Re: [PATCH] fs/kernfs: Fix lockdep warning in kernfs_active()

Greg KH
2022-11-10T18:01:50Z

On Wed, Nov 09, 2022 at 08:04:15PM +0800, Chengming Zhou wrote:
> syzbot found a lockdep warning in kernfs_find_and_get_node_by_id(),
> bisected to the commit c25491747b21 ("kernfs: Add KERNFS_REMOVING flags"),
> which didn't hold kernfs_rwsem before call kernfs_active(kn).
>
> Since kernfs_find_and_get_node_by_id() doesn't have to get active count
> of kn, only need to get a stable refcount of kn, so it should be enough
> to just check kn has been KERNFS_ACTIVATED.
>
> Reported-by: syzbot+2fdf66e68f5f882c1074@syzkaller.appspotmail.com
> Fixes: c25491747b21 ("kernfs: Add KERNFS_REMOVING flags")
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
> fs/kernfs/dir.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 6acd9c3d4cff..08f0f1570cd7 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -705,7 +705,13 @@ struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
> goto err_unlock;
> }
>
> - if (unlikely(!kernfs_active(kn) || !atomic_inc_not_zero(&kn->count)))
> + /*
> + * ACTIVATED is protected with kernfs_mutex but it was clear when
> + * @kn was added to idr and we just wanna see it set. No need to
> + * grab kernfs_mutex.
> + */
> + if (unlikely(!(kn->flags & KERNFS_ACTIVATED) ||
> + !atomic_inc_not_zero(&kn->count)))
> goto err_unlock;
>
> spin_unlock(&kernfs_idr_lock);
> --
> 2.37.2
>

Shouldn't:
https://lore.kernel.org/r/Y0SwqBsZ9BMmZv6x@slm.duckdns.org fix this
instead?