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>

4 files changed, 72 insertions, 11 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) {
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 372c7fb66531..302a2056422e 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -1351,8 +1351,9 @@ static int mount_ubifs(struct ubifs_info *c)
         x = (long long)c->log_lebs * c->leb_size + c->max_bud_bytes;
         ubifs_msg("journal size:       %lld bytes (%lld KiB, %lld MiB, %d "
                   "LEBs)", x, x >> 10, x >> 20, c->log_lebs + c->max_bud_cnt);
-        ubifs_msg("media format:       %d (latest is %d)",
-                  c->fmt_version, UBIFS_FORMAT_VERSION);
+        ubifs_msg("media format:       w%d/r%d (latest is w%d/r%d)",
+                  c->fmt_version, c->ro_compat_version,
+                  UBIFS_FORMAT_VERSION, UBIFS_RO_COMPAT_VERSION);
         ubifs_msg("default compressor: %s", ubifs_compr_name(c->default_compr));
         ubifs_msg("reserved for root:  %llu bytes (%llu KiB)",
                 c->report_rp_size, c->report_rp_size >> 10);
@@ -1492,6 +1493,15 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 {
         int err, lnum;
 
+        if (c->rw_incompat) {
+                ubifs_err("the file-system is not R/W-compatible");
+                ubifs_msg("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);
+                return -EROFS;
+        }
+
         mutex_lock(&c->umount_mutex);
         dbg_save_space_info(c);
         c->remounting_rw = 1;
diff --git a/fs/ubifs/ubifs-media.h b/fs/ubifs/ubifs-media.h
index b25fc36cf72f..3eee07e0c495 100644
--- a/fs/ubifs/ubifs-media.h
+++ b/fs/ubifs/ubifs-media.h
@@ -36,9 +36,31 @@
 /* UBIFS node magic number (must not have the padding byte first or last) */
 #define UBIFS_NODE_MAGIC  0x06101831
 
-/* UBIFS on-flash format version */
+/*
+ * UBIFS on-flash format version. This version is increased when the on-flash
+ * format is changing. If this happens, UBIFS is will support older versions as
+ * well. But older UBIFS code will not support newer formats. Format changes
+ * will be rare and only when absolutely necessary, e.g. to fix a bug or to add
+ * a new feature.
+ *
+ * UBIFS went into mainline kernel with format version 4. The older formats
+ * were development formats.
+ */
 #define UBIFS_FORMAT_VERSION 4
 
+/*
+ * Read-only compatibility version. If the UBIFS format is changed, older UBIFS
+ * implementations will not be able to mount newer formats in read-write mode.
+ * However, depending on the change, it may be possible to mount newer formats
+ * in R/O mode. This is indicated by the R/O compatibility version which is
+ * stored in the super-block.
+ *
+ * This is needed to support boot-loaders which only need R/O mounting. With
+ * this flag it is possible to do UBIFS format changes without a need to update
+ * boot-loaders.
+ */
+#define UBIFS_RO_COMPAT_VERSION 0
+
 /* Minimum logical eraseblock size in bytes */
 #define UBIFS_MIN_LEB_SZ (15*1024)
 
@@ -53,7 +75,7 @@
 
 /*
  * If compressed data length is less than %UBIFS_MIN_COMPRESS_DIFF bytes
- * shorter than uncompressed data length, UBIFS preferes to leave this data
+ * shorter than uncompressed data length, UBIFS prefers to leave this data
  * node uncompress, because it'll be read faster.
  */
 #define UBIFS_MIN_COMPRESS_DIFF 64
@@ -586,6 +608,7 @@ struct ubifs_pad_node {
  * @padding2: reserved for future, zeroes
  * @time_gran: time granularity in nanoseconds
  * @uuid: UUID generated when the file system image was created
+ * @ro_compat_version: UBIFS R/O compatibility version
  */
 struct ubifs_sb_node {
         struct ubifs_ch ch;
@@ -612,7 +635,8 @@ struct ubifs_sb_node {
         __le64 rp_size;
         __le32 time_gran;
         __u8 uuid[16];
-        __u8 padding2[3972];
+        __le32 ro_compat_version;
+        __u8 padding2[3968];
 } __attribute__ ((packed));
 
 /**
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index a53b9a6df2be..0a8341e14088 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -934,6 +934,7 @@ struct ubifs_debug_info;
  *          by @commit_sem
  * @cnt_lock: protects @highest_inum and @max_sqnum counters
  * @fmt_version: UBIFS on-flash format version
+ * @ro_compat_version: R/O compatibility version
  * @uuid: UUID from super block
  *
  * @lhead_lnum: log head logical eraseblock number
@@ -966,6 +967,7 @@ struct ubifs_debug_info;
  *                   recovery)
  * @bulk_read: enable bulk-reads
  * @default_compr: default compression algorithm (%UBIFS_COMPR_LZO, etc)
+ * @rw_incompat: the media is not R/W compatible
  *
  * @tnc_mutex: protects the Tree Node Cache (TNC), @zroot, @cnext, @enext, and
  *             @calc_idx_sz
@@ -1179,6 +1181,7 @@ struct ubifs_info {
         unsigned long long cmt_no;
         spinlock_t cnt_lock;
         int fmt_version;
+        int ro_compat_version;
         unsigned char uuid[16];
 
         int lhead_lnum;
@@ -1207,6 +1210,7 @@ struct ubifs_info {
         unsigned int no_chk_data_crc:1;
         unsigned int bulk_read:1;
         unsigned int default_compr:2;
+        unsigned int rw_incompat:1;
 
         struct mutex tnc_mutex;
         struct ubifs_zbranch zroot;