Date: Fri, 9 Oct 2009 23:39:35 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Bryan Donlan <bdonlan@gmail.com>, linux-kernel@vger.kernel.org,
       Ulrich Drepper <drepper@redhat.com>, linux-api@vger.kernel.org,
       Timo Sirainen <tss@iki.fi>
Subject: Re: [resend][PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()
Message-Id: <20091009233935.1be0edf9.akpm@linux-foundation.org>
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On Sat, 10 Oct 2009 15:32:35 +0900 KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> >>> The solution is to use the seqlock to detect this, and prevent the
> >>> secret information from ever making it back to process B's userspace.
> >>> Note that it's not enough to just recheck arg_start, as process A may
> >>> reassign the proctitle area back to its original position after having
> >>> it somewhere else for a while.
> >>
> >> Well seqlock is _a_ solution. __Another is to use a mutex or an rwsem
> >> around the whole operation.
> >>
> >> With the code as you propose it, what happens if a process sits in a
> >> tight loop running setproctitle? __Do other processes running `ps' get
> >> stuck in a livelock until the offending process gets scheduled out?
> >
> > It does seem like a maximum spin count should be put in there - and
> > maybe a timeout as well (since with FUSE etc it's possible to engineer
> > page faults that take arbitrarily long).
> > Also, it occurs to me that:
> 
> makes sense.
> I like maximum spin rather than timeout.

Start simple.  What's wrong with mutex_lock() on the reader and writer
sides?  rwsems might be OK too.

In both cases we should think about whether persistent readers can
block the writer excessively though.
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