Re: [PATCH] ovl: check before dereferencing s_root field

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

On Wed, Sep 17, 2025 at 09:42:00PM +0100, Al Viro wrote:
> On Wed, Sep 17, 2025 at 01:07:45PM +0200, Amir Goldstein wrote:
> 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 60046ae23d514..8c9d0d6bb0045 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -1999,10 +1999,12 @@ struct dentry *d_make_root(struct inode *root_inode)
> > 
> >         if (root_inode) {
> >                 res = d_alloc_anon(root_inode->i_sb);
> > -               if (res)
> > +               if (res) {
> > +                       root_inode->i_opflags |= IOP_ROOT;
> >                         d_instantiate(res, root_inode);
> 
> Umm...  Not a good idea - if nothing else, root may end up
> being attached someplace (normal with nfs, for example).
> 
> But more fundamentally, once we are into ->kill_sb(), let alone
> generic_shutdown_super(), nobody should be playing silly buggers
> with the filesystem.  Sure, RCU accesses are possible, but messing
> around with fhandles?  ->s_root is not the only thing that might
> be no longer there.
> 
> What the fuck is fsnotify playing at?

PS: there is a whole lot of the logics in e.g. shrink_dcache_for_umount()
that relies on nobody else messing with dentry tree by that point.