[OS] e2fsprogs 코드 분석 : ext2fs_open
e2fsprogs의 깃허브 코드를 참조해 분석한 글입니다.
📌 e2fsprogs 코드 분석 : ext2fs_open
🫧 ext2fs_open()
- 슈퍼블록을 읽어올 때 사용
🫧 과정
- 매직 넘버 체크
- 구조체 초기화 및 io 장치 오픈
- 슈퍼블록 읽기
- 블록 사이즈 검사
- inode table이 차지하는 블록 수 계산
- 그룹 디스크립터 읽기
🫧 특징
inode 테이블이 차지하는 블록 수계산 (각 그룹 당 inode 수 * inode 사이즈 (inode table) + (블록크기-1)/블록크기),각 블록 그룹의 블록 수 계산등의 작업을 진행함
🫧 사용
ext4magic에서는 다음과 같이 사용한다.
// 호출
open_filesystem(argv[optind], open_flags,superblock, blocksize, magicscan, data_filename);
// open_filesystem 정의
static void open_filesystem(char *device, int open_flags, blk_t superblock,
blk_t blocksize, int magicscan,
char *data_filename)
// 내부적으로 e2fsprogs 호출
// !! 핵심 함수 !! (분석하고자 하는 함수)
retval = ext2fs_open(device, open_flags, superblock, blocksize, unix_io_manager, ¤t_fs);
device: 파일시스템이 존재하는 경로 (ex, /dev/loop13 또는 디스크 덤프 이미지)open_flags: -r과 같은 인자에 따라 case를 나누면서 플래그를 설정해줌.
case 'z': //experimental not activ at default
data_filename = optarg;
break;
case 'y': //experimental not activ at default
open_flags |= EXT2_FLAG_IMAGE_FILE;
break;
blk_t superblock: 0으로 초기화blk_t blocksize: 0으로 초기화unix_io_manager: 뭔지 모르겠음. 안 보여요..ext2_filsys current_fs: 초기화된 (NULL) 포인터
🫧 코드
✨ ext2fs_open()
- libs/ext2fs/openfs.c, $88
errcode_t ext2fs_open(const char *name, int flags, int superblock,
unsigned int block_size, io_manager manager,
ext2_filsys *ret_fs)
{
return ext2fs_open2(name, 0, flags, superblock, block_size,
manager, ret_fs);
}
✨ ext2fs_open()
- libs/ext2fs/openfs.c, $88
blk64_t ext2fs_descriptor_block_loc2(ext2_filsys fs, blk64_t group_block,
dgrp_t i)
{
int bg;
int has_super = 0, group_zero_adjust = 0;
blk64_t ret_blk;
/*
* On a bigalloc FS with 1K blocks, block 0 is reserved for non-ext4
* stuff, so adjust for that if we're being asked for group 0.
*/
if (i == 0 && fs->blocksize == 1024 && EXT2FS_CLUSTER_RATIO(fs) > 1)
group_zero_adjust = 1;
if (!ext2fs_has_feature_meta_bg(fs->super) ||
(i < fs->super->s_first_meta_bg))
return group_block + i + 1 + group_zero_adjust;
bg = EXT2_DESC_PER_BLOCK(fs->super) * i;
if (ext2fs_bg_has_super(fs, bg))
has_super = 1;
ret_blk = ext2fs_group_first_block2(fs, bg);
/*
* If group_block is not the normal value, we're trying to use
* the backup group descriptors and superblock --- so use the
* alternate location of the second block group in the
* metablock group. Ideally we should be testing each bg
* descriptor block individually for correctness, but we don't
* have the infrastructure in place to do that.
*/
if (group_block != fs->super->s_first_data_block &&
((ret_blk + has_super + fs->super->s_blocks_per_group) <
ext2fs_blocks_count(fs->super))) {
ret_blk += fs->super->s_blocks_per_group;
/*
* If we're going to jump forward a block group, make sure
* that we adjust has_super to account for the next group's
* backup superblock (or lack thereof).
