From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2] fail dentry revalidation after namespace change
Date: Mon, 9 Jul 2012 17:47:05 -0700
Message-ID: <20120709174705.0e2078c8.akpm@linux-foundation.org>
References: <1341565747-15374-1-git-send-email-glommer@parallels.com>
	<20120709161336.0ec23592.akpm@linux-foundation.org>
	<87txxgxxs7.fsf@xmission.com>
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Cc: Glauber Costa <glommer@parallels.com>,
	<linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
	Greg Thelen <gthelen@google.com>,
	Serge Hallyn <serge.hallyn@canonical.com>,
	Tejun Heo <tj@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: ebiederm@xmission.com (Eric W. Biederman)
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On Mon, 09 Jul 2012 17:30:48 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> >>  {
> >>  	struct sysfs_dirent *sd;
> >>  	int is_dir;
> >> +	int type;
> >>  
> >>  	if (nd->flags & LOOKUP_RCU)
> >>  		return -ECHILD;
> >> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> >>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
> >>  		goto out_bad;
> >>  
> >> +	/* The sysfs dirent has been moved to a different namespace */
> >> +	type = KOBJ_NS_TYPE_NONE;
> >> +	if (sd->s_parent)
> >> +		type = sysfs_ns_type(sd->s_parent);
> >> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> >
> > eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
> > Don't do that; it smells bad.
> 
> Gag.  An incomplete change in idiom.
> 
> KOBJ_NS_TYPE_NONE is explicitly defined as 0 so that it can be used
> this way, and every where else in fs/sysfs/dir.c uses this idiom.

One man's idiom is another man's idiocy.

Seriously.  What sort of idea is that?  Create an enumerated type and
then just ignore it?

> Pray tell in what parallel universe is that monstrosity above more
> readable than the line it replaces?

Don't be silly, it is not a "monstrosity".  The code it is modifying
contains an unneeded test-and-branch.  It's a test and branch which the
compiler might be able to avoid.  If we can demonstrate that the
compiler does indeed optimise it, or if we can find a less monstrous
way of implementing it then fine.  Otherwise, efficiency wins.