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Subject: Re: [RFC PATCH v2 1/3] restartable sequences: user-space per-cpu
 critical sections
From: Johannes Berg <johannes@sipsolutions.net>
To: Paul Turner <commonly@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
        Andrew Hunter <ahh@google.com>, Andy Lutomirski <luto@amacapital.net>,
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        Dave Watson <davejwatson@fb.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>, Ingo Molnar <mingo@redhat.com>,
        Chris Lameter <cl@linux.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 19 Nov 2015 17:38:10 +0100
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On Tue, 2015-10-27 at 16:56 -0700, Paul Turner wrote:
> 
> + *  flags is currently unused.
> + */
> +SYSCALL_DEFINE3(restartable_sequences,
> +> 	> 	> int, flags, long, event_and_cpu, long, post_commit_instr_addr)
	
> +{
> +	return rseq_configure_current((__user u64 *)event_and_cpu,
> +				      (__user void *)post_commit_instr_addr);

It seems that, for forward compatibility and actually usefulness of the
'flags' argument, you need to reject the syscall if it's passed non-
zero right now. Otherwise applications can (inadvertedly) pass garbage
and you'll not be able to use it in the future.

johannes