Re: pkeys: Support setting access rights for signal handlers

[Dave Hansen <dave.hansen@intel.com>]

On 12/13/2017 07:08 AM, Florian Weimer wrote:
> Okay, this model is really quite different from x86.  Is there a
> good reason for the difference?

Yes, both implementations are simple and take the "natural" behavior.
x86 changes XSAVE-controlled register values on entering a signal, so we
let them be changed (including PKRU).  POWER hardware does not do this
to its PKRU-equivalent, so we do not force it to.

x86 didn't have to do this for *signals*.  But, we kinda went on this
trajectory when we decided to clear/restore FPU state on
entering/exiting signals before XSAVE even existed.

FWIW, I do *not* think we have to do this for future XSAVE states.  But,
if we do that, we probably need an interface for apps to tell us which
states to save/restore and which state to set upon entering a signal
handler.  That's what I was trying to get you to consider instead of
just a one-off hack to fix this for pkeys.

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