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>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 7fc2f3f07c16..a2420c900275 100644
--- a/fs/ubifs/super.c
+++ b/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.