enforce ->sync_fs is only called for rw superblock

Make sure a superblock really is writeable by checking MS_RDONLY
under s_umount.  sync_filesystems needed some re-arragement for
that, but all but one sync_filesystem caller had the correct locking
already so that we could add that check there.  cachefiles grew
s_umount locking.

I've also added a WARN_ON to sync_filesystem to assert this for
future callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 16 insertions, 7 deletions

diff --git a/fs/sync.c b/fs/sync.c
index 4487b5560dc8..89c37f732afa 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -51,6 +51,18 @@ int sync_filesystem(struct super_block *sb)
 {
         int ret;
 
+        /*
+         * We need to be protected against the filesystem going from
+         * r/o to r/w or vice versa.
+         */
+        WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
+        /*
+         * No point in syncing out anything if the filesystem is read-only.
+         */
+        if (sb->s_flags & MS_RDONLY)
+                return 0;
+
         ret = __sync_filesystem(sb, 0);
         if (ret < 0)
                 return ret;
@@ -79,25 +91,22 @@ static void sync_filesystems(int wait)
 
         mutex_lock(&mutex);             /* Could be down_interruptible */
         spin_lock(&sb_lock);
-        list_for_each_entry(sb, &super_blocks, s_list) {
-                if (sb->s_flags & MS_RDONLY)
-                        continue;
+        list_for_each_entry(sb, &super_blocks, s_list)
                 sb->s_need_sync = 1;
-        }
 
 restart:
         list_for_each_entry(sb, &super_blocks, s_list) {
                 if (!sb->s_need_sync)
                         continue;
                 sb->s_need_sync = 0;
-                if (sb->s_flags & MS_RDONLY)
-                        continue;       /* hm.  Was remounted r/o meanwhile */
                 sb->s_count++;
                 spin_unlock(&sb_lock);
+
                 down_read(&sb->s_umount);
-                if (sb->s_root)
+                if (!(sb->s_flags & MS_RDONLY) && sb->s_root)
                         __sync_filesystem(sb, wait);
                 up_read(&sb->s_umount);
+
                 /* restart only when sb is no longer on the list */
                 spin_lock(&sb_lock);
                 if (__put_super_and_need_restart(sb))