[PATCH 18/74] VFS: Add hard read-only users count to superblock

[Valerie Aurora <valerie.aurora@gmail.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.

Signed-off-by: Valerie Aurora <valerie.aurora@gmail.com>
---
 fs/super.c         |    8 ++++++++
 include/linux/fs.h |    7 +++++++
 2 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 8819e3a..d02a4d6 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -126,6 +126,7 @@ 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);
 	kfree(s->s_subtype);
 	kfree(s->s_options);
@@ -577,6 +578,9 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 			return -EBUSY;
 	}
 
+	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)
@@ -963,6 +967,10 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
 	WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
 		"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
 
+	error = -EROFS;
+	if (!(flags & MS_RDONLY) && mnt->mnt_sb->s_hard_readonly_users)
+		goto out_sb;
+
 	mnt->mnt_mountpoint = mnt->mnt_root;
 	mnt->mnt_parent = mnt;
 	up_write(&mnt->mnt_sb->s_umount);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 92d248b..469e0ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1388,6 +1388,13 @@ struct super_block {
 	 * generic_show_options()
 	 */
 	char *s_options;
+
+	/*
+	 * 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;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);
-- 
1.7.0.4