Re: [PATCH] x86, fpu: fix 32-bit signal frame handling

[Dave Hansen <dave@sr71.net>]

On 11/10/2015 04:23 PM, Dave Hansen wrote:
> For MPX, this leads to the most permissive state and means we
> silently lose bounds violations.  I think this would also mean
> that we could lose *ANY* FPU/SSE/AVX state.  I'm not sure why
> no one has spotted this bug.

FWIW, I looked at this a little more today.

We lose all extended state for our "extended xfeatures", also known as
state component numbers >=2 (AVX, MPX, AVX-512, PKEYs)...  But we retain
the state for FP/SSE state.  So we lose the top half of the AVX
registers (the bottom half are SSE state).

I also did a little objdump'ing and grep'ing in a 32-bit distro.
There's no sign of actual use of the ymm registers.

Basically, it appears nobody has taken a 64-bit Sandybridge or later
CPU, put a 32-bit distro on it that had a >=3.7 kernel on it and tried
to use AVX instructions.  Or, if they did, they got random corruption
and gave up before actually diagnosing the problem. :)