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

[Mark Rutland <mark.rutland@arm.com>]

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?

> > > 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

... 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?

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.

> 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
> 
> 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,
Mark.