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Date: Tue, 14 Jun 2016 00:10:18 +0200
From: Ralf Baechle <ralf@linux-mips.org>
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Subject: Re: [Qemu-devel] qemu-img segfaults on MIPS hosts due to not having
 an executable stack
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Daniel P. Berrange" <berrange@redhat.com>, Leon Alrae <leon.alrae@imgtec.com>, Michael Tokarev <mjt@tls.msk.ru>, QEMU Developers <qemu-devel@nongnu.org>, Aurelien Jarno <aurelien@aurel32.net>, James Hogan <james.hogan@imgtec.com>

On Mon, Jun 13, 2016 at 04:16:02PM +0100, Peter Maydell wrote:

> On 13 June 2016 at 15:45, Daniel P. Berrange <berrange@redhat.com> wrote:
> > On Mon, Jun 13, 2016 at 03:11:08PM +0100, Peter Maydell wrote:
> >> QEMU currently allocates coroutine stacks with a plain g_malloc(),
> >> which makes them r/w but not exec. That's a bug in QEMU which we
> >> should fix (though I'm not sure how best to identify the required
> >> permissions for stacks). It's a bit unhelpful of the kernel to
> >> assume an executable stack and not give a useful diagnostic or
> >> failure mode if it's not true, though.
> >
> > I'd suggest we just #ifdef the code base on architecture, on that basis
> > all platforms except mips are probably happy with non-exec stack.
> 
> Have they really all got rid of signal handler trampolines?

Apparently Android wants a non-executable stack for security reasons.

That said, some special code such as GCC's nested functions may require
stack trampolines.  For such code there is the option to use the p_flags
of the PT_GNU_STACK program header to mark the stack executable.  One
way to do so is to pass the option "-z execstack" to ld or a ".section
.note.GNU-stack,"",@progbits" into the assembler code which is what GCC
will do when generating trampolines.

  Ralf