bcachefs: make offline fsck set read_only fs flag

A subsequent change will remove "read_only" as a mount option in favor
of the standard option "ro", meaning the userspace fsck command cannot
pass it to the fsck ioctl. Instead, in offline fsck, set "read_only"
kernel-side without trying to parse it as a mount option.

For compatibility with versions of the "bcachefs fsck" command that try
to pass the "read_only" mount opt, remove it from the mount options
string prior to parsing when it is present.
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/chardev.c

@@ -213,6 +213,18 @@ static long bch2_ioctl_fsck_offline(struct bch_ioctl_fsck_offline __user *user_a
 
 	if (arg.opts) {
 		char *optstr = strndup_user((char __user *)(unsigned long) arg.opts, 1 << 16);
+		char *ro, *rest;
+
+		/*
+		 * If passed a "read_only" mount option, remove it because it is
+		 * no longer a valid mount option, and the filesystem will be
+		 * set "read_only" regardless.
+		 */
+		ro = strstr(optstr, "read_only");
+		if (ro) {
+			rest = ro + strlen("read_only");
+			memmove(ro, rest, strlen(rest) + 1);
+		}
 
 		ret =   PTR_ERR_OR_ZERO(optstr) ?:
 			bch2_parse_mount_opts(NULL, &thr->opts, optstr);
@@ -224,6 +236,7 @@ static long bch2_ioctl_fsck_offline(struct bch_ioctl_fsck_offline __user *user_a
 	}
 
 	opt_set(thr->opts, stdio, (u64)(unsigned long)&thr->thr.stdio);
+	opt_set(thr->opts, read_only, 1);
 
 	/* We need request_key() to be called before we punt to kthread: */
 	opt_set(thr->opts, nostart, true);