ksys_umount was refactored to into split into another function (path_umount) to enable sharing code. This changed the order that flags and permissions are validated in, and made it so that user_path_at was called before validating flags.

Unfortunately, libfuse2[1] and libmount[2] rely on the old flag validation behaviour to determine whether or not the kernel supports UMOUNT_NOFOLLOW. The other path that this validation is being checked on is init_umount->path_umount->can_umount. That's all internal to the kernel. We can safely move flag checking to ksys_umount, and let other users of path_umount know they need to perform their own validation.

[1]: https://github.com/libfuse/libfuse/blob/9bfbeb576c5901b62a171d35510f0d1a9220... [2]: https://github.com/karelzak/util-linux/blob/7ed579523b556b1270f28dbdb7ee07de...

Signed-off-by: Sargun Dhillon sargun@sargun.me Cc: Christoph Hellwig hch@lst.de Cc: stable@vger.kernel.org Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: linux-fsdevel@vger.kernel.org Fixes: 41525f56e256 ("fs: refactor ksys_umount") --- fs/namespace.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c index cebaa3e81794..752f82121dd4 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1710,8 +1710,6 @@ static int can_umount(const struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt);

- if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) - return -EINVAL; if (!may_mount()) return -EPERM; if (path->dentry != path->mnt->mnt_root) @@ -1725,6 +1723,13 @@ static int can_umount(const struct path *path, int flags) return 0; }

+ +/* + * path_umount - unmount by path + * + * path_umount does not check the validity of flags. It is up to the caller + * to ensure that it only contains valid umount options. + */ int path_umount(struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt); @@ -1746,6 +1751,10 @@ static int ksys_umount(char __user *name, int flags) struct path path; int ret;

+ /* Check flag validity first to allow probing of supported flags */ + if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) + return -EINVAL; + if (!(flags & UMOUNT_NOFOLLOW)) lookup_flags |= LOOKUP_FOLLOW; ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);