linux-kernel - Re: [PATCH] fix race in AF

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Message-ID: <20070618130152.50215201@the-village.bc.nu>
Date:	Mon, 18 Jun 2007 13:01:52 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Miklos Szeredi <miklos@...redi.hu>, davem@...emloft.net,
	akpm@...ux-foundation.org, viro@....linux.org.uk,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix race in AF_UNIX

On Mon, 18 Jun 2007 12:43:40 +0200
Thomas Graf <tgraf@...g.ch> wrote:

> * Miklos Szeredi <miklos@...redi.hu> 2007-06-18 12:39
> > You are wrong.  Look in unix_release_sock():
> > 
> > 	if (atomic_read(&unix_tot_inflight))
> > 		unix_gc();		/* Garbage collect fds */
> > 
> > 
> > unix_tot_inflight is the number of AF_UNIX sockets currently being
> > transferred over some AF_UNIX sockets.
> > 
> > That means that just sending (non-unix socket) fds over unix sockets
> > will never invoke the gc.
> 
> That's what I meant, I'm sorry, I should have written unix socket
> file descriptor to not leave any room for misinterpretation.

You can bound the worst case on this and I think stay within the specs
(as the specs don't say a lot about it). One way would be to make
unix_gc() kick off a thread/tasklet and if the last unix_gc was within 5
seconds then use a timer to defer it - that prevents any user driven
"lets cause a ton of gc" cases.
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