linux-kernel - Re: [PATCH 11/13] ovl: use is_subdir() for testing if one thing is a subdir of another

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Message-ID: <CAOQ4uxiYUTi=8BjRFbY_GdBZR_5CuP6680me=_xQaPcQk7EFuQ@mail.gmail.com>
Date: Thu, 5 Feb 2026 10:37:18 +0100
From: Amir Goldstein <amir73il@...il.com>
To: NeilBrown <neil@...wn.name>
Cc: Christian Brauner <brauner@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	David Howells <dhowells@...hat.com>, Jan Kara <jack@...e.cz>, Chuck Lever <chuck.lever@...cle.com>, 
	Jeff Layton <jlayton@...nel.org>, Miklos Szeredi <miklos@...redi.hu>, 
	John Johansen <john.johansen@...onical.com>, Paul Moore <paul@...l-moore.com>, 
	James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
	Stephen Smalley <stephen.smalley.work@...il.com>, linux-kernel@...r.kernel.org, 
	netfs@...ts.linux.dev, linux-fsdevel@...r.kernel.org, 
	linux-nfs@...r.kernel.org, linux-unionfs@...r.kernel.org, 
	apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org, 
	selinux@...r.kernel.org
Subject: Re: [PATCH 11/13] ovl: use is_subdir() for testing if one thing is a
 subdir of another

On Wed, Feb 4, 2026 at 6:09 AM NeilBrown <neilb@...mail.net> wrote:
>
> From: NeilBrown <neil@...wn.name>
>
> Rather than using lock_rename(), use the more obvious is_subdir() for
> ensuring that neither upper nor workdir contain the other.
> Also be explicit in the comment that the two directories cannot be the
> same.
>
> As this is a point-it-time sanity check and does not provide any
> on-going guarantees, the removal of locking does not introduce any
> interesting races.
>
> Signed-off-by: NeilBrown <neil@...wn.name>

Looks reasonable

Reviewed-by: Amir Goldstein <amir73il@...il.com>

> ---
>  fs/overlayfs/super.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index ba9146f22a2c..2fd3e0aee50e 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -451,18 +451,13 @@ static int ovl_lower_dir(const char *name, const struct path *path,
>         return 0;
>  }
>
> -/* Workdir should not be subdir of upperdir and vice versa */
> +/*
> + * Workdir should not be subdir of upperdir and vice versa, and
> + * they should not be the same.
> + */
>  static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
>  {
> -       bool ok = false;
> -
> -       if (workdir != upperdir) {
> -               struct dentry *trap = lock_rename(workdir, upperdir);
> -               if (!IS_ERR(trap))
> -                       unlock_rename(workdir, upperdir);
> -               ok = (trap == NULL);
> -       }
> -       return ok;
> +       return !is_subdir(workdir, upperdir) && !is_subdir(upperdir, workdir);
>  }
>
>  static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,
> --
> 2.50.0.107.gf914562f5916.dirty
>