Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

From: Mark Rutland <mark.rutland@arm.com>
To: Marco Elver <elver@google.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
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends
Date: Thu, 4 Mar 2021 16:59:23 +0000	[thread overview]
Message-ID: <20210304165923.GA60457@C02TD0UTHF1T.local> (raw)
In-Reply-To: <CANpmjNOSpFbbDaH9hNucXrpzG=HpsoQpk5w-24x8sU_G-6cz0Q@mail.gmail.com>

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?

> 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.