rangel 
---
M src/cpu/x86/smm/smm_stub.S
1 file changed, 2 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/26/27226/1

diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S
index 5162c95..3fea32c 100644
--- a/src/cpu/x86/smm/smm_stub.S
+++ b/src/cpu/x86/smm/smm_stub.S
@@ -135,6 +135,8 @@
    movl	stack_top, %edx
    subl	%eax, %edx # %edx(stack_top) - %eax(offset) = %edx(stack_top)
    mov	%edx, %esp
+
+	pushl	$0x0 # push a NULL stack base pointer
    mov	%esp, %ebp

subl	$0x4, %esp # Allocate locals (fxsave)