Re: [PATCH v2] fail dentry revalidation after namespace change

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.