ext4: support read-only images

Add a rocompat feature, "readonly" to mark a FS image as read-only.
The feature prevents the kernel and e2fsprogs from changing the image;
the flag can be toggled by tune2fs.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1530,6 +1530,7 @@ static inline void ext4_clear_state_flags(struct ext4_inode_info *ei)
  * GDT_CSUM bits are mutually exclusive.
  */
 #define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM	0x0400
+#define EXT4_FEATURE_RO_COMPAT_READONLY		0x1000
 
 #define EXT4_FEATURE_INCOMPAT_COMPRESSION	0x0001
 #define EXT4_FEATURE_INCOMPAT_FILETYPE		0x0002

fs/ext4/super.c

@@ -2776,6 +2776,12 @@ static int ext4_feature_set_ok(struct super_block *sb, int readonly)
 	if (readonly)
 		return 1;
 
+	if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_READONLY)) {
+		ext4_msg(sb, KERN_INFO, "filesystem is read-only");
+		sb->s_flags |= MS_RDONLY;
+		return 1;
+	}
+
 	/* Check that feature set is OK for a read-write mount */
 	if (EXT4_HAS_RO_COMPAT_FEATURE(sb, ~EXT4_FEATURE_RO_COMPAT_SUPP)) {
 		ext4_msg(sb, KERN_ERR, "couldn't mount RDWR because of "
@@ -4929,7 +4935,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 				ext4_mark_recovery_complete(sb, es);
 		} else {
 			/* Make sure we can mount this feature set readwrite */
-			if (!ext4_feature_set_ok(sb, 0)) {
+			if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
+					EXT4_FEATURE_RO_COMPAT_READONLY) ||
+			    !ext4_feature_set_ok(sb, 0)) {
 				err = -EROFS;
 				goto restore_opts;
 			}