[PATCH] btrfs: refuse to remount read-write with unsupported compat-ro features

[Chung-Chiang Cheng <cccheng@synology.com>]

btrfs with unsupported compat-ro features can only be mounted as
read-only, but we can make it read-write indirectly through remount.
Just add the missing check to refuse it.

  mount -o ro /dev/vdb /mnt
  mount -o remount,rw /mnt

Reported-by: Johnny Chang <johnnyc@synology.com>
Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
---
 fs/btrfs/super.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 5942b9384088..45836a426499 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1991,6 +1991,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	unsigned old_flags = sb->s_flags;
 	unsigned long old_opts = fs_info->mount_opt;
 	unsigned long old_compress_type = fs_info->compress_type;
+	u64 compat_ro = btrfs_super_compat_ro_flags(fs_info->super_copy);
+	u64 compat_ro_unsupp = compat_ro & ~BTRFS_FEATURE_COMPAT_RO_SUPP;
 	u64 old_max_inline = fs_info->max_inline;
 	u32 old_thread_pool_size = fs_info->thread_pool_size;
 	u32 old_metadata_ratio = fs_info->metadata_ratio;
@@ -2107,6 +2109,13 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		if (ret)
 			goto restore;
 	} else {
+		if (compat_ro_unsupp) {
+			btrfs_err(fs_info,
+				"cannot remount read-write because of unknown compat_ro features (0x%llx)",
+				compat_ro);
+			ret = -EINVAL;
+			goto restore;
+		}
 		if (BTRFS_FS_ERROR(fs_info)) {
 			btrfs_err(fs_info,
 				"Remounting read-write after error is not allowed");
-- 
2.34.1