Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

[Andy Lutomirski]

On Wed, Jul 13, 2016 at 12:56 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> > If we push a PKRU value into a thread between the rdpkru() and wrpkru(), we'll
>> > lose the content of that "push".  I'm not sure there's any way to guarantee
>> > this with a user-controlled register.
>>
>> We could try to insist that user code uses some vsyscall helper that tracks
>> which bits are as-yet-unassigned.  That's quite messy, though.
>
> Actually, if we turned the vDSO into something more like a minimal user-space
> library with the ability to run at process startup as well to prepare stuff then
> it's painful to get right only *once*, and there will be tons of other areas where
> a proper per thread data storage on the user-space side would be immensely useful!

Doing this could be tricky: how exactly is the vDSO supposed to find
per-thread data without breaking existing glibc?