mnt: Refactor fs_fully_visible into mount_too_revealing

Replace the call of fs_fully_visible in do_new_mount from before the
new superblock is allocated with a call of mount_too_revealing after
the superblock is allocated.   This winds up being a much better location
for maintainability of the code.

The first change this enables is the replacement of FS_USERNS_VISIBLE
with SB_I_USERNS_VISIBLE.  Moving the flag from struct filesystem_type
to sb_iflags on the superblock.

Unfortunately mount_too_revealing fundamentally needs to touch
mnt_flags adding several MNT_LOCKED_XXX flags at the appropriate
times.  If the mnt_flags did not need to be touched the code
could be easily moved into the filesystem specific mount code.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd288148a6b1..71988dd3af95 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1328,6 +1328,9 @@ struct mm_struct;
 #define SB_I_CGROUPWB   0x00000001      /* cgroup-aware writeback enabled */
 #define SB_I_NOEXEC     0x00000002      /* Ignore executables on this fs */
 
+/* sb->s_iflags to limit user namespace mounts */
+#define SB_I_USERNS_VISIBLE             0x00000010 /* fstype already mounted */
+
 /* Possible states of 'frozen' field */
 enum {
         SB_UNFROZEN = 0,                /* FS is unfrozen */
@@ -2011,7 +2014,6 @@ struct file_system_type {
 #define FS_HAS_SUBTYPE          4
 #define FS_USERNS_MOUNT         8       /* Can be mounted by userns root */
 #define FS_USERNS_DEV_MOUNT     16 /* A userns mount does not imply MNT_NODEV */
-#define FS_USERNS_VISIBLE       32      /* FS must already be visible */
 #define FS_RENAME_DOES_D_MOVE   32768   /* FS will handle d_move() during rename() internally. */
         struct dentry *(*mount) (struct file_system_type *, int,
                        const char *, void *);