Re: [PATCH] tst-setcontext2: avoid bug from compiler optimization

[Florian Weimer <fweimer at redhat dot com>]

On 01/27/2017 10:53 PM, Chris Metcalf wrote:
> On 1/27/2017 4:30 PM, Florian Weimer wrote:
> > On 01/27/2017 09:34 PM, Chris Metcalf wrote:
> > > I'd like to commit the one-line patch below that just declares the
> > > context
> > > variables for test-setcontext2 as static.  I think the consensus is that
> > > this is necessary,
> >
> > No, I think the test is supposed to work as-is; it is valid.
> >
> > So if this test is broken, it is a bug in the implementation of the
> > context switching functions (as they are visible to applications, i.e.
> > the fix might not be in the functions themselves, but adjusting their
> > calling conventions, inhibiting GCC optimizations which make incorrect
> > assumptions, etc.).
> >
> > If we do not have a fix right now, it is better to let the test keep
> > failing (perhaps XFAIL it), rather than tweaking it so that it tests
> > something else.
>
> We would be waiting on functionality from gcc that doesn't currently
> exist, correct?  The current returns_twice semantics is insufficient to
> describe the problem that we currently see with shared stack frames.

I think this could still be a GCC regression in the handling of the 
returns_twice attribute, introduced when GCC started sharing stack slots 
more aggressively.  When returns_twice was initially implemented and 
documented, the question of stack slot reuse did not arise because GCC 
probably did not do it at all.

> It's possible that returns_twice could be extended to also have the
> semantics of "don't share my arguments' stack space with any other
> variables in this function" pretty reasonably, but I don't want to try to
> guess what will make most sense to the gcc folks.

GCC already has this behavior on x86_64.  This could be an accident.

> I can certainly let the test keep failing and just open a bug to reference
> it and capture some of this discussion.

Yes, would you please file a GCC bug with a preprocessed input file?

Thanks,
Florian