[PATCH 5/7] VFS: Add read-only users count to superblock

[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
remounted read-write at any time; it's also conceivable that a file
system can be mounted a second time and converted to read-write if the
underlying fs allows it.  This is a problem for union mounts, which
require the underlying file system be read-only.  Add a read-only
users count and don't allow remounts to change the file system to
read-write or read-write mounts if there are any read-only users.

Signed-off-by: Valerie Aurora <vaurora@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
---
 fs/super.c         |   18 ++++++++++++++++--
 include/linux/fs.h |    5 +++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 19eb70b..8a12bab 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -583,8 +583,10 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 	shrink_dcache_sb(sb);
 	sync_filesystem(sb);
 
-	/* If we are remounting RDONLY and current sb is read/write,
-	   make sure there are no rw files opened */
+	/*
+	 * If we are remounting RDONLY and current sb is read/write,
+	 * make sure there are no rw files opened.
+	 */
 	if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {
 		if (force)
 			mark_files_ro(sb);
@@ -596,6 +598,14 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 	}
 	remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
 
+	/*
+	 * If we are remounting read/write, deny write access if the
+	 * file system is being used by anything that requires it to
+	 * stay read-only (e.g., union mounts).
+	 */
+	if (remount_rw && sb->s_readonly_users)
+		return -EROFS;
+
 	if (sb->s_op->remount_fs) {
 		retval = sb->s_op->remount_fs(sb, &flags, data);
 		if (retval)
@@ -953,6 +963,10 @@ 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 (mnt->mnt_sb->s_readonly_users && !(flags & MS_RDONLY))
+		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 2620a8c..4070eac 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1380,6 +1380,11 @@ struct super_block {
 	 * generic_show_options()
 	 */
 	char *s_options;
+
+	/*
+	 * Users who require read-only access - e.g., union mounts
+	 */
+	int s_readonly_users;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);
-- 
1.5.6.5