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

[Dave Hansen]

On 12/21/2015 03:02 PM, Andy Lutomirski wrote:
> 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?

Yep, but that's with a concrete benefit in mind.

> 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.

Yep, agreed.
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