From: Marco Elver <elver@google.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: 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@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>,
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 19:22:53 +0100 [thread overview]
Message-ID: <CANpmjNOZWuhqXATDjH3F=DMbpg2xOy0XppVJ+Wv2XjFh_crJJg@mail.gmail.com> (raw)
In-Reply-To: <20210304180154.GD60457@C02TD0UTHF1T.local>
On Thu, 4 Mar 2021 at 19:02, Mark Rutland <mark.rutland@arm.com> wrote:
> On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver 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.
>
> I'd assumed that those cases were benign, e.g. for livepatching what
> matters is what can be returned to, so B disappearing from the trace
> isn't a problem there.
>
> Is the concern debugability, or is there a functional issue you have in
> mind?
For me, it's just been debuggability, and reliable test cases.
> > > > 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.
>
> Yup; that's what I meant then I said you could do that on the caller but
> not the callee.
>
> I don't follow why you'd want to put this on the callee, though, so I
> think I'm missing something. Considering a set of functions in different
> compilation units:
>
> A->B->C->D->E->F->G->H->I->J->K
I was having this problem with KCSAN, where the compiler would
tail-call-optimize __tsan_X instrumentation. This would mean that
KCSAN runtime functions ended up in the trace, but the function where
the access happened would not. However, I don't care about the runtime
functions, and instead want to see the function where the access
happened. In that case, I'd like to just mark __tsan_X and any other
kcsan instrumentation functions as do-not-tail-call-optimize, which
would solve the problem.
The solution today is that when you compile a kernel with KCSAN, every
instrumented TU is compiled with -fno-optimize-sibling-calls. The
better solution would be to just mark KCSAN runtime functions somehow,
but permit tail calling other things. Although, I probably still want
to see the full trace, and would decide that having
-fno-optimize-sibling-calls is a small price to pay in a
debug-only-kernel to get complete traces.
> ... if K were marked in this way, and J was compiled with visibility of
> this, J would stick around, but J's callers might not, and so the a
> trace might see:
>
> A->J->K
>
> ... do you just care about the final caller, i.e. you just need
> certainty that J will be in the trace?
Yes. But maybe it's a special problem that only sanitizers have.
> If so, we can somewhat bodge that by having K have an __always_inline
> wrapper which has a barrier() or similar after the real call to K, so
> the call couldn't be TCO'd.
>
> Otherwise I'd expect we'd probably need to disable TCO generally.
Thanks,
-- Marco
next prev parent reply other threads:[~2021-03-04 18:24 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
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 [this message]
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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