linux-kernel - Re: [PATCH v1] powerpc: Include running function as first entry in save_stack

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Date:   Thu, 4 Mar 2021 16:59:23 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Marco Elver <elver@...gle.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        kasan-dev <kasan-dev@...glegroups.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        broonie@...nel.org
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in
 save_stack_trace() and friends

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@....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?

> 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.

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

> 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.

Thanks,
Mark.