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

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Date:	Fri, 27 Nov 2015 11:06:27 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
	x86@...nel.org, luto@...nel.org, fenghua.yu@...el.com,
	yu-cheng.yu@...el.com
Subject: Re: [PATCH] x86, fpu: fix 32-bit signal frame handling


* Dave Hansen <dave@...1.net> wrote:

> 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. :)

Weird: putting a 32-bit distro on such a fine piece of 64-bit hardware is pure 
masochism - and such masochism would also imply the willingness to track down 
random corruptions! ;-)

Thanks,

	Ingo
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