[PATCH v5 1/2] ARM: kprobes: fix UNPREDICTABLE warnings

[Ard Biesheuvel]

On Thu, 11 Feb 2021 at 03:52, Nick Desaulniers <ndesaulniers at google.com> wrote:
>
> GNU as warns twice for this file:
> Warning: using r15 results in unpredictable behaviour
>
> via the Arm ARM:
> K1.1.1 Overview of the constraints on Armv7 UNPREDICTABLE behaviors
>
>   The term UNPREDICTABLE describes a number of cases where the
>   architecture has a feature that software must not use.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1271
> Link: https://reviews.llvm.org/D95586
> Reported-by: kernelci.org bot <bot at kernelci.org>
> Suggested-by: Peter Smith <peter.smith at arm.com>
> Suggested-by: Renato Golin <rengolin at systemcall.eu>
> Suggested-by: David Spickett <david.spickett at linaro.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>

Acked-by: Ard Biesheuvel <ardb at kernel.org>

But can we add a bit more context to the commit log, please? It is not
obvious to the casual reader why it is OK to emit UNPREDICTABLE
opcodes, i.e., that these are selftests that aim to ensure that kprobe
never attempts to replace the opcodes in question with a probe, but
that they are not actually executed, or expected to occur in real
code.

> ---
>  arch/arm/probes/kprobes/test-arm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/probes/kprobes/test-arm.c b/arch/arm/probes/kprobes/test-arm.c
> index 977369f1aa48..2543106a203e 100644
> --- a/arch/arm/probes/kprobes/test-arm.c
> +++ b/arch/arm/probes/kprobes/test-arm.c
> @@ -166,10 +166,10 @@ void kprobe_arm_test_cases(void)
>
>         /* Data-processing with PC as a target and status registers updated */
>         TEST_UNSUPPORTED("movs  pc, r1")
> -       TEST_UNSUPPORTED("movs  pc, r1, lsl r2")
> +       TEST_UNSUPPORTED(__inst_arm(0xe1b0f211) "       @movs   pc, r1, lsl r2")
>         TEST_UNSUPPORTED("movs  pc, #0x10000")
>         TEST_UNSUPPORTED("adds  pc, lr, r1")
> -       TEST_UNSUPPORTED("adds  pc, lr, r1, lsl r2")
> +       TEST_UNSUPPORTED(__inst_arm(0xe09ef211) "       @adds   pc, lr, r1, lsl r2")
>         TEST_UNSUPPORTED("adds  pc, lr, #4")
>
>         /* Data-processing with SP as target */
> --
> 2.30.0.478.g8a0d178c01-goog
>