*/
if (ext2fs_bg_has_super(fs, bg + 1))
has_super = 1;
else
has_super = 0;
}
return ret_blk + has_super + group_zero_adjust;
}
blk_t ext2fs_descriptor_block_loc(ext2_filsys fs, blk_t group_block, dgrp_t i)
{
return ext2fs_descriptor_block_loc2(fs, group_block, i);
}
errcode_t ext2fs_open(const char *name, int flags, int superblock,
unsigned int block_size, io_manager manager,
ext2_filsys *ret_fs)
{
return ext2fs_open2(name, 0, flags, superblock, block_size,
manager, ret_fs);
}
static void block_sha_map_free_entry(void *data)
{
free(data);
return;
}
/*
* Note: if superblock is non-zero, block-size must also be non-zero.
* Superblock and block_size can be zero to use the default size.
*
* Valid flags for ext2fs_open()
*
* EXT2_FLAG_RW - Open the filesystem for read/write.
* EXT2_FLAG_FORCE - Open the filesystem even if some of the
* features aren't supported.
* EXT2_FLAG_JOURNAL_DEV_OK - Open an ext3 journal device
* EXT2_FLAG_SKIP_MMP - Open without multi-mount protection check.
* EXT2_FLAG_64BITS - Allow 64-bit bitfields (needed for large
* filesystems)
*/
/*
* 참고: 슈퍼블록이 0이 아닌 경우 블록 크기도 0이 아닌 값이어야 합니다.
* 기본 크기를 사용하려면 Superblock과 block_size를 0으로 설정할 수 있습니다.
*
* ext2fs_open()에 유효한 플래그
*
* EXT2_FLAG_RW - 읽기/쓰기를 위해 파일 시스템을 엽니다.
* EXT2_FLAG_FORCE - 파일 시스템 중 일부라도 열기 (기능이 지원되지 않습니다)
* EXT2_FLAG_Journal_DEV_OK - ext3 저널 장치 열기
* EXT2_FLAG_SKIP_MMP - 멀티 마운트 보호 검사 없이 열립니다.
* EXT2_FLAG_64BITS - 64비트 비트 필드 허용(대형에 필요)
* 파일 시스템)
*/
// io_options는 0으로 들어옴
errcode_t ext2fs_open2(const char *name, const char *io_options,
int flags, int superblock,
unsigned int block_size, io_manager manager,
ext2_filsys *ret_fs)
{
ext2_filsys fs;
errcode_t retval;
unsigned long i, first_meta_bg;
__u32 features;
unsigned int blocks_per_group, io_flags;
blk64_t group_block, blk;
char *dest, *cp;
int group_zero_adjust = 0;
unsigned int inode_size;
__u64 groups_cnt;
#ifdef WORDS_BIGENDIAN
unsigned int groups_per_block;
struct ext2_group_desc *gdp;
int j;
#endif
char *time_env;
int csum_retries = 0;
// 1. 매직 넘버 체크
EXT2_CHECK_MAGIC(manager, EXT2_ET_MAGIC_IO_MANAGER);
// 2. 구조체 초기화? 정확히 뭘 하는 건지 모르겠음 (오픈 불가)
retval = ext2fs_get_mem(sizeof(struct struct_ext2_filsys), &fs);
if (retval)
return retval;
memset(fs, 0, sizeof(struct struct_ext2_filsys));
fs->magic = EXT2_ET_MAGIC_EXT2FS_FILSYS;
fs->flags = flags;
/* don't overwrite sb backups unless flag is explicitly cleared */
fs->flags |= EXT2_FLAG_MASTER_SB_ONLY;
fs->umask = 022;
time_env = ext2fs_safe_getenv("SOURCE_DATE_EPOCH");
if (time_env) {
fs->now = strtoul(time_env, NULL, 0);
fs->flags2 |= EXT2_FLAG2_USE_FAKE_TIME;
} else {
time_env = ext2fs_safe_getenv("E2FSPROGS_FAKE_TIME");
if (time_env)
fs->now = strtoul(time_env, NULL, 0);
}
retval = ext2fs_get_mem(strlen(name)+1, &fs->device_name);
if (retval)
goto cleanup;
strcpy(fs->device_name, name);
