Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

[Andy Lutomirski]

On Mon, Dec 21, 2015 at 3:00 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 12/21/2015 02:52 PM, Andy Lutomirski wrote:
>> Perhaps this is silly, but what if the default were changed to deny
>> reads and writes for unallocated keys?  Is there a use case that
>> breaks?
>
> It's probably a reasonable debugging feature.
>
> But, anything that takes an XSAVE feature out of its "init state" has
> the potential to do a bit of harm because it increases the potential
> size of writes during XSAVE.  XSAVEOPT will _help_ here, but we probably
> don't want to go out of our way to take things out of the init state
> when we're unsure of the benefits.

Aren't you already doing that with your magic execute-only thing?

Also, if we ever do the deferred-xstate-restore thing that Rik was
playing with awhile back, then we'll want to switch to using rdpkru
and wrpkru in-kernel directly, and we'll explicitly mask PKRU out of
the XRSTOR and XSAVEOPT state, and this particular issue will become
irrelevant.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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