Re: [PATCH 1/7] autofs4: Save autofs trigger's vfsmount in super block info

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Jan 02, 2010 at 08:44:25AM +0800, Ian Kent wrote:
> On Wed, Dec 23, 2009 at 03:36:57PM -0800, Valerie Aurora wrote:
> > From: Jan Blunck <jblunck@suse.de>
> > 
> > This is a bugfix/replacement for commit
> > 051d381259eb57d6074d02a6ba6e90e744f1a29f:
> > 
> >     During a path walk if an autofs trigger is mounted on a dentry,
> >     when the follow_link method is called, the nameidata struct
> >     contains the vfsmount and mountpoint dentry of the parent mount
> >     while the dentry that is passed in is the root of the autofs
> >     trigger mount.  I believe it is impossible to get the vfsmount of
> >     the trigger mount, within the follow_link method, when only the
> >     parent vfsmount and the root dentry of the trigger mount are
> >     known.
> > 
> > The solution in this commit was to replace the path embedded in the
> > parent's nameidata with the path of the link itself in
> > __do_follow_link().  This is a relatively harmless misuse of the
> > field, but union mounts ran into a bug during follow_link() caused by
> > the nameidata containing the wrong path (we count on it being what it
> > is all other places - the path of the parent).
> > 
> > A cleaner and easier to understand solution is to save the necessary
> > vfsmount in the autofs superblock info when it is mounted.  Then we
> > can easily update the vfsmount in autofs4_follow_link().
> > 
> > Signed-off-by: Jan Blunck <jblunck@suse.de>
> > Signed-off-by: Valerie Aurora <vaurora@redhat.com>
> Acked-by: <raven@themaw.net>
> 
> Don't know how I missed such an obvious solution when I did this.
> Thanks, Ian

TBH, I don't like either variant (both the in-tree one and that).
The reason why vfsmount does *NOT* belong in superblock, TYVM: you've
messed the lifetime rules.  You can't pin it down, or the damn thing will
be impossible to kill.  OTOH, you have no promise whatsoever that superblock
won't outlive the initial vfsmount.  You might get another vfsmount over
the same thing and once the original one is gone...

So this is simply broken.