Re: [PATCH 26/26] x86, pkeys: Documentation

From: Dave Hansen
Date: Fri Oct 16 2015 - 11:12:55 EST


On 10/03/2015 12:27 AM, Ingo Molnar wrote:
> - Along similar considerations, also add a sys_pkey_query() system call to query
> the mapping of a specific pkey. (returns -EBADF or so if the key is not mapped
> at the moment.) This too could be vDSO accelerated in the future.
>
> I.e. something like:
>
> unsigned long sys_pkey_alloc (unsigned long flags, unsigned long init_val)
> unsigned long sys_pkey_set (int pkey, unsigned long new_val)
> unsigned long sys_pkey_get (int pkey)
> unsigned long sys_pkey_free (int pkey)

The pkey_set() operation is going to get a wee bit interesting with signals.

pkey_set() will modify the _current_ context's PKRU which includes the
register itself and the kernel XSAVE buffer (if active). But, since the
PKRU state is saved/restored with the XSAVE state, we will blow away any
state set during the signal.

I _think_ the right move here is to either keep a 'shadow' version of
PKRU inside the kernel (for each thread) and always update the task's
XSAVE PKRU state when returning from a signal handler. Or, _copy_ the
signal's PKRU state in to the main process's PKRU state when returning
from a signal.
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