[coreboot] New patch to review for coreboot: 4715d6b Poison the stack to uncover programming errors

Ronald G. Minnich (rminnich@gmail.com) gerrit at coreboot.org
Thu Jul 12 20:24:34 CEST 2012

Ronald G. Minnich (rminnich at gmail.com) just uploaded a new patch set to gerrit, which you can find at http://review.coreboot.org/1221


commit 4715d6b4706fb78e9073295b878b69a6cf37ab6f
Author: Ronald G. Minnich <rminnich at gmail.com>
Date:   Thu Jul 12 11:21:51 2012 -0700

    Poison the stack to uncover programming errors
    Code can easily make the mistake of using uninitialized
    values or, in assembly, mistakenly dereferencing stack pointers
    when an address is desired.
    Set the stack to a non-zero value which is also (by testing)
    a pointer which will crash coreboot if used. This poisoning
    has uncovered at least one bug.
    Change-Id: I4affb9a14b96611e8bf83cb82636e47913025a5d
    Signed-off-by: Ronald G. Minnich <rminnich at gmail.com>
 src/arch/x86/lib/c_start.S |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/arch/x86/lib/c_start.S b/src/arch/x86/lib/c_start.S
index fd9dce7..9c1fdb1 100644
--- a/src/arch/x86/lib/c_start.S
+++ b/src/arch/x86/lib/c_start.S
@@ -16,17 +16,21 @@ _start:
 	post_code(POST_ENTRY_C_START)		/* post 13 */
-	/** clear stack */
+	/** poison the stack. Code should not count on the
+	 * stack being full of zeros. This stack poisoning
+	 * recently uncovered a bug in the broadcast SIPI
+	 * code.
+	 */
 	leal	_stack, %edi
 	movl	$_estack, %ecx
 	subl	%edi, %ecx
 	shrl	$2, %ecx   /* it is 32 bit aligned, right? */
-	xorl	%eax, %eax
+	movl	$0xDEADBEEF, %eax
-	/** clear bss */
+	/** clear bss, which unlike the stack is zero by definition */
 	leal	_bss, %edi
 	movl	$_ebss, %ecx
 	subl	%edi, %ecx

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