btrfs: tree-checker: reject inline extent items with 0 ref count

[BUG]
There is a bug report in the mailing list where btrfs_run_delayed_refs()
failed to drop the ref count for logical 25870311358464 num_bytes
2113536.

The involved leaf dump looks like this:

  item 166 key (25870311358464 168 2113536) itemoff 10091 itemsize 50
    extent refs 1 gen 84178 flags 1
    ref#0: shared data backref parent 32399126528000 count 0 <<<
    ref#1: shared data backref parent 31808973717504 count 1

Notice the count number is 0.

[CAUSE]
There is no concrete evidence yet, but considering 0 -> 1 is also a
single bit flipped, it's possible that hardware memory bitflip is
involved, causing the on-disk extent tree to be corrupted.

[FIX]
To prevent us reading such corrupted extent item, or writing such
damaged extent item back to disk, enhance the handling of
BTRFS_EXTENT_DATA_REF_KEY and BTRFS_SHARED_DATA_REF_KEY keys for both
inlined and key items, to detect such 0 ref count and reject them.

CC: stable@vger.kernel.org # 5.4+
Link: https://lore.kernel.org/linux-btrfs/7c69dd49-c346-4806-86e7-e6f863a66f48@app.fastmail.com/
Reported-by: Frankie Fisher <frankie@terrorise.me.uk>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

fs/btrfs/tree-checker.c

@@ -1527,6 +1527,11 @@ static int check_extent_item(struct extent_buffer *leaf,
 					   dref_offset, fs_info->sectorsize);
 				return -EUCLEAN;
 			}
+			if (unlikely(btrfs_extent_data_ref_count(leaf, dref) == 0)) {
+				extent_err(leaf, slot,
+			"invalid data ref count, should have non-zero value");
+				return -EUCLEAN;
+			}
 			inline_refs += btrfs_extent_data_ref_count(leaf, dref);
 			break;
 		/* Contains parent bytenr and ref count */
@@ -1539,6 +1544,11 @@ static int check_extent_item(struct extent_buffer *leaf,
 					   inline_offset, fs_info->sectorsize);
 				return -EUCLEAN;
 			}
+			if (unlikely(btrfs_shared_data_ref_count(leaf, sref) == 0)) {
+				extent_err(leaf, slot,
+			"invalid shared data ref count, should have non-zero value");
+				return -EUCLEAN;
+			}
 			inline_refs += btrfs_shared_data_ref_count(leaf, sref);
 			break;
 		case BTRFS_EXTENT_OWNER_REF_KEY:
@@ -1611,8 +1621,18 @@ static int check_simple_keyed_refs(struct extent_buffer *leaf,
 {
 	u32 expect_item_size = 0;
 
-	if (key->type == BTRFS_SHARED_DATA_REF_KEY)
+	if (key->type == BTRFS_SHARED_DATA_REF_KEY) {
+		struct btrfs_shared_data_ref *sref;
+
+		sref = btrfs_item_ptr(leaf, slot, struct btrfs_shared_data_ref);
+		if (unlikely(btrfs_shared_data_ref_count(leaf, sref) == 0)) {
+			extent_err(leaf, slot,
+		"invalid shared data backref count, should have non-zero value");
+			return -EUCLEAN;
+		}
+
 		expect_item_size = sizeof(struct btrfs_shared_data_ref);
+	}
 
 	if (unlikely(btrfs_item_size(leaf, slot) != expect_item_size)) {
 		generic_err(leaf, slot,
@@ -1689,6 +1709,11 @@ static int check_extent_data_ref(struct extent_buffer *leaf,
 				   offset, leaf->fs_info->sectorsize);
 			return -EUCLEAN;
 		}
+		if (unlikely(btrfs_extent_data_ref_count(leaf, dref) == 0)) {
+			extent_err(leaf, slot,
+	"invalid extent data backref count, should have non-zero value");
+			return -EUCLEAN;
+		}
 	}
 	return 0;
 }