Re: pkeys on POWER: Default AMR, UAMOR values

On Fri, May 18, 2018 at 11:13:30PM +0200, Florian Weimer wrote:

On 05/18/2018 09:39 PM, Andy Lutomirski wrote:

quoted

The difference is that x86 starts out with deny-all instead of allow-all.

Ah!. this explains the discrepency. But still does not explain one
thing.. see below.

quoted

The POWER semantics make it very hard for a multithreaded program to
meaningfully use protection keys to prevent accidental access to important
memory.

And you can change access rights for unallocated keys (unallocated
at thread start time, allocated later) on x86.  I have extended the
misc/tst-pkeys test to verify that, and it passes on x86, but not on
POWER, where the access rights are stuck.

This is something I do not understand. How can a thread change permissions
on a key, that is not even allocated in the first place. Do you consider a key
allocated in some other thread's context, as allocated in this threads
context? If not, does that mean -- On x86, you can activate a key just
by changing its permission?


RP