Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>
> In preparation for allowing mountpoints to be renamed and unlinked
> in remote filesystems and in other mount namespaces test if on a dentry
> there is a mount in the local mount namespace before allowing it to
> be renamed or unlinked.
>
> The primary motivation here are old versions of fusermount unmount
> which is not safe if the a path can be renamed or unlinked while it is
> verifying the mount is safe to unmount. More recent versions are simpler
> and safer by simply using UMOUNT_NOFOLLOW when unmounting a mount
> in a directory owned by an arbitrary user.
>
> Miklos Szeredi <miklos@xxxxxxxxxx> reports this is approach is good
> enough to remove concerns about new kernels mixed with old versions
> of fusermount.
>
> A secondary motivation for restrictions here is that it removing empty
> directories that have non-empty mount points on them appears to
> violate the rule that rmdir can not remove empty directories. As
> Linus Torvalds pointed out this is useful for programs (like git) that
> test if a directory is empty with rmdir.
>
> Therefore this patch arranges to enforce the existing mount point
> semantics for local mount namespace.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> ---
> fs/namei.c | 25 +++++++++++++++++++++++++
> 1 files changed, 25 insertions(+), 0 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 645268f23eb6..df7bd6e9c7b6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3547,6 +3547,20 @@ void dentry_unhash(struct dentry *dentry)
> spin_unlock(&dentry->d_lock);
> }
>
> +static bool covered(struct vfsmount *mnt, struct dentry *dentry)
> +{
> + /* test to see if a dentry is covered with a mount in
> + * the current mount namespace.
> + */
> + bool is_covered;
> +
> + rcu_read_lock();
> + is_covered = d_mountpoint(dentry) && __lookup_mnt(mnt, dentry, 1);
> + rcu_read_unlock();
> +
> + return is_covered;
> +}
> +
> int vfs_rmdir(struct inode *dir, struct dentry *dentry)
> {
> int error = may_delete(dir, dentry, 1);
> @@ -3622,6 +3636,9 @@ retry:
> error = -ENOENT;
> goto exit3;
> }
> + error = -EBUSY;
> + if (covered(nd.path.mnt, dentry))
> + goto exit3;
> error = security_path_rmdir(&nd.path, dentry);
> if (error)
> goto exit3;
> @@ -3716,6 +3733,9 @@ retry:
> inode = dentry->d_inode;
> if (!inode)
> goto slashes;
> + error = -EBUSY;
> + if (covered(nd.path.mnt, dentry))
> + goto exit2;
> ihold(inode);
> error = security_path_unlink(&nd.path, dentry);
> if (error)
> @@ -4164,6 +4184,11 @@ retry:
> error = -ENOTEMPTY;
> if (new_dentry == trap)
> goto exit5;
> + error = -EBUSY;
> + if (covered(oldnd.path.mnt, old_dentry))
> + goto exit5;
> + if (covered(newnd.path.mnt, new_dentry))
> + goto exit5;
>
> error = security_path_rename(&oldnd.path, old_dentry,
> &newnd.path, new_dentry);
> --
> 1.7.5.4
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