[PATCH] [ARM] Fix stack alignment when processing backtraces

[jgunthorpe@obsidianresearch.com (Jason Gunthorpe)]

The dumpstm helper within c_backtrace pushed 5 dwords onto the stack
causing the stack to become unaligned and then calls printk. This
causes memory corruption in the kernel which assumes AAPCS calling
convention.

Since this bit of asm doesn't use the standard prologue just add
another register to restore alignment.

Fixes: 7ab3f8d595a1b ("[ARM] Add ability to dump exception stacks to kernel backtraces")
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
---
 arch/arm/lib/backtrace.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

In my case the kernel was hitting a WARN_ON during boot and then
reliably failed to start the compiled-in initramfs.

I'm inferring that the stack misalignment caused some kind of memory
corruption which wiped out the unpacked initramfs.

Saw with gcc 5.4.0 on a kirkwood armv5te

diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
index fab5a50503ae..25e1cce19991 100644
--- a/arch/arm/lib/backtrace.S
+++ b/arch/arm/lib/backtrace.S
@@ -116,7 +116,8 @@ ENDPROC(c_backtrace)
 #define reg   r5
 #define stack r6
 
-.Ldumpstm:	stmfd	sp!, {instr, reg, stack, r7, lr}
+	        /* Must maintain 8 byte stack alignment */
+.Ldumpstm:	stmfd	sp!, {r3, instr, reg, stack, r7, lr}
 		mov	stack, r0
 		mov	instr, r1
 		mov	reg, #10
@@ -140,7 +141,7 @@ ENDPROC(c_backtrace)
 		teq	r7, #0
 		adrne	r0, .Lcr
 		blne	printk
-		ldmfd	sp!, {instr, reg, stack, r7, pc}
+		ldmfd	sp!, {r3, instr, reg, stack, r7, pc}
 
 .Lfp:		.asciz	" r%d:%08x%s"
 .Lcr:		.asciz	"\n"
-- 
2.1.4