On Fri, May 18, 2018 at 11:13:30PM +0200, Florian Weimer wrote: > On 05/18/2018 09:39 PM, Andy Lutomirski wrote: > >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. > >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
