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Subject[PATCH 4.4 19/78] arm: kprobes: Align stack to 8-bytes in test code
4.4-stable review patch.  If anyone has any objections, please let me know.


From: Jon Medhurst <>

[ Upstream commit 974310d047f3c7788a51d10c8d255eebdb1fa857 ]

kprobes test cases need to have a stack that is aligned to an 8-byte
boundary because they call other functions (and the ARM ABI mandates
that alignment) and because test cases include 64-bit accesses to the
stack. Unfortunately, GCC doesn't ensure this alignment for inline
assembler and for the code in question seems to always misalign it by
pushing just the LR register onto the stack. We therefore need to
explicitly perform stack alignment at the start of each test case.

Without this fix, some test cases will generate alignment faults on
systems where alignment is enforced. Even if the kernel is configured to
handle these faults in software, triggering them is ugly. It also
exposes limitations in the fault handling code which doesn't cope with
writes to the stack. E.g. when handling this instruction

strd r6, [sp, #-64]!

the fault handling code will write to a stack location below the SP
value at the point the fault occurred, which coincides with where the
exception handler has pushed the saved register context. This results in
corruption of those registers.

Signed-off-by: Jon Medhurst <>
Signed-off-by: Sasha Levin <>
Signed-off-by: Greg Kroah-Hartman <>
arch/arm/probes/kprobes/test-core.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

--- a/arch/arm/probes/kprobes/test-core.c
+++ b/arch/arm/probes/kprobes/test-core.c
@@ -976,7 +976,10 @@ static void coverage_end(void)
void __naked __kprobes_test_case_start(void)
__asm__ __volatile__ (
- "stmdb sp!, {r4-r11} \n\t"
+ "mov r2, sp \n\t"
+ "bic r3, r2, #7 \n\t"
+ "mov sp, r3 \n\t"
+ "stmdb sp!, {r2-r11} \n\t"
"sub sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
"bic r0, lr, #1 @ r0 = inline data \n\t"
"mov r1, sp \n\t"
@@ -996,7 +999,8 @@ void __naked __kprobes_test_case_end_32(
"movne pc, r0 \n\t"
"mov r0, r4 \n\t"
"add sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
- "ldmia sp!, {r4-r11} \n\t"
+ "ldmia sp!, {r2-r11} \n\t"
+ "mov sp, r2 \n\t"
"mov pc, r0 \n\t"
@@ -1012,7 +1016,8 @@ void __naked __kprobes_test_case_end_16(
"bxne r0 \n\t"
"mov r0, r4 \n\t"
"add sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
- "ldmia sp!, {r4-r11} \n\t"
+ "ldmia sp!, {r2-r11} \n\t"
+ "mov sp, r2 \n\t"
"bx r0 \n\t"

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