From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] restartable sequences: user-space per-cpu
 critical sections
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:
>=C2=A0
> + *=C2=A0=C2=A0flags is currently unused.
> + */
> +SYSCALL_DEFINE3(restartable_sequences,
> +> 	> 	> int, flags, long, event_and_cpu, long, post_commit_instr_add=
r)
=09
> +{
> +	return rseq_configure_current((__user u64 *)event_and_cpu,
> +				=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0(__user void *)post_commit_i=
nstr_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