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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 11 Feb 2021 11:05:17 -0800
Message-ID: <CAKwvOdkduHO357SPzwtN5TAaa_NnJtcJJ89BEe-50WFbRFDdUA@mail.gmail.com>
Subject: Re: [PATCH v5 1/2] ARM: kprobes: fix UNPREDICTABLE warnings
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Russell King <linux@armlinux.org.uk>,
        Arnd Bergmann <arnd@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Peter Smith <peter.smith@arm.com>,
        Renato Golin <rengolin@systemcall.eu>,
        David Spickett <david.spickett@linaro.org>,
        "kernelci . org bot" <bot@kernelci.org>
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On Thu, Feb 11, 2021 at 12:15 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Thu, 11 Feb 2021 at 03:52, Nick Desaulniers <ndesaulniers@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@kernelci.org>
> > Suggested-by: Peter Smith <peter.smith@arm.com>
> > Suggested-by: Renato Golin <rengolin@systemcall.eu>
> > Suggested-by: David Spickett <david.spickett@linaro.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>
> Acked-by: Ard Biesheuvel <ardb@kernel.org>
>
> But can we add a bit more context to the commit log, please? It is not

Sure, that's a good idea.

> 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.

I'll add:

Ard notes:
  These are selftests that aim to ensure that kprobe
  never attempts to replace the opcodes in question with a probe, but
  they are not actually executed, or expected to occur in real
  code.

to the commit message, when submitting to RMK's queue.

Thanks for taking a look and the feedback.

>
> > ---
> >  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
> >


-- 
Thanks,
~Nick Desaulniers