pkeys on POWER: Default AMR, UAMOR values

[Florian Weimer]

On 05/19/2018 02:52 AM, Ram Pai wrote:

>>> 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.

It was allocated by another thread, and there is synchronization so that 
the allocation happens before the change in access rights.

> Do you consider a key
> allocated in some other thread's context, as allocated in this threads
> context?

Yes, x86 does that.

> If not, does that mean -- On x86, you can activate a key just
> by changing its permission?

This also true on x86, but just an artifact of the implementation.  You 
are supposed to call pkey_alloc before changing the flag.

Thanks,
Florian