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Subject: Re: [PATCH 26/26] x86, pkeys: Documentation
To: Ingo Molnar <mingo@kernel.org>
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From: Dave Hansen <dave@sr71.net>
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Date: Fri, 16 Oct 2015 08:12:48 -0700
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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.