From: Mark Rutland <mark.rutland@arm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Marco Elver <elver@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
broonie@kernel.org, Paul Mackerras <paulus@samba.org>,
kasan-dev <kasan-dev@googlegroups.com>,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends
Date: Tue, 9 Mar 2021 16:05:23 +0000 [thread overview]
Message-ID: <20210309160505.GA4979@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210304215448.GU29191@gate.crashing.org>
On Thu, Mar 04, 2021 at 03:54:48PM -0600, Segher Boessenkool wrote:
> Hi!
Hi Segher,
> On Thu, Mar 04, 2021 at 02:57:30PM +0000, Mark Rutland wrote:
> > It looks like GCC is happy to give us the function-entry-time FP if we use
> > __builtin_frame_address(1),
>
> From the GCC manual:
> 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.
>
> It *does* warn (the warning is in -Wall btw), on both powerpc and
> aarch64. Furthermore, using this builtin causes lousy code (it forces
> the use of a frame pointer, which we normally try very hard to optimise
> away, for good reason).
>
> And, that warning is not an idle warning. Non-zero arguments to
> __builtin_frame_address can crash the program. It won't on simpler
> functions, but there is no real definition of what a simpler function
> *is*. It is meant for debugging, not for production use (this is also
> why no one has bothered to make it faster).
>
> On Power it should work, but on pretty much any other arch it won't.
I understand this is true generally, and cannot be relied upon in
portable code. However as you hint here for Power, I believe that on
arm64 __builtin_frame_address(1) shouldn't crash the program due to the
way frame records work on arm64, but I'll go check with some local
compiler folk. I agree that __builtin_frame_address(2) and beyond
certainly can, e.g. by NULL dereference and similar.
For context, why do you think this would work on power specifically? I
wonder if our rationale is similar.
Are you aware of anything in particular that breaks using
__builtin_frame_address(1) in non-portable code, or is this just a
general sentiment of this not being a supported use-case?
> > Unless we can get some strong guarantees from compiler folk such that we
> > can guarantee a specific function acts boundary for unwinding (and
> > doesn't itself get split, etc), the only reliable way I can think to
> > solve this requires an assembly trampoline. Whatever we do is liable to
> > need some invasive rework.
>
> You cannot get such a guarantee, other than not letting the compiler
> see into the routine at all, like with assembler code (not inline asm,
> real assembler code).
If we cannot reliably ensure this then I'm happy to go write an assembly
trampoline to snapshot the state at a function call boundary (where our
procedure call standard mandates the state of the LR, FP, and frame
records pointed to by the FP). This'll require reworking a reasonable
amount of code cross-architecture, so I'll need to get some more
concrete justification (e.g. examples of things that can go wrong in
practice).
> The real way forward is to bite the bullet and to no longer pretend you
> can do a full backtrace from just the stack contents. You cannot.
I think what you mean here is that there's no reliable way to handle the
current/leaf function, right? If so I do agree.
Beyond that I believe that arm64's frame records should be sufficient.
Thanks,
Mark.
next prev parent reply other threads:[~2021-03-09 16:06 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 14:09 [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Christophe Leroy
2021-03-03 14:38 ` Marco Elver
2021-03-03 14:52 ` Christophe Leroy
2021-03-03 15:20 ` Marco Elver
2021-03-04 14:57 ` Mark Rutland
2021-03-04 15:30 ` Marco Elver
2021-03-04 16:59 ` Mark Rutland
2021-03-04 17:25 ` Marco Elver
2021-03-04 17:54 ` Nick Desaulniers
2021-03-04 19:24 ` Segher Boessenkool
2021-03-05 6:38 ` Christophe Leroy
2021-03-05 18:16 ` Segher Boessenkool
2021-03-04 18:01 ` Mark Rutland
2021-03-04 18:22 ` Marco Elver
2021-03-04 18:51 ` Mark Rutland
2021-03-04 19:01 ` Marco Elver
2021-03-05 12:04 ` Mark Rutland
2021-03-04 21:54 ` Segher Boessenkool
2021-03-09 16:05 ` Mark Rutland [this message]
2021-03-09 22:05 ` Segher Boessenkool
2021-03-10 11:32 ` Mark Rutland
2021-03-10 17:37 ` Segher Boessenkool
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