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

[Dave Hansen]

On 07/07/2016 07:45 AM, Mel Gorman wrote:
> On Thu, Jul 07, 2016 at 05:47:28AM -0700, Dave Hansen wrote:
>> > 
>> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> > 
>> > 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.