cp = strchr(fs->device_name, '?');
if (!io_options && cp) {
*cp++ = 0;
io_options = cp;
}
io_flags = 0;
if (flags & EXT2_FLAG_RW)
io_flags |= IO_FLAG_RW;
if (flags & EXT2_FLAG_EXCLUSIVE)
io_flags |= IO_FLAG_EXCLUSIVE;
if (flags & EXT2_FLAG_DIRECT_IO)
io_flags |= IO_FLAG_DIRECT_IO;
if (flags & EXT2_FLAG_THREADS)
io_flags |= IO_FLAG_THREADS;
// 2. i/o 장치 오픈
retval = manager->open(fs->device_name, io_flags, &fs->io);
if (retval)
goto cleanup;
if (io_options &&
(retval = io_channel_set_options(fs->io, io_options)))
goto cleanup;
fs->image_io = fs->io;
fs->io->app_data = fs;
retval = io_channel_alloc_buf(fs->io, -SUPERBLOCK_SIZE, &fs->super);
if (retval)
goto cleanup;
if (flags & EXT2_FLAG_IMAGE_FILE) {
retval = ext2fs_get_mem(sizeof(struct ext2_image_hdr),
&fs->image_header);
if (retval)
goto cleanup;
retval = io_channel_read_blk(fs->io, 0,
-(int)sizeof(struct ext2_image_hdr),
fs->image_header);
if (retval)
goto cleanup;
if (ext2fs_le32_to_cpu(fs->image_header->magic_number) != EXT2_ET_MAGIC_E2IMAGE)
return EXT2_ET_MAGIC_E2IMAGE;
superblock = 1;
block_size = ext2fs_le32_to_cpu(fs->image_header->fs_blocksize);
}
/*
* If the user specifies a specific block # for the
* superblock, then he/she must also specify the block size!
* Otherwise, read the master superblock located at offset
* SUPERBLOCK_OFFSET from the start of the partition.
*
* Note: we only save a backup copy of the superblock if we
* are reading the superblock from the primary superblock location.
*/
// 3. 슈퍼블록 읽기
// 장치가 오픈된 경우
if (superblock) {
if (!block_size) {
retval = EXT2_ET_INVALID_ARGUMENT;
goto cleanup;
}
io_channel_set_blksize(fs->io, block_size);
group_block = superblock;
fs->orig_super = 0;
} else {
io_channel_set_blksize(fs->io, SUPERBLOCK_OFFSET);
// 없는 경우 슈퍼블록을 1로 만들어 다음 실행
superblock = 1;
group_block = 0;
retval = ext2fs_get_mem(SUPERBLOCK_SIZE, &fs->orig_super);
if (retval)
goto cleanup;
}
retry:
retval = io_channel_read_blk(fs->io, superblock, -SUPERBLOCK_SIZE,
fs->super);
if (retval)
goto cleanup;
if (fs->orig_super)
memcpy(fs->orig_super, fs->super, SUPERBLOCK_SIZE);
if (!(fs->flags & EXT2_FLAG_IGNORE_CSUM_ERRORS)) {
retval = 0;
if (!ext2fs_verify_csum_type(fs, fs->super))
retval = EXT2_ET_UNKNOWN_CSUM;
if (!ext2fs_superblock_csum_verify(fs, fs->super)) {
if (csum_retries++ < 3)
goto retry;
retval = EXT2_ET_SB_CSUM_INVALID;
}
}
#ifdef WORDS_BIGENDIAN
fs->flags |= EXT2_FLAG_SWAP_BYTES;
ext2fs_swap_super(fs->super);
#else
if (fs->flags & EXT2_FLAG_SWAP_BYTES) {
retval = EXT2_ET_UNIMPLEMENTED;
goto cleanup;
}
#endif
if (fs->super->s_magic != EXT2_SUPER_MAGIC)
retval = EXT2_ET_BAD_MAGIC;
if (retval)
goto cleanup;
if (fs->super->s_rev_level > EXT2_LIB_CURRENT_REV) {
retval = EXT2_ET_REV_TOO_HIGH;
goto cleanup;
}
/*
* Check for feature set incompatibility
*/
if (!(flags & EXT2_FLAG_FORCE)) {
