ext4 block group descriptor implementation in disk layout package.

PiperOrigin-RevId: 254482180

1 files changed, 135 insertions, 0 deletions

diff --git a/pkg/sentry/fs/ext4/disklayout/block_group.go b/pkg/sentry/fs/ext4/disklayout/block_group.go
new file mode 100644
index 000000000..7359df5b9
--- /dev/null
+++ b/pkg/sentry/fs/ext4/disklayout/block_group.go
@@ -0,0 +1,135 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package disklayout provides ext4 disk level structures which can be directly
+// filled with bytes from the underlying device. All structures on disk are in
+// little-endian order. Only jbd2 (journal) structures are in big-endian order.
+// Structs aim to emulate structures `exactly` how they are layed out on disk.
+//
+// Note: All fields in these structs are exported because binary.Read would
+// panic otherwise.
+package disklayout
+
+// BlockGroup represents Linux struct ext4_group_desc which is internally
+// called a block group descriptor. An ext4 file system is split into a series
+// of block groups. This provides an access layer to information needed to
+// access and use a block group.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#block-group-descriptors.
+type BlockGroup interface {
+	// InodeTable returns the absolute block number of the block containing the
+	// inode table. This points to an array of Inode structs. Inode tables are
+	// statically allocated at mkfs time. The superblock records the number of
+	// inodes per group (length of this table).
+	InodeTable() uint64
+
+	// BlockBitmap returns the absolute block number of the block containing the
+	// block bitmap. This bitmap tracks the usage of data blocks within this block
+	// group and has its own checksum.
+	BlockBitmap() uint64
+
+	// InodeBitmap returns the absolute block number of the block containing the
+	// inode bitmap. This bitmap tracks the usage of this group's inode table
+	// entries and has its own checksum.
+	InodeBitmap() uint64
+
+	// ExclusionBitmap returns the absolute block number of the snapshot exclusion
+	// bitmap.
+	ExclusionBitmap() uint64
+
+	// FreeBlocksCount returns the number of free blocks in the group.
+	FreeBlocksCount() uint32
+
+	// FreeInodesCount returns the number of free inodes in the group.
+	FreeInodesCount() uint32
+
+	// DirectoryCount returns the number of inodes that represent directories
+	// under this block group.
+	DirectoryCount() uint32
+
+	// UnusedInodeCount returns the number of unused inodes beyond the last used
+	// inode in this group's inode table. As a result, we needn’t scan past the
+	// (InodesPerGroup - UnusedInodeCount())th entry in the inode table.
+	UnusedInodeCount() uint32
+
+	// BlockBitmapChecksum returns the block bitmap checksum. This is calculated
+	// using crc32c(FS UUID + group number + entire bitmap).
+	BlockBitmapChecksum() uint32
+
+	// InodeBitmapChecksum returns the inode bitmap checksum. This is calculated
+	// using crc32c(FS UUID + group number + entire bitmap).
+	InodeBitmapChecksum() uint32
+
+	// Checksum returns this block group's checksum.
+	//
+	// If RO_COMPAT_METADATA_CSUM feature is set:
+	//     - checksum is crc32c(FS UUID + group number + group descriptor
+	//       structure) & 0xFFFF.
+	//
+	// If RO_COMPAT_GDT_CSUM feature is set:
+	//     - checksum is crc16(FS UUID + group number + group descriptor
+	//       structure).
+	//
+	// RO_COMPAT_METADATA_CSUM and RO_COMPAT_GDT_CSUM should not be both set.
+	// If they are, Linux warns and asks to run fsck.
+	Checksum() uint16
+
+	// Flags returns BGFlags which represents the block group flags.
+	Flags() BGFlags
+}
+
+// These are the different block group flags.
+const (
+	// BgInodeUninit indicates that inode table and bitmap are not initialized.
+	BgInodeUninit uint16 = 0x1
+
+	// BgBlockUninit indicates that block bitmap is not initialized.
+	BgBlockUninit uint16 = 0x2
+
+	// BgInodeZeroed indicates that inode table is zeroed.
+	BgInodeZeroed uint16 = 0x4
+)
+
+// BGFlags represents all the different combinations of block group flags.
+type BGFlags struct {
+	InodeUninit bool
+	BlockUninit bool
+	InodeZeroed bool
+}
+
+// ToInt converts a BGFlags struct back to its 16-bit representation.
+func (f BGFlags) ToInt() uint16 {
+	var res uint16
+
+	if f.InodeUninit {
+		res |= BgInodeUninit
+	}
+	if f.BlockUninit {
+		res |= BgBlockUninit
+	}
+	if f.InodeZeroed {
+		res |= BgInodeZeroed
+	}
+
+	return res
+}
+
+// BGFlagsFromInt converts the 16-bit flag representation to a BGFlags struct.
+func BGFlagsFromInt(flags uint16) BGFlags {
+	return BGFlags{
+		InodeUninit: (flags & BgInodeUninit) > 0,
+		BlockUninit: (flags & BgBlockUninit) > 0,
+		InodeZeroed: (flags & BgInodeZeroed) > 0,
+	}
+}