Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

On 07/07/2016 07:45 AM, Mel Gorman wrote:

On Thu, Jul 07, 2016 at 05:47:28AM -0700, Dave Hansen wrote:

quoted

quoted

From: Dave Hansen <dave.hansen@linux.intel.com>

This establishes two more system calls for protection key management:

	unsigned long pkey_get(int pkey);
	int pkey_set(int pkey, unsigned long access_rights);

The return value from pkey_get() and the 'access_rights' passed
to pkey_set() are the same format: a bitmask containing
PKEY_DENY_WRITE and/or PKEY_DENY_ACCESS, or nothing set at all.

These can replace userspace's direct use of the new rdpkru/wrpkru
instructions.

...

This one feels like something that can or should be implemented in
glibc.

I generally agree, except that glibc doesn't have any visibility into
whether a pkey is currently valid or not.

There is no real enforcement of the values yet looking them up or
setting them takes mmap_sem for write.

There are checks for mm_pkey_is_allocated().  That's the main thing
these syscalls add on top of the raw instructions.

Applications that frequently get
called will get hammed into the ground with serialisation on mmap_sem
not to mention the cost of the syscall entry/exit.

I think we can do both of them without mmap_sem, as long as we resign
ourselves to this just being fundamentally racy (which it is already, I
think).  But, is it worth performance-tuning things that we don't expect
performance-sensitive apps to be using in the first place?  They'll just
use the RDPKRU/WRPKRU instructions directly.

Ingo, do you still feel strongly that these syscalls (pkey_set/get())
should be included?  Of the 5, they're definitely the two with the
weakest justification.