features = fs->super->s_feature_incompat;
#ifdef EXT2_LIB_SOFTSUPP_INCOMPAT
if (flags & EXT2_FLAG_SOFTSUPP_FEATURES)
features &= ~EXT2_LIB_SOFTSUPP_INCOMPAT;
#endif
if (features & ~EXT2_LIB_FEATURE_INCOMPAT_SUPP) {
retval = EXT2_ET_UNSUPP_FEATURE;
goto cleanup;
}
features = fs->super->s_feature_ro_compat;
#ifdef EXT2_LIB_SOFTSUPP_RO_COMPAT
if (flags & EXT2_FLAG_SOFTSUPP_FEATURES)
features &= ~EXT2_LIB_SOFTSUPP_RO_COMPAT;
#endif
if ((flags & EXT2_FLAG_RW) &&
(features & ~EXT2_LIB_FEATURE_RO_COMPAT_SUPP)) {
retval = EXT2_ET_RO_UNSUPP_FEATURE;
goto cleanup;
}
if (!(flags & EXT2_FLAG_JOURNAL_DEV_OK) &&
ext2fs_has_feature_journal_dev(fs->super)) {
retval = EXT2_ET_UNSUPP_FEATURE;
goto cleanup;
}
}
// 4. 블록 사이즈 검사
if ((fs->super->s_log_block_size >
(unsigned) (EXT2_MAX_BLOCK_LOG_SIZE - EXT2_MIN_BLOCK_LOG_SIZE)) ||
(fs->super->s_log_cluster_size >
(unsigned) (EXT2_MAX_CLUSTER_LOG_SIZE - EXT2_MIN_CLUSTER_LOG_SIZE)) ||
(fs->super->s_log_block_size > fs->super->s_log_cluster_size) ||
(fs->super->s_log_groups_per_flex > 31)) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
/*
* bigalloc requires cluster-aware bitfield operations, which at the
* moment means we need EXT2_FLAG_64BITS.
*/
if (ext2fs_has_feature_bigalloc(fs->super) &&
!(flags & EXT2_FLAG_64BITS)) {
retval = EXT2_ET_CANT_USE_LEGACY_BITMAPS;
goto cleanup;
}
if (!ext2fs_has_feature_bigalloc(fs->super) &&
(fs->super->s_log_block_size != fs->super->s_log_cluster_size)) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
fs->fragsize = fs->blocksize = EXT2_BLOCK_SIZE(fs->super);
inode_size = EXT2_INODE_SIZE(fs->super);
if ((inode_size < EXT2_GOOD_OLD_INODE_SIZE) ||
(inode_size > fs->blocksize) ||
(inode_size & (inode_size - 1))) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
/* Enforce the block group descriptor size */
// 슈퍼블록에서 그룹 디스크립터 사이즈 가져오기
if (ext2fs_has_feature_64bit(fs->super)) {
unsigned desc_size = fs->super->s_desc_size;
if (desc_size == 0 ||
(!(flags & EXT2_FLAG_IGNORE_SB_ERRORS) &&
((desc_size > EXT2_MAX_DESC_SIZE) ||
(desc_size < EXT2_MIN_DESC_SIZE_64BIT) ||
(desc_size & (desc_size - 1)) != 0))) {
retval = EXT2_ET_BAD_DESC_SIZE;
goto cleanup;
}
}
fs->cluster_ratio_bits = fs->super->s_log_cluster_size -
fs->super->s_log_block_size;
if (EXT2_BLOCKS_PER_GROUP(fs->super) !=
EXT2_CLUSTERS_PER_GROUP(fs->super) << fs->cluster_ratio_bits) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
// 한 블록에 inode table이 차지하는 블록 수 계산
// 각 그룹 당 inode 수 * inode 사이즈 (inode table) + (블록크기-1)/블록크기
fs->inode_blocks_per_group = ((EXT2_INODES_PER_GROUP(fs->super) *
EXT2_INODE_SIZE(fs->super) +
EXT2_BLOCK_SIZE(fs->super) - 1) /
EXT2_BLOCK_SIZE(fs->super));
if (block_size) {
if (block_size != fs->blocksize) {
retval = EXT2_ET_UNEXPECTED_BLOCK_SIZE;
goto cleanup;
}
}
/*
* Set the blocksize to the filesystem's blocksize.
