arm: kprobes: Align stack to 8-bytes in test code

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 <tixy@linaro.org>

diff --git a/arch/arm/probes/kprobes/test-core.c b/arch/arm/probes/kprobes/test-core.c
index c893726aa52d8da16e56d9df56ad1e6ca8685b29..1c98a87786ca768adb622f1720df442a272bfb75 100644 (file)
--- a/arch/arm/probes/kprobes/test-core.c
+++ b/arch/arm/probes/kprobes/test-core.c
@@ -977,7 +977,10 @@ 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"
@@ -997,7 +1000,8 @@ void __naked __kprobes_test_case_end_32(void)
                "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"
        );
 }
@@ -1013,7 +1017,8 @@ void __naked __kprobes_test_case_end_16(void)
                "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"
        );
 }