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

[Andy Lutomirski]

On Thu, Jul 14, 2016 at 1:07 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> 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?
>
> So I think the way this could be done is by allocating it itself. The vDSO vma
> itself is 'external' to glibc as well to begin with - this would be a small
> extension to that concept.

But how does the vdso code find it?  FS and GS are both spoken for by
existing userspace.

--Andy