*/
io_channel_set_blksize(fs->io, fs->blocksize);
/*
* If this is an external journal device, don't try to read
* the group descriptors, because they're not there.
*/
// 외부 저널 장치인 경우
if (ext2fs_has_feature_journal_dev(fs->super)) {
fs->group_desc_count = 0;
*ret_fs = fs;
return 0;
}
if (EXT2_INODES_PER_GROUP(fs->super) == 0) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
/* Precompute the FS UUID to seed other checksums */
ext2fs_init_csum_seed(fs);
/*
* Read group descriptors
*/
// 5. 그룹 디스크립터 읽기
blocks_per_group = EXT2_BLOCKS_PER_GROUP(fs->super);
// 오류 나지 않기 위한 조건
// 블록그룹 당 최소 블록 수는 8을 넘어야 함
// 최대 블록 그룹 수보다 작아야 함
// 그룹당 최대 inode 수보다 작아야 함
// 한 블록 그룹 안 desc 개수가 0이면 안 됨
if (blocks_per_group < 8 ||
blocks_per_group > EXT2_MAX_BLOCKS_PER_GROUP(fs->super) ||
fs->inode_blocks_per_group > EXT2_MAX_INODES_PER_GROUP(fs->super) ||
EXT2_DESC_PER_BLOCK(fs->super) == 0 ||
fs->super->s_first_data_block >= ext2fs_blocks_count(fs->super)) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
groups_cnt = ext2fs_div64_ceil(ext2fs_blocks_count(fs->super) -
fs->super->s_first_data_block,
blocks_per_group);
if (groups_cnt >> 32) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
fs->group_desc_count = groups_cnt;
if (!(flags & EXT2_FLAG_IGNORE_SB_ERRORS) &&
(__u64)fs->group_desc_count * EXT2_INODES_PER_GROUP(fs->super) !=
fs->super->s_inodes_count) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
fs->desc_blocks = ext2fs_div_ceil(fs->group_desc_count,
EXT2_DESC_PER_BLOCK(fs->super));
if (ext2fs_has_feature_meta_bg(fs->super) &&
(fs->super->s_first_meta_bg > fs->desc_blocks) &&
!(flags & EXT2_FLAG_IGNORE_SB_ERRORS)) {
retval = EXT2_ET_CORRUPT_SUPERBLOCK;
goto cleanup;
}
if (flags & EXT2_FLAG_SUPER_ONLY)
goto skip_read_bg;
retval = ext2fs_get_array(fs->desc_blocks, fs->blocksize,
&fs->group_desc);
if (retval)
goto cleanup;
if (!group_block)
group_block = fs->super->s_first_data_block;
/*
* On a FS with a 1K blocksize, block 0 is reserved for bootloaders
* so we must increment block numbers to any group 0 items.
*
* However, we cannot touch group_block directly because in the meta_bg
* case, the ext2fs_descriptor_block_loc2() function will interpret
* group_block != s_first_data_block to mean that we want to access the
* backup group descriptors. This is not what we want if the caller
* set superblock == 0 (i.e. auto-detect the superblock), which is
* what's going on here.
