From: Nick Desaulniers <ndesaulniers@google.com>
To: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
LKML <linux-kernel@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
kasan-dev <kasan-dev@googlegroups.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Mark Brown <broonie@kernel.org>,
linux-toolchains@vger.kernel.org
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends
Date: Thu, 4 Mar 2021 09:54:44 -0800 [thread overview]
Message-ID: <CAKwvOd=wBArMwvtDC8zV-QjQa5UuwWoxksQ8j+hUCZzbEAn+Fw@mail.gmail.com> (raw)
In-Reply-To: <YEEYDSJeLPvqRAHZ@elver.google.com>
On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@google.com> wrote:
>
> On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > [adding Mark Brown]
> > > >
> > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > this is still liable to break in some cases. One big concern is that
> > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > or outline functions, causing the skipp value to be too large or too
> > > > small. That's liable to happen to callers, and in theory (though
> > > > unlikely in practice), portions of arch_stack_walk() or
> > > > stack_trace_save() could get outlined too.
> > > >
> > > > 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.
> > >
> > > Will LTO and friends respect 'noinline'?
> >
> > I hope so (and suspect we'd have more problems otherwise), but I don't
> > know whether they actually so.
> >
> > I suspect even with 'noinline' the compiler is permitted to outline
> > portions of a function if it wanted to (and IIUC it could still make
> > specialized copies in the absence of 'noclone').
> >
> > > One thing I also noticed is that tail calls would also cause the stack
> > > trace to appear somewhat incomplete (for some of my tests I've
> > > disabled tail call optimizations).
> >
> > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > trace A->C? ... or is A going missing too?
>
> Correct, it's just the A->C outcome.
>
> > > Is there a way to also mark a function non-tail-callable?
> >
> > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > function-local optimization options), but I don't expect there's any way
> > to mark a callee as not being tail-callable.
>
> I don't think this is reliable. It'd be
> __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> work if applied to the function we do not want to tail-call-optimize,
> but would have to be applied to the function that does the tail-calling.
> So it's a bit backwards, even if it worked.
>
> > Accoding to the GCC documentation, GCC won't TCO noreturn functions, but
> > obviously that's not something we can use generally.
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
include/linux/compiler.h:246:
prevent_tail_call_optimization
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
>
> Perhaps we can ask the toolchain folks to help add such an attribute. Or
> maybe the feature already exists somewhere, but hidden.
>
> +Cc linux-toolchains@vger.kernel.org
>
> > > But I'm also not sure if with all that we'd be guaranteed the code we
> > > want, even though in practice it might.
> >
> > True! I'd just like to be on the least dodgy ground we can be.
>
> It's been dodgy for a while, and I'd welcome any low-cost fixes to make
> it less dodgy in the short-term at least. :-)
>
> Thanks,
> -- Marco
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2021-03-04 17:56 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <e2e8728c4c4553bbac75a64b148e402183699c0c.1614780567.git.christophe.leroy@csgroup.eu>
[not found] ` <CANpmjNOvgbUCf0QBs1J-mO0yEPuzcTMm7aS1JpPB-17_LabNHw@mail.gmail.com>
[not found] ` <1802be3e-dc1a-52e0-1754-a40f0ea39658@csgroup.eu>
[not found] ` <YD+o5QkCZN97mH8/@elver.google.com>
[not found] ` <20210304145730.GC54534@C02TD0UTHF1T.local>
[not found] ` <CANpmjNOSpFbbDaH9hNucXrpzG=HpsoQpk5w-24x8sU_G-6cz0Q@mail.gmail.com>
[not found] ` <20210304165923.GA60457@C02TD0UTHF1T.local>
2021-03-04 17:25 ` [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Marco Elver
2021-03-04 17:54 ` Nick Desaulniers [this message]
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
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