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 <[e-mail]>
Acked-by: Adrian Hunter <[e-mail]>

fs/ubifs/sb.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);
···
 	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) {

fs/ubifs/super.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);
···
 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);

fs/ubifs/ubifs-media.h

···
 /* 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)
···
 
 /*
  * 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
···
  * @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;
···
 	__le64 rp_size;
 	__le32 time_gran;
 	__u8 uuid[16];
-	__u8 padding2[3972];
+	__le32 ro_compat_version;
+	__u8 padding2[3968];
 } __attribute__ ((packed));
 
 /**

fs/ubifs/ubifs.h

···
  *          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
···
  *                   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
···
 	unsigned long long cmt_no;
 	spinlock_t cnt_lock;
 	int fmt_version;
+	int ro_compat_version;
 	unsigned char uuid[16];
 
 	int lhead_lnum;
···
 	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;