Re: pkeys: Support setting access rights for signal handlers

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On 12/10/2017 01:17 AM, Dave Hansen wrote:
On 12/09/2017 01:16 PM, Florian Weimer wrote:
The attached patch addresses a problem with the current x86 pkey
implementation, which makes default-readable pkeys unusable from signal
handlers because the default init_pkru value blocks access.

Thanks for looking into this!

What do you mean by "default-readable pkeys"?

I think you mean that, for any data that needs to be accessed to enter a
signal handler, it must be set to pkey=0 with the current
implementation.  All other keys are inaccessible when entering a signal
handler because the "init" value disables access.

Right, and for keys which are readable (but not writable) most of the time, so that date is readable, this breaks things.

My only nit with this is whether it is the *right* interface.  The
signal vs. XSAVE state thing is pretty x86 specific and I doubt that
this will be the last feature that we encounter that needs special
signal behavior.

The interface is not specific to XSAVE. To generic code, only the two signal mask manipulation functions are exposed. And I expect that we're going to need that for other (non-x86) implementations because they will have the same issue because the signal handler behavior will be identical.

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