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: <c7445f68809ea699c73da0282a9e2b3f99ff3bcf.camel@kernel.org>
Date: Thu, 05 Feb 2026 07:38:52 -0500
From: Jeff Layton <jlayton@...nel.org>
To: NeilBrown <neil@...wn.name>, 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>, Miklos Szeredi <miklos@...redi.hu>, Amir
 Goldstein	 <amir73il@...il.com>, 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>
Cc: 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, 2026-02-04 at 15:57 +1100, NeilBrown 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>
> ---
>  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,

Reviewed-by: Jeff Layton <jlayton@...nel.org>