linux-kernel - Re: [patch 3/8] procfs: Add ability to plug in auxiliary fdinfo providers

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Date:	Thu, 16 Aug 2012 01:31:03 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>
Subject: Re: [patch 3/8] procfs: Add ability to plug in auxiliary fdinfo
 providers

On Wed, Aug 15, 2012 at 10:16:28PM +0100, Al Viro wrote:
> On Wed, Aug 15, 2012 at 01:21:19PM +0400, Cyrill Gorcunov wrote:
> > -static int fdinfo_open_helper(struct inode *inode, int *f_flags, struct path *path)
> > +static int fdinfo_open_helper(struct inode *inode, int *f_flags, struct file **f_file, struct path *path)
> 
> Bloody bad taste, that...  This kind of optional arguments is almost always
> a bad sign - tends to happen when you have two barely related functions
> crammed into one.  And yes, proc_fd_info() suffers the same braindamage.
> Trying to avoid code duplication is generally a good thing, but it's not
> always worth doing - less obfuscated code wins.

Sure I'll update. Thanks.

> >  static int seq_show(struct seq_file *m, void *v)
> >  {
> >  	struct proc_fdinfo *fdinfo = m->private;
> > -	seq_printf(m, "pos:\t%lli\nflags:\t0%o\n",
> > -		   (long long)fdinfo->f_pos,
> > -		   fdinfo->f_flags);
> > -	return 0;
> > +	int ret;
> > +
> > +	ret = seq_printf(m, "pos:\t%lli\nflags:\t0%o\n",
> > +			 (long long)fdinfo->f_file->f_pos,
> > +			 fdinfo->f_flags);
> 
> Realistically, that one is not going to overflow; you are almost certainly
> wasting more cycles on that check of !ret just below than you'll save on
> not going into ->show_fdinfo() in case of full buffer.

Yes, this is redundant, thanks. Will fix.

> 
> > +	if (!ret && fdinfo->f_file->f_op->show_fdinfo)
> > +		ret = fdinfo->f_file->f_op->show_fdinfo(m, fdinfo->f_file);
> > +
> > +	return ret;
> >  }
>   
> > +	ret = single_open(file, seq_show, fdinfo);
> > +	if (ret) {
> > +		put_filp(fdinfo->f_file);
> 
> Excuse me?  We should *never* do put_filp() on anything that has already
> been opened.  Think what happens if you race with close(); close() would
> rip the reference from descriptor table and do fput(), leaving you with
> the last reference to that struct file.  You really don't want to just
> go and free it.  IOW, that one should be fput().
> 
> > +	put_filp(fdinfo->f_file);
> Ditto. 

It seems I indeed missed this scenario, thanks Al, will update!

	Cyrill
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