linux-kernel - Re: [resend][PATCH] Added PR_SET_PROCTITLE

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Date:	Fri, 9 Oct 2009 23:39:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Bryan Donlan <bdonlan@...il.com>, linux-kernel@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>, linux-api@...r.kernel.org,
	Timo Sirainen <tss@....fi>
Subject: Re: [resend][PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()

On Sat, 10 Oct 2009 15:32:35 +0900 KOSAKI Motohiro <kosaki.motohiro@...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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