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SubjectRe: framebuffer corruption due to overlapping stp instructions on arm64


On Fri, 3 Aug 2018, Florian Weimer wrote:

> On 08/03/2018 09:11 AM, Andrew Pinski wrote:
> > Yes fix Links not to use memcpy on the framebuffer.
> > It is undefined behavior to use device memory with memcpy.
>
> Some (de facto) ABIs require that it is supported, though. For example,
> the POWER string functions avoid unaligned loads and stores for this
> reason because the platform has the same issue with device memory. And
> yes, GCC will expand memcpy on POWER to something that is incompatible
> with device memory. 8-(
>
> If we don't want people to use memcpy, we probably need to provide a
> credible alternative.
>
> Thanks,
> Florian

And what does POWER do with code like this?

void write_merge(int *x)
{
x[0] = x[1] = 0;
}

With -O2, gcc-8 translates it into:

li 9,0
std 9,0(3)
blr

And that std instruction may end up being unaligned (the C ABI mandates
that x is aligned to 4 bytes, not 8).

If this piece of code is inside some graphics driver and writes to
framebuffer memory, what do you do with it?

Mikulas

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