138 lines
4.7 KiB
Go
138 lines
4.7 KiB
Go
// Copyright 2019 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package disklayout
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// BlockGroup represents a Linux ext block group descriptor. An ext file system
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// is split into a series of block groups. This provides an access layer to
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// information needed to access and use a block group.
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//
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// Location:
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// - The block group descriptor table is always placed in the blocks
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// immediately after the block containing the superblock.
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// - The 1st block group descriptor in the original table is in the
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// (sb.FirstDataBlock() + 1)th block.
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// - See SuperBlock docs to see where the block group descriptor table is
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// replicated.
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// - sb.BgDescSize() must be used as the block group descriptor entry size
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// while reading the table from disk.
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//
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// See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#block-group-descriptors.
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type BlockGroup interface {
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// InodeTable returns the absolute block number of the block containing the
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// inode table. This points to an array of Inode structs. Inode tables are
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// statically allocated at mkfs time. The superblock records the number of
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// inodes per group (length of this table) and the size of each inode struct.
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InodeTable() uint64
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// BlockBitmap returns the absolute block number of the block containing the
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// block bitmap. This bitmap tracks the usage of data blocks within this block
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// group and has its own checksum.
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BlockBitmap() uint64
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// InodeBitmap returns the absolute block number of the block containing the
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// inode bitmap. This bitmap tracks the usage of this group's inode table
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// entries and has its own checksum.
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InodeBitmap() uint64
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// ExclusionBitmap returns the absolute block number of the snapshot exclusion
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// bitmap.
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ExclusionBitmap() uint64
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// FreeBlocksCount returns the number of free blocks in the group.
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FreeBlocksCount() uint32
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// FreeInodesCount returns the number of free inodes in the group.
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FreeInodesCount() uint32
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// DirectoryCount returns the number of inodes that represent directories
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// under this block group.
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DirectoryCount() uint32
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// UnusedInodeCount returns the number of unused inodes beyond the last used
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// inode in this group's inode table. As a result, we needn’t scan past the
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// (InodesPerGroup - UnusedInodeCount())th entry in the inode table.
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UnusedInodeCount() uint32
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// BlockBitmapChecksum returns the block bitmap checksum. This is calculated
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// using crc32c(FS UUID + group number + entire bitmap).
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BlockBitmapChecksum() uint32
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// InodeBitmapChecksum returns the inode bitmap checksum. This is calculated
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// using crc32c(FS UUID + group number + entire bitmap).
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InodeBitmapChecksum() uint32
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// Checksum returns this block group's checksum.
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//
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// If SbMetadataCsum feature is set:
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// - checksum is crc32c(FS UUID + group number + group descriptor
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// structure) & 0xFFFF.
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//
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// If SbGdtCsum feature is set:
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// - checksum is crc16(FS UUID + group number + group descriptor
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// structure).
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//
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// SbMetadataCsum and SbGdtCsum should not be both set.
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// If they are, Linux warns and asks to run fsck.
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Checksum() uint16
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// Flags returns BGFlags which represents the block group flags.
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Flags() BGFlags
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}
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// These are the different block group flags.
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const (
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// BgInodeUninit indicates that inode table and bitmap are not initialized.
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BgInodeUninit uint16 = 0x1
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// BgBlockUninit indicates that block bitmap is not initialized.
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BgBlockUninit uint16 = 0x2
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// BgInodeZeroed indicates that inode table is zeroed.
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BgInodeZeroed uint16 = 0x4
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)
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// BGFlags represents all the different combinations of block group flags.
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type BGFlags struct {
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InodeUninit bool
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BlockUninit bool
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InodeZeroed bool
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}
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// ToInt converts a BGFlags struct back to its 16-bit representation.
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func (f BGFlags) ToInt() uint16 {
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var res uint16
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if f.InodeUninit {
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res |= BgInodeUninit
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}
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if f.BlockUninit {
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res |= BgBlockUninit
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}
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if f.InodeZeroed {
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res |= BgInodeZeroed
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}
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return res
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}
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// BGFlagsFromInt converts the 16-bit flag representation to a BGFlags struct.
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func BGFlagsFromInt(flags uint16) BGFlags {
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return BGFlags{
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InodeUninit: flags&BgInodeUninit > 0,
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BlockUninit: flags&BgBlockUninit > 0,
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InodeZeroed: flags&BgInodeZeroed > 0,
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}
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}
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