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

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Date:	Thu, 7 Jul 2016 15:45:08 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, dave.hansen@...ux.intel.com,
	arnd@...db.de, hughd@...gle.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

On Thu, Jul 07, 2016 at 05:47:28AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.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.
> 
> With current hardware, the kernel can not enforce that it has
> control over a given key.  But, this at least allows the kernel
> to indicate to userspace that userspace does not control a given
> protection key.  This makes it more likely that situations like
> using a pkey after sys_pkey_free() can be detected.
> 
> The kernel does _not_ enforce that this interface must be used for
> changes to PKRU, whether or not a key has been "allocated".
> 
> This syscall interface could also theoretically be replaced with a
> pair of vsyscalls.  The vsyscalls would just call WRPKRU/RDPKRU
> directly in situations where they are drop-in equivalents for
> what the kernel would be doing.
> 

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

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

RIght now, I'm seeing a lot of cost and not much benefit with this
specific patch.

-- 
Mel Gorman
SUSE Labs