Re: pkeys: Support setting access rights for signal handlers

[Florian Weimer <fweimer@redhat.com>]

On 12/13/2017 12:13 AM, Ram Pai wrote:

> On POWER, the value of the pkey_read() i.e contents the AMR
> register(pkru equivalent), is always the same regardless of its
> context; signal handler or not.
> 
> In other words, the permission of any allocated key will not
> reset in a signal handler context.

That's certainly the simpler semantics, but I don't like how they differ 
from x86.

Is the AMR register reset to the original value upon (regular) return 
from the signal handler?

> I was not aware that x86 would reset the key permissions in signal
> handler.  I think, the proposed behavior for PKEY_ALLOC_SETSIGNAL should
> actually be the default behavior.

Note that PKEY_ALLOC_SETSIGNAL does something different: It requests 
that the kernel sets the access rights for the key to the bits specified 
at pkey_alloc time when the signal handler is invoked.  So there is 
still a reset with PKEY_ALLOC_SETSIGNAL, but to a different value.  It 
did not occur to me that it might be desirable to avoid resetting the 
value on a per-key basis.

Thanks,
Florian

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