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>

2 files changed, 10 insertions, 1 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b7f393df2e4c..7fec2efd8635 100644
--- a/fs/ext4/ext4.h
+++ b/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
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 95b388cae0ab..60db5a11fbea 100644
--- a/fs/ext4/super.c
+++ b/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;
                         }