better error handling

1 files changed, 20 insertions, 14 deletions

diff --git a/pkg/btrfsmisc/fsck.go b/pkg/btrfsmisc/fsck.go
index 8d20e6e..a771cf4 100644
--- a/pkg/btrfsmisc/fsck.go
+++ b/pkg/btrfsmisc/fsck.go
@@ -36,26 +36,37 @@ func ScanForNodes(dev *btrfs.Device, sb btrfs.Superblock, fn func(*util.Ref[btrf
 			continue
 		}
 
-		// does it look like a node?
+		// parse (early)
 
+		nodeRef := &util.Ref[btrfs.PhysicalAddr, btrfs.Node]{
+			File: dev,
+			Addr: pos,
+			Data: btrfs.Node{
+				Size: sb.NodeSize,
+			},
+		}
 		var nodeHeader btrfs.NodeHeader
 		if _, err := binstruct.Unmarshal(nodeBuf, &nodeHeader); err != nil {
 			fn(nil, fmt.Errorf("sector@%d: %w", pos, err))
 		}
+
+		// sanity checking
+
 		if nodeHeader.MetadataUUID != sb.EffectiveMetadataUUID() {
 			//fmt.Printf("sector@%d does not look like a node\n", pos)
 			continue
 		}
 
-		// ok, it looks like a node; go ahead and read it as a node
-
-		nodeRef := &util.Ref[btrfs.PhysicalAddr, btrfs.Node]{
-			File: dev,
-			Addr: pos,
-			Data: btrfs.Node{
-				Size: sb.NodeSize,
-			},
+		stored := nodeRef.Data.Head.Checksum
+		calced := btrfs.CRC32c(nodeBuf[binstruct.StaticSize(btrfs.CSum{}):])
+		if stored != calced {
+			fn(nodeRef, fmt.Errorf("sector@%d: looks like a node but is corrupt: checksum doesn't match: stored=%s calculated=%s",
+				pos, stored, calced))
+			continue
 		}
+
+		// parse (main)
+
 		if _, err := nodeRef.Data.UnmarshalBinary(nodeBuf); err != nil {
 			fn(nil, fmt.Errorf("sector@%d: %w", pos, err))
 			continue
@@ -63,11 +74,6 @@ func ScanForNodes(dev *btrfs.Device, sb btrfs.Superblock, fn func(*util.Ref[btrf
 
 		// finally, process the node
 
-		if nodeRef.Data.Head.Checksum != btrfs.CRC32c(nodeBuf[0x20:]) {
-			fn(nodeRef, fmt.Errorf("sector@%d looks like a node but is corrupt (checksum doesn't match)", pos))
-			continue
-		}
-
 		fn(nodeRef, nil)
 
 		pos += btrfs.PhysicalAddr(sb.NodeSize) - btrfs.PhysicalAddr(sb.SectorSize)