UBIFS: add R/O compatibility

Now UBIFS is supported by u-boot. If we ever decide to change the
media format, then people will have to upgrade their u-boots to
mount new format images. However, very often it is possible to
preserve R/O forward-compatibility, even though the write
forward-compatibility is not preserved.

This patch introduces a new super-block field which stores the
R/O compatibility version.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Adrian Hunter <Adrian.Hunter@nokia.com>

1 files changed, 29 insertions, 6 deletions

diff --git a/fs/ubifs/sb.c b/fs/ubifs/sb.c
index 0dec47c87c6d..57085e43320f 100644
--- a/fs/ubifs/sb.c
+++ b/fs/ubifs/sb.c
@@ -193,6 +193,7 @@ static int create_default_filesystem(struct ubifs_info *c)
         if (tmp64 > DEFAULT_MAX_RP_SIZE)
                 tmp64 = DEFAULT_MAX_RP_SIZE;
         sup->rp_size = cpu_to_le64(tmp64);
+        sup->ro_compat_version = cpu_to_le32(UBIFS_RO_COMPAT_VERSION);
 
         err = ubifs_write_node(c, sup, UBIFS_SB_NODE_SZ, 0, 0, UBI_LONGTERM);
         kfree(sup);
@@ -532,17 +533,39 @@ int ubifs_read_superblock(struct ubifs_info *c)
         if (IS_ERR(sup))
                 return PTR_ERR(sup);
 
+        c->fmt_version = le32_to_cpu(sup->fmt_version);
+        c->ro_compat_version = le32_to_cpu(sup->ro_compat_version);
+
         /*
          * The software supports all previous versions but not future versions,
          * due to the unavailability of time-travelling equipment.
          */
-        c->fmt_version = le32_to_cpu(sup->fmt_version);
         if (c->fmt_version > UBIFS_FORMAT_VERSION) {
-                ubifs_err("on-flash format version is %d, but software only "
-                          "supports up to version %d", c->fmt_version,
-                          UBIFS_FORMAT_VERSION);
-                err = -EINVAL;
-                goto out;
+                struct super_block *sb = c->vfs_sb;
+                int mounting_ro = sb->s_flags & MS_RDONLY;
+
+                ubifs_assert(!c->ro_media || mounting_ro);
+                if (!mounting_ro ||
+                    c->ro_compat_version > UBIFS_RO_COMPAT_VERSION) {
+                        ubifs_err("on-flash format version is w%d/r%d, but "
+                                  "software only supports up to version "
+                                  "w%d/r%d", c->fmt_version,
+                                  c->ro_compat_version, UBIFS_FORMAT_VERSION,
+                                  UBIFS_RO_COMPAT_VERSION);
+                        if (c->ro_compat_version <= UBIFS_RO_COMPAT_VERSION) {
+                                ubifs_msg("only R/O mounting is possible");
+                                err = -EROFS;
+                        } else
+                                err = -EINVAL;
+                        goto out;
+                }
+
+                /*
+                 * The FS is mounted R/O, and the media format is
+                 * R/O-compatible with the UBIFS implementation, so we can
+                 * mount.
+                 */
+                c->rw_incompat = 1;
         }
 
         if (c->fmt_version < 3) {