[PATCH] ovl: minimal remount support.

[NeilBrown]

As overlayfs reflects the 'readonly' mount status in write-access to
the upper filesystem, we must handle remount and either drop or take
write access when the ro status changes.

Signed-off-by: NeilBrown <neilb@xxxxxxx>

diff --git a/fs/overlayfs/overlayfs.c b/fs/overlayfs/overlayfs.c
index 0ddfeec..4e032e8 100644
--- a/fs/overlayfs/overlayfs.c
+++ b/fs/overlayfs/overlayfs.c
@@ -1685,8 +1685,28 @@ static void ovl_put_super(struct super_block *sb)
 	kfree(ufs);
 }
 
+static int ovl_remount_fs(struct super_block *sb, int *flagsp, char *data)
+{
+	int flags = *flagsp;
+	struct ovl_fs *ufs = sb->s_fs_info;
+
+	/* When remounting rw or ro, we need to adjust the write access to the
+	 * upper fs.
+	 */
+	if (((flags ^ sb->s_flags) & MS_RDONLY) == 0)
+		/* No change to readonly status */
+		return 0;
+
+	if (flags & MS_RDONLY) {
+		mnt_drop_write(ufs->upper_mnt);
+		return 0;
+	} else
+		return mnt_want_write(ufs->upper_mnt);
+}
+
 static const struct super_operations ovl_super_operations = {
 	.put_super	= ovl_put_super,
+	.remount_fs	= ovl_remount_fs,
 };
 
 struct ovl_config {
--
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