*/
if (group_block == 0 && fs->blocksize == 1024)
group_zero_adjust = 1;
dest = (char *) fs->group_desc;
#ifdef WORDS_BIGENDIAN
groups_per_block = EXT2_DESC_PER_BLOCK(fs->super);
#endif
if (ext2fs_has_feature_meta_bg(fs->super) &&
!(flags & EXT2_FLAG_IMAGE_FILE)) {
first_meta_bg = fs->super->s_first_meta_bg;
if (first_meta_bg > fs->desc_blocks)
first_meta_bg = fs->desc_blocks;
} else
first_meta_bg = fs->desc_blocks;
if (first_meta_bg) {
retval = io_channel_read_blk(fs->io, group_block +
group_zero_adjust + 1,
first_meta_bg, dest);
if (retval)
goto cleanup;
#ifdef WORDS_BIGENDIAN
gdp = (struct ext2_group_desc *) dest;
for (j=0; j < groups_per_block*first_meta_bg; j++) {
gdp = ext2fs_group_desc(fs, fs->group_desc, j);
if (gdp)
ext2fs_swap_group_desc2(fs, gdp);
}
#endif
dest += fs->blocksize*first_meta_bg;
}
for (i = first_meta_bg ; i < fs->desc_blocks; i++) {
blk = ext2fs_descriptor_block_loc2(fs, group_block, i);
io_channel_cache_readahead(fs->io, blk, 1);
}
for (i=first_meta_bg ; i < fs->desc_blocks; i++) {
blk = ext2fs_descriptor_block_loc2(fs, group_block, i);
retval = io_channel_read_blk64(fs->io, blk, 1, dest);
if (retval)
goto cleanup;
#ifdef WORDS_BIGENDIAN
for (j=0; j < groups_per_block; j++) {
gdp = ext2fs_group_desc(fs, fs->group_desc,
i * groups_per_block + j);
if (gdp)
ext2fs_swap_group_desc2(fs, gdp);
}
#endif
dest += fs->blocksize;
}
fs->stride = fs->super->s_raid_stride;
/*
* If recovery is from backup superblock, Clear _UNININT flags &
* reset bg_itable_unused to zero
*/
if (superblock > 1 && ext2fs_has_group_desc_csum(fs)) {
dgrp_t group;
for (group = 0; group < fs->group_desc_count; group++) {
ext2fs_bg_flags_clear(fs, group, EXT2_BG_BLOCK_UNINIT);
ext2fs_bg_flags_clear(fs, group, EXT2_BG_INODE_UNINIT);
ext2fs_bg_itable_unused_set(fs, group, 0);
/* The checksum will be reset later, but fix it here
* anyway to avoid printing a lot of spurious errors. */
ext2fs_group_desc_csum_set(fs, group);
}
if (fs->flags & EXT2_FLAG_RW)
ext2fs_mark_super_dirty(fs);
}
skip_read_bg:
if (ext2fs_has_feature_mmp(fs->super) &&
!(flags & EXT2_FLAG_SKIP_MMP) &&
(flags & (EXT2_FLAG_RW | EXT2_FLAG_EXCLUSIVE))) {
retval = ext2fs_mmp_start(fs);
if (retval) {
fs->flags |= EXT2_FLAG_SKIP_MMP; /* just do cleanup */
ext2fs_mmp_stop(fs);
goto cleanup;
}
}
if (fs->flags & EXT2_FLAG_SHARE_DUP) {
fs->block_sha_map = ext2fs_hashmap_create(ext2fs_djb2_hash,
block_sha_map_free_entry, 4096);
if (!fs->block_sha_map) {
retval = EXT2_ET_NO_MEMORY;
goto cleanup;
}
ext2fs_set_feature_shared_blocks(fs->super);
}
if (ext2fs_has_feature_casefold(fs->super))
fs->encoding = ext2fs_load_nls_table(fs->super->s_encoding);
fs->flags &= ~EXT2_FLAG_NOFREE_ON_ERROR;
*ret_fs = fs;
return 0;
cleanup:
if (!(flags & EXT2_FLAG_NOFREE_ON_ERROR)) {
ext2fs_free(fs);
fs = NULL;
}
*ret_fs = fs;
return retval;
}