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

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

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?