linux-kernel - Re: perf: overflow signal survives an exec call starting in 3.0

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Date:	Thu, 25 Aug 2011 16:47:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	acme@...stprotocols.net, Stephane Eranian <eranian@...il.com>,
	krentel@...rice.edu
Subject: Re: perf: overflow signal survives an exec call starting in 3.0

On Thu, 2011-08-25 at 16:37 +0200, Peter Zijlstra wrote:
> On Tue, 2011-08-23 at 11:36 -0400, Vince Weaver wrote:
> > I'm guessing this was an unintended change, although what to do in
> > this situation is a bit vague.
> 
> > 
> > commit f506b3dc0ec454a16d40cab9ee5d75435b39dc50
> > Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date:   Thu May 26 17:02:53 2011 +0200
> > 
> >     perf: Fix SIGIO handling 
> 
> Ah, quite... its a combination of way too many weird things.
> 
> So that change made it possible (as requested) to receive a signal
> without having to mmap() the buffer, which seems like a perfectly sane
> thing.
> 
> Now, looking at fs/exec.c:setup_new_exec() we see that we only call
> perf_event_exit_task() when we cross a security domain. 
> 
> Previously the exec() would have wiped out the mmap, and hence the
> counter would have stopped sending signals (but not stopped counting).
> 
> Now exec() doesn't normally close fds, so the counter stays around and
> with the new scheme continues sending signals.
> 
> I think the message is, don't do that then.. and use fcntl(fd, F_SETFD,
> FD_CLOEXEC) or so.

That is, I suspect you can get into the same trouble by creating a pipe,
setting F_SETOWN on the read side and then calling exec() while ensuring
someone writes to the thing at the right moment.


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