fs: add is_idmapped_mnt() helper

Multiple places open-code the same check to determine whether a given
mount is idmapped. Introduce a simple helper function that can be used
instead. This allows us to get rid of the fragile open-coding. We will
later change the check that is used to determine whether a given mount
is idmapped. Introducing a helper allows us to do this in a single
place instead of doing it for multiple places.

Link: https://lore.kernel.org/r/20211123114227.3124056-2-brauner@kernel.org (v1)
Link: https://lore.kernel.org/r/20211130121032.3753852-2-brauner@kernel.org (v2)
Link: https://lore.kernel.org/r/20211203111707.3901969-2-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 659a8f39c61a..4994b816a74c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3936,7 +3936,7 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	 * mapping. It makes things simpler and callers can just create
 	 * another bind-mount they can idmap if they want to.
 	 */
-	if (mnt_user_ns(m) != &init_user_ns)
+	if (is_idmapped_mnt(m))
 		return -EPERM;
 
 	/* The underlying filesystem doesn't support idmapped mounts yet. */