[PATCH 07/73] VFS: Add hard read-only users count to superblock [ver #2]

[David Howells <dhowells@redhat.com>]

From: Valerie Aurora <vaurora@redhat.com>

While we can check if a file system is currently read-only, we can't
guarantee that it will stay read-only.  The file system can be mounted
or remounted read-write at any time.  This is a problem for union
mounts, which require the underlying file system be read-only for the
entire duration of the union mount.

Add a hard read-only users count to the superblock.  When this count
is non-zero, don't allow any read-write mounts of this super, or any
read-write remounts of existing mounts.

Original-author: Valerie Aurora <vaurora@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/super.c         |   11 +++++++++++
 include/linux/fs.h |    6 ++++++
 2 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index f343eda..732e19b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -202,6 +202,7 @@ static inline void destroy_super(struct super_block *s)
 #ifdef CONFIG_SMP
 	free_percpu(s->s_files);
 #endif
+	BUG_ON(s->s_hard_readonly_users);
 	security_sb_free(s);
 	WARN_ON(!list_empty(&s->s_mounts));
 	kfree(s->s_subtype);
@@ -758,6 +759,9 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 		}
 	}
 
+	if (!(flags & MS_RDONLY) && sb->s_hard_readonly_users)
+		return -EROFS;
+
 	if (sb->s_op->remount_fs) {
 		retval = sb->s_op->remount_fs(sb, &flags, data);
 		if (retval) {
@@ -1150,9 +1154,16 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
 	WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
 		"negative value (%lld)\n", type->name, sb->s_maxbytes);
 
+	if (!(flags & MS_RDONLY) && sb->s_hard_readonly_users)
+		goto out_sb_is_hard_ro;
+
 	up_write(&sb->s_umount);
 	free_secdata(secdata);
 	return root;
+
+out_sb_is_hard_ro:
+	up_write(&sb->s_umount);
+	error = -EROFS;
 out_sb:
 	dput(root);
 	deactivate_locked_super(sb);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d851be9..b8276c0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1496,6 +1496,12 @@ struct super_block {
 
 	/* Being remounted read-only */
 	int s_readonly_remount;
+
+	/* Number of mounts requiring that the underlying file system never
+	 * transition to read-write.  Protected by s_umount.  Decremented by
+	 * free_vfsmnt() if MNT_HARD_READONLY is set.
+	 */
+	int s_hard_readonly_users;
 };
 
 /* superblock cache pruning functions */