linux-kernel - Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space pipe readers to complete (v4)

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Message-ID: <20090701141220.GC29601@hmsreliant.think-freely.org>
Date:	Wed, 1 Jul 2009 10:12:20 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	earl_chew@...lent.com, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v4)

On Wed, Jul 01, 2009 at 02:25:33PM +0200, Oleg Nesterov wrote:
> On 07/01, Neil Horman wrote:
> >
> > On Wed, Jul 01, 2009 at 07:52:57AM +0200, Oleg Nesterov wrote:
> > > > This extra count is reclaimed in
> > > > +		 * wait_for_dump_helpers
> > > > +		 */
> > > > +		pipe = file->f_path.dentry->d_inode->i_pipe;
> > > > +		pipe_lock(pipe);
> > > > +		pipe->readers++;
> > > > +		pipe_unlock(pipe);
> > >
> > > why should we inc ->readers in advance?
> > >
> > Read the comment immediately above it and look at the filp_close path.  We inc
> > ->readers in advance so as to prevent pipe_inode_info getting freed between the
> > time we write out the core file and the time we wait on the pipe.
> 
> Can't understand.
> 
> call_usermodehelper_stdinpipe() creates 2 files, both share the same
> inode/pipe_inode_info (->f_path actually).
> 
> Until we close the file returned by call_usermodehelper_pipe(),
> pipe_inode_info can't go away.
> 
> > If the
> > userspace helper exits in between those points we inode->i_pipe will be null by
> > the time we get to wait_for_dump_helpers.
> 
> See above. Can't understand how this can happen.
> 
Yes, I apologize, in reviewing that code, I don't see how it could happen
either.  I think that I must have changed to things at once and gotten erroneous
results in my testing (most likely I did the wait_for_dump_helpers after the
filp_close and that cause the race).  I'm reworking the patch now.

> > > > +	wait_for_dump_helpers(file);
> > >
> > > why do we call it unconditionally and then check ISFIFO? We only need to wait
> > > when ispipe = T, and in that case we know that this file is pipe.
> > >
> > Cosmetic, I can call it unconditionally here and then check if its a fifo in the
> > function, so that in do_coredump I don't have to do the following:
> > if (is_pipe)
> > 	wait_for_dump_helpers(file);
> 
> I think the above is better. More straightforward and clean.
> 
> > This is exactly the sort of crap your cleanups to do_coredump attemtped to
> > remove.  I thought it best not to undo that work :)
> 
> Well. I tried to remove unnecessary "if (ispipe)" checks, yes. But in that
> case we can't avoid this check. And your patch still does this check, but
> instead of simple "ispipe == T" we check
> S_ISFIFO(file->f_path.dentry->d_inode->i_mode), doesn't look as a cleanup ;)
> 
Ok, whatever, I'm not interested in debating what looks prettier, so I'll just
use the ispipe test and be done with it.

I'm testing version 5 and will post the set shortly.
Neil

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