ubifs: add option to specify version for new file systems

Instead of creating ubifs file systems with UBIFS_FORMAT_VERSION
by default, add a module parameter ubifs.default_version to allow
the user to specify the desired version. Valid values are 4 to
UBIFS_FORMAT_VERSION (currently 5).

This way, one can for example create a file system with version 4
on kernel 4.19 which can still be mounted rw when downgrading to
kernel 4.9.
Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/sb.c

@@ -174,7 +174,8 @@ static int create_default_filesystem(struct ubifs_info *c)
 	tmp64 = (long long)max_buds * c->leb_size;
 	if (big_lpt)
 		sup_flags |= UBIFS_FLG_BIGLPT;
-	sup_flags |= UBIFS_FLG_DOUBLE_HASH;
+	if (ubifs_default_version > 4)
+		sup_flags |= UBIFS_FLG_DOUBLE_HASH;
 
 	if (ubifs_authenticated(c)) {
 		sup_flags |= UBIFS_FLG_AUTHENTICATION;
@@ -200,7 +201,7 @@ static int create_default_filesystem(struct ubifs_info *c)
 	sup->jhead_cnt     = cpu_to_le32(DEFAULT_JHEADS_CNT);
 	sup->fanout        = cpu_to_le32(DEFAULT_FANOUT);
 	sup->lsave_cnt     = cpu_to_le32(c->lsave_cnt);
-	sup->fmt_version   = cpu_to_le32(UBIFS_FORMAT_VERSION);
+	sup->fmt_version   = cpu_to_le32(ubifs_default_version);
 	sup->time_gran     = cpu_to_le32(DEFAULT_TIME_GRAN);
 	if (c->mount_opts.override_compr)
 		sup->default_compr = cpu_to_le16(c->mount_opts.compr_type);

fs/ubifs/super.c

@@ -26,6 +26,24 @@
 #include <linux/writeback.h>
 #include "ubifs.h"
 
+static int ubifs_default_version_set(const char *val, const struct kernel_param *kp)
+{
+	int n = 0, ret;
+
+	ret = kstrtoint(val, 10, &n);
+	if (ret != 0 || n < 4 || n > UBIFS_FORMAT_VERSION)
+		return -EINVAL;
+	return param_set_int(val, kp);
+}
+
+static const struct kernel_param_ops ubifs_default_version_ops = {
+	.set = ubifs_default_version_set,
+	.get = param_get_int,
+};
+
+int ubifs_default_version = UBIFS_FORMAT_VERSION;
+module_param_cb(default_version, &ubifs_default_version_ops, &ubifs_default_version, 0600);
+
 /*
  * Maximum amount of memory we may 'kmalloc()' without worrying that we are
  * allocating too much.

fs/ubifs/ubifs.h

@@ -1504,6 +1504,7 @@ extern const struct file_operations ubifs_dir_operations;
 extern const struct inode_operations ubifs_dir_inode_operations;
 extern const struct inode_operations ubifs_symlink_inode_operations;
 extern struct ubifs_compressor *ubifs_compressors[UBIFS_COMPR_TYPES_CNT];
+extern int ubifs_default_version;
 
 /* auth.c */
 static inline int ubifs_authenticated(const struct ubifs_info *c)