Non 64-bit ext4 gdt (#311)

* ext4: GDT size set to 32 unless 64bit flag set

Signed-off-by: Avi Deitcher <[email protected]>

* handle GDT size 0 error

Signed-off-by: Avi Deitcher <[email protected]>

---------

Signed-off-by: Avi Deitcher <[email protected]>

Author: deitch

filesystem/ext4/ext4.go

@@ -663,6 +663,10 @@ func Read(b backend.Storage, size, start, sectorsize int64) (*FileSystem, error)
 	// how big should the GDT be?
 	gdtSize := uint64(sb.groupDescriptorSize) * sb.blockGroupCount()
 
+	if gdtSize == 0 {
+		return nil, errors.New("calculated Group Descriptor Table size is zero")
+	}
+
 	gdtBytes := make([]byte, gdtSize)
 	// where do we find the GDT?
 	// - if blocksize is 1024, then 1024 padding for BootSector is block 0, 1024 for superblock is block 1

filesystem/ext4/groupdescriptors.go

@@ -87,6 +87,9 @@ func (gds *groupDescriptors) equal(a *groupDescriptors) bool {
 
 // groupDescriptorsFromBytes create a groupDescriptors struct from bytes
 func groupDescriptorsFromBytes(b []byte, gdSize uint16, hashSeed uint32, checksumType gdtChecksumType) (*groupDescriptors, error) {
+	if gdSize == 0 {
+		return nil, fmt.Errorf("group descriptor size cannot be zero")
+	}
 	gds := groupDescriptors{}
 	gdSlice := make([]groupDescriptor, 0, 10)
 

filesystem/ext4/superblock.go

@@ -347,7 +347,10 @@ func superblockFromBytes(b []byte) (*superblock, error) {
 
 	sb.hashVersion = hashAlgorithm(b[0xfc])
 
-	sb.groupDescriptorSize = binary.LittleEndian.Uint16(b[0xfe:0x100])
+	sb.groupDescriptorSize = 32
+	if sb.features.fs64Bit {
+		sb.groupDescriptorSize = binary.LittleEndian.Uint16(b[0xfe:0x100])
+	}
 
 	sb.defaultMountOptions = parseMountOptions(binary.LittleEndian.Uint32(b[0x100:0x104]))
 	sb.firstMetablockGroup = binary.LittleEndian.Uint32(b[0x104:0x108])