btrfs: fix compat_ro checks against remount

[BUG]
Even with commit 81d5d614 ("btrfs: enhance unsupported compat RO
flags handling"), btrfs can still mount a fs with unsupported compat_ro
flags read-only, then remount it RW:

  # btrfs ins dump-super /dev/loop0 | grep compat_ro_flags -A 3
  compat_ro_flags		0x403
			( FREE_SPACE_TREE |
			  FREE_SPACE_TREE_VALID |
			  unknown flag: 0x400 )

  # mount /dev/loop0 /mnt/btrfs
  mount: /mnt/btrfs: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error.
         dmesg(1) may have more information after failed mount system call.
  ^^^ RW mount failed as expected ^^^

  # dmesg -t | tail -n5
  loop0: detected capacity change from 0 to 1048576
  BTRFS: device fsid cb5b82f5-0fdd-4d81-9b4b-78533c324afa devid 1 transid 7 /dev/loop0 scanned by mount (1146)
  BTRFS info (device loop0): using crc32c (crc32c-intel) checksum algorithm
  BTRFS info (device loop0): using free space tree
  BTRFS error (device loop0): cannot mount read-write because of unknown compat_ro features (0x403)
  BTRFS error (device loop0): open_ctree failed

  # mount /dev/loop0 -o ro /mnt/btrfs
  # mount -o remount,rw /mnt/btrfs
  ^^^ RW remount succeeded unexpectedly ^^^

[CAUSE]
Currently we use btrfs_check_features() to check compat_ro flags against
our current mount flags.

That function get reused between open_ctree() and btrfs_remount().

But for btrfs_remount(), the super block we passed in still has the old
mount flags, thus btrfs_check_features() still believes we're mounting
read-only.

[FIX]
Replace the existing @sb argument with @is_rw_mount.

As originally we only use @sb to determine if the mount is RW.

Now it's callers' responsibility to determine if the mount is RW, and
since there are only two callers, the check is pretty simple:

- caller in open_ctree()
  Just pass !sb_rdonly().

- caller in btrfs_remount()
  Pass !(*flags & SB_RDONLY), as our check should be against the new
  flags.

Now we can correctly reject the RW remount:

  # mount /dev/loop0 -o ro /mnt/btrfs
  # mount -o remount,rw /mnt/btrfs
  mount: /mnt/btrfs: mount point not mounted or bad option.
         dmesg(1) may have more information after failed mount system call.
  # dmesg -t | tail -n 1
  BTRFS error (device loop0: state M): cannot mount read-write because of unknown compat_ro features (0x403)
Reported-by: Chung-Chiang Cheng <shepjeng@gmail.com>
Fixes: 81d5d614 ("btrfs: enhance unsupported compat RO flags handling")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3384,6 +3384,8 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
 /*
  * Do various sanity and dependency checks of different features.
  *
+ * @is_rw_mount:	If the mount is read-write.
+ *
  * This is the place for less strict checks (like for subpage or artificial
  * feature dependencies).
  *
@@ -3394,7 +3396,7 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
  * (space cache related) can modify on-disk format like free space tree and
  * screw up certain feature dependencies.
  */
-int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb)
+int btrfs_check_features(struct btrfs_fs_info *fs_info, bool is_rw_mount)
 {
 	struct btrfs_super_block *disk_super = fs_info->super_copy;
 	u64 incompat = btrfs_super_incompat_flags(disk_super);
@@ -3433,7 +3435,7 @@ int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb)
 	if (btrfs_super_nodesize(disk_super) > PAGE_SIZE)
 		incompat |= BTRFS_FEATURE_INCOMPAT_BIG_METADATA;
 
-	if (compat_ro_unsupp && !sb_rdonly(sb)) {
+	if (compat_ro_unsupp && is_rw_mount) {
 		btrfs_err(fs_info,
 	"cannot mount read-write because of unknown compat_ro features (0x%llx)",
 		       compat_ro);
@@ -3636,7 +3638,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		goto fail_alloc;
 	}
 
-	ret = btrfs_check_features(fs_info, sb);
+	ret = btrfs_check_features(fs_info, !sb_rdonly(sb));
 	if (ret < 0) {
 		err = ret;
 		goto fail_alloc;

fs/btrfs/disk-io.h

@@ -50,7 +50,7 @@ int __cold open_ctree(struct super_block *sb,
 void __cold close_ctree(struct btrfs_fs_info *fs_info);
 int btrfs_validate_super(struct btrfs_fs_info *fs_info,
 			 struct btrfs_super_block *sb, int mirror_num);
-int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb);
+int btrfs_check_features(struct btrfs_fs_info *fs_info, bool is_rw_mount);
 int write_all_supers(struct btrfs_fs_info *fs_info, int max_mirrors);
 struct btrfs_super_block *btrfs_read_dev_super(struct block_device *bdev);
 struct btrfs_super_block *btrfs_read_dev_one_super(struct block_device *bdev,

fs/btrfs/super.c

@@ -1705,7 +1705,7 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto restore;
 
-	ret = btrfs_check_features(fs_info, sb);
+	ret = btrfs_check_features(fs_info, !(*flags & SB_RDONLY));
 	if (ret < 0)
 		goto restore;