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

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 
Signed-off-by: Jason Gunthorpe <>
 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"

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