umount(2): move the flag validity checks first

Unfortunately, there's userland code that used to rely upon these
checks being done before anything else to check for UMOUNT_NOFOLLOW
support.  That broke in 41525f56e256 ("fs: refactor ksys_umount").
Separate those from the rest of checks and move them to ksys_umount();
unlike everything else in there, this can be sanely done there.

Reported-by: Sargun Dhillon <sargun@sargun.me>
Fixes: 41525f56e256 ("fs: refactor ksys_umount")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index d2db7df..9d33909 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1713,8 +1713,6 @@ 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)
@@ -1728,6 +1726,7 @@ static int can_umount(const struct path *path, int flags)
        return 0;
 }
 
+// caller is responsible for flags being sane
 int path_umount(struct path *path, int flags)
 {
        struct mount *mnt = real_mount(path->mnt);
@@ -1749,6 +1748,10 @@ static int ksys_umount(char __user *name, int flags)
        struct path path;
        int ret;
 
+       // basic validity checks done first
+       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);