Re: Strange Application bug, race in MSG_PEEK complaints (was: Bug#513695: fetchmail: race in MSG_PEEK)

["Ilpo Järvinen" <ilpo.jarvinen@helsinki.fi>]

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On Thu, 7 May 2009, Frans Pop wrote:

> On Thursday 07 May 2009, Ilpo Järvinen wrote:
> > On Thu, 7 May 2009, Matthias Andree wrote:
> > > I've applied the following patch to net/ipv4/tcp.c for the kernel
> > > running on my server (2.6.29-rc8):
> > > @@ -1499,8 +1499,9 @@ do_prequeue:
> > >    		}
> > >    		if ((flags & MSG_PEEK) && peek_seq != tp->copied_seq) {
> > >    			if (net_ratelimit())
> > > -				printk(KERN_DEBUG "TCP(%s:%d): Application bug, race in MSG_PEEK.\n",
> > > -				       current->comm, task_pid_nr(current));
> > > +				printk(KERN_DEBUG "TCP(%s:%d): Application bug, race in MSG_PEEK: %x, %x.\n",
> > > +				       current->comm, task_pid_nr(current)),
> > > +				       peek_seq, tp->copied_seq;
> >
> > I cannot resist myself from noting that this certainly wasn't the patch
> > one got those printks below... It might happily compile though :-).
> 
> Can you please elaborate why you think that? It may be horribly broken
> (I've never claimed to be a C coder, and probably never will), but it
> also really is the patch that generates the printks...

...This was mainly meant to be a joke... :-)

The parenthesis won't match how a printk with string and 4 args should 
be called, so with this version you have in the mail peek_seq and 
tp->copied_seq are not put into stack or you were just super lucky.

> > >    			peek_seq = tp->copied_seq;
> > >    		}
> > >    		continue;
> > >
> > > So, the values you see at the end of the warning are peek_seq and
> > > tp->copied_seq. This gives messages like:
> > > kernel: TCP(fetchmail:9311): Application bug, race in MSG_PEEK: 156233, 16a.
> > > kernel: TCP(fetchmail:9311): Application bug, race in MSG_PEEK: 71259ac8, 5b4.
> > > kernel: TCP(fetchmail:31216): Application bug, race in MSG_PEEK: 833fe5, c0.
> 
> [...]
> 
> > What would you think about the following, untested patch... I suppose
> > it is enough to capture the racy situations except with that crazy urg
> > hole, grr (I suppose that will need just another variable to do the
> > offset of one).
> 
> I'll give your patch a try and report back.

Thanks. If it works I can add that urg hole madness handling too there.

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 i.