Re: [Qemu-devel] qemu-img segfaults on MIPS hosts due to not having an executable stack

[Peter Maydell <peter.maydell@linaro.org>]

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?

(Also, probably breaks running QEMU under QEMU linux-user, because
our linux-user code still assumes trampolines generally :-))

> eg, just change the g_malloc(size) to
>
>   mmap(0, size,
>   #ifdef (TARGET_MIPS)

This wants to be per *host*, not per target.

>        /* FP emulation in linux requires executable stack */
>        PROT_READ | PROT_WRITE | PROT_EXEC,
>   #else
>        PROT_READ | PROT_WRITE,
>   #endif
>        MAP_PRIVATE | MAP_ANONYMOUS,
>        -1, 0);

I have a definite dislike of architecture-ifdef ladders if
we can possibly avoid them. (We have too many already.)

thanks
-- PMM