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

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Date:	Wed, 1 Jul 2009 14:25:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Neil Horman <nhorman@...driver.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 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.

> > > +	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 ;)

And please note this relies on do_coredump()->S_ISREG() check which can be
removed.

> > IOW, could you explain why the (much simpler) patch I sent doesn't work ?
> >
> In short, because the much simpler patch that you sent is broken.  I in fact
> tried it as is, and ran across the exact race that I described above, in which
> the user space helepr exited before we waited on it, resulting in an oops when
> we tried to manipulate the i_pipe pointer, which had become NULL;

I must have missed something. And yes, as I said I didn't test my patch.

But I don't understand how this can happen, see above. And look, if this
is possible then dump_write()->pipe_write() should oops too, it doesn't
check inode/pipe != NULL.


OK, I have to check this all. But perhaps you can explain where I am wrong?

Perhaps I should actually apply my patch and test ;)

Oleg.

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