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

On Thu, Jul 14, 2016 at 1:07 AM, Ingo Molnar [off-list ref] wrote:

* Andy Lutomirski [off-list ref] wrote:

quoted

On Wed, Jul 13, 2016 at 12:56 AM, Ingo Molnar [off-list ref] wrote:

quoted

* Andy Lutomirski [off-list ref] wrote:

quoted

quoted

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