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Date: Fri, 23 Mar 2012 13:53:56 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hughd@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] swapon: check validity of swap_flags
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On Fri, 23 Mar 2012 13:48:35 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

> Most system calls taking flags first check that the flags passed in are
> valid, and that helps userspace to detect when new flags are supported.
> 
> But swapon never did so: start checking now, to help if we ever want to
> support more swap_flags in future.
> 
> It's difficult to get stray bits set in an int, and swapon is not widely
> used, so this is most unlikely to break any userspace; but we can just
> revert if it turns out to do so.

It would be safer to emit a nasty message then let the swapon proceed
as before.