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References: <20210319184106.5688-1-mark.rutland@arm.com> <161641697038.3900410.16800195752008718733.b4-ty@kernel.org>
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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 22 Mar 2021 17:05:02 +0100
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Subject: Re: [PATCH] arm64: stacktrace: don't trace arch_stack_walk()
To:     Will Deacon <will@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Android Kernel Team <kernel-team@android.com>,
        Mark Brown <broonie@kernel.org>,
        Marco Elver <elver@google.com>,
        Chen Jun <chenjun102@huawei.com>
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On Mon, 22 Mar 2021 at 16:57, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Mon, 22 Mar 2021 at 14:26, Will Deacon <will@kernel.org> wrote:
> >
> > On Fri, 19 Mar 2021 18:41:06 +0000, Mark Rutland wrote:
> > > We recently converted arm64 to use arch_stack_walk() in commit:
> > >
> > >   5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
> > >
> > > The core stacktrace code expects that (when tracing the current task)
> > > arch_stack_walk() starts a trace at its caller, and does not include
> > > itself in the trace. However, arm64's arch_stack_walk() includes itself,
> > > and so traces include one more entry than callers expect. The core
> > > stacktrace code which calls arch_stack_walk() tries to skip a number of
> > > entries to prevent itself appearing in a trace, and the additional entry
> > > prevents skipping one of the core stacktrace functions, leaving this in
> > > the trace unexpectedly.
> > >
> > > [...]
> >
> > Applied to arm64 (for-next/fixes), thanks!
> >
> > [1/1] arm64: stacktrace: don't trace arch_stack_walk()
> >       https://git.kernel.org/arm64/c/c607ab4f916d
> >
>
> Ehm, did anyone check if the following caveat regarding
> __builtin_frame_address() applies on arm64? (from the GCC man page
> [0])
>
> """
> Calling this function with a nonzero argument can have unpredictable
> effects, including crashing the calling program. As a result, calls
> that are considered unsafe are diagnosed when the -Wframe-address
> option is in effect. Such calls should only be made in debugging
> situations.
> """
>
> [0] https://gcc.gnu.org/onlinedocs/gcc/Return-Address.html

Never mind, failed to read the entire thread. Apologies ...