Re: [PATCH] arm64: stacktrace: don't trace arch_stack_walk()

From: Ard Biesheuvel <ardb@kernel.org>
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>
Subject: Re: [PATCH] arm64: stacktrace: don't trace arch_stack_walk()
Date: Mon, 22 Mar 2021 16:57:22 +0100	[thread overview]
Message-ID: <CAMj1kXGpFW73sO0viR7_kwuVDgKSUzbn-FeZZi14tYj2DpDjpA@mail.gmail.com> (raw)
In-Reply-To: <161641697038.3900410.16800195752008718733.b4-ty@kernel.org>

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