ext2/3/4 magic相同，怎么区分是哪个类型呢？
24 Oct 2014大家都知道，文件系统类型是靠磁盘上superblock区块里的magic number不同来区分的， 但是，今天同事遇到相关问题，引出这个问题: ext2/3/4 使用的magic numner都一样，怎么判断分区类型到底是哪一个呢？
google 到一些工具可以判断，但是，具体他们根据什么判断的呢？ 多年不看文件系统了，但是还有一点印象:
1.找磁盘superblock结构定义，hexdump -C -n 128 -s 1024 查看验证
2.找到 ext4 的超级块定义代码，对比ext3/2，有 incompatible feature 的字样， 就是它了:
//ext2/3/4 magic相同，区分靠 feature;
//不兼容 feature 信息对应的偏移地址 /*60*/ (代码如下)
__le16 s_magic; /* Magic signature */ /*38*/ <<<---magic
__le16 s_state; /* File system state */
__le16 s_errors; /* Behaviour when detecting errors */
__le16 s_minor_rev_level; /* minor revision level */
/*40*/ __le32 s_lastcheck; /* time of last check */
__le32 s_checkinterval; /* max. time between checks */
__le32 s_creator_os; /* OS */
__le32 s_rev_level; /* Revision level */
/*50*/ __le16 s_def_resuid; /* Default uid for reserved blocks */
__le16 s_def_resgid; /* Default gid for reserved blocks */
/*
* These fields are for EXT4_DYNAMIC_REV superblocks only.
*
* Note: the difference between the compatible feature set and
* the incompatible feature set is that if there is a bit set
* in the incompatible feature set that the kernel doesn't
* know about, it should refuse to mount the filesystem.
*
* e2fsck's requirements are more strict; if it doesn't know
* about a feature in either the compatible or incompatible
* feature set, it must abort and not try to meddle with
* things it doesn't understand...
*/
__le32 s_first_ino; /* First non-reserved inode */
__le16 s_inode_size; /* size of inode structure */
__le16 s_block_group_nr; /* block group # of this superblock */
__le32 s_feature_compat; /* compatible feature set */
/*60*/ __le32 s_feature_incompat; /* incompatible feature set */ <<<--- ext2/3/4 magic相同，区分主要是feature,
__le32 s_feature_ro_compat; /* readonly-compatible feature set */
/*68*/ __u8 s_uuid[16]; /* 128-bit uuid for volume */
进一步看代码ext4 superblock初始化代码，会发现s_feature_incompat赋值里面 有EXT4FEATURE_INCOMPAT_EXTENTS等多个ext4独有的特性， 所以使用magic+s_feature*判断应该是可以的
验证： hexdump -C 查看超级块 feature 相关信息比较ext 4/3/2: ext4 ext3比较s_feature_incompat不同， ext3 ext2比较s_feature_compat 不同:
[root@dhcp12-241 rdma.sunrpc]# hexdump -C -s 1024 ext4.image -n $((0x68))
00000400 10 f4 01 00 00 d0 07 00 00 64 00 00 d6 67 07 00 |.........d...g..|
00000410 05 f4 01 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000420 00 20 00 00 00 20 00 00 f0 07 00 00 00 00 00 00 |. ... ..........|
00000430 00 1e 4a 54 00 00 ff ff 53 ef 01 00 01 00 00 00 |..JT....S.......|
00000440 00 1e 4a 54 00 00 00 00 00 00 00 00 01 00 00 00 |..JT............|
00000450 00 00 00 00 0b 00 00 00 80 00 00 00 3c 00 00 00 |............<...|
00000460 42 02 00 00 79 00 00 00 |B...y...| ^^ ^^
00000468 ^^^^^
[root@dhcp12-241 rdma.sunrpc]# hexdump -C -s 1024 ext3.image -n $((0x68))
00000400 10 f4 01 00 00 d0 07 00 00 64 00 00 b5 67 07 00 |.........d...g..|
00000410 05 f4 01 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000420 00 20 00 00 00 20 00 00 f0 07 00 00 00 00 00 00 |. ... ..........|
00000430 72 13 4a 54 00 00 ff ff 53 ef 01 00 01 00 00 00 |r.JT....S.......|
00000440 72 13 4a 54 00 00 00 00 00 00 00 00 01 00 00 00 |r.JT............|
00000450 00 00 00 00 0b 00 00 00 80 00 00 00 3c 00 00 00 |............<...|
00000460 02 00 00 00 01 00 00 00 |........| ^^ ^^
00000468
[root@dhcp12-241 rdma.sunrpc]# hexdump -C -s 1024 ext2.image -n $((0x68))
00000400 10 f4 01 00 00 d0 07 00 00 64 00 00 d6 87 07 00 |.........d......|
00000410 05 f4 01 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000420 00 20 00 00 00 20 00 00 f0 07 00 00 00 00 00 00 |. ... ..........|
00000430 61 13 4a 54 00 00 ff ff 53 ef 01 00 01 00 00 00 |a.JT....S.......|
00000440 61 13 4a 54 00 00 00 00 00 00 00 00 01 00 00 00 |a.JT............|
00000450 00 00 00 00 0b 00 00 00 80 00 00 00 38 00 00 00 |............8...|
00000460 02 00 00 00 01 00 00 00 |........| ^^ ^^
00000468
手动改一个ext3的的superblock /5c~60*/位置的值，再用file查看是否文件系统类型发生变化 将ext4.image superblock的 s_feature_incompat 值覆盖到ext3.image, 再 file ext3.image 查看 类型就变成ext4了!
[root@dhcp12-241 rdma.sunrpc]# dd if=ext4.image skip=$((1024+0x60))c bs=8 count=1 iflag=skip_bytes of=ext3.image seek=$((1024+0x60))c oflag=seek_bytes
1+0 records in
1+0 records out
8 bytes (8 B) copied, 0.000174478 s, 45.9 kB/s
[root@dhcp12-241 rdma.sunrpc]# file ext3.image
ext3.image: Linux rev 1.0 ext4 filesystem data (extents) (huge files)
[root@dhcp12-241 rdma.sunrpc]# file *.image
ext2.image: Linux rev 1.0 ext2 filesystem data, UUID=b63e742f-e5cc-4610-a702-f36d563e782c
ext3.image: Linux rev 1.0 ext3 filesystem data, UUID=d986c054-9cc1-4b38-bfed-09caa75ee142
ext4.image: Linux rev 1.0 ext4 filesystem data, UUID=f1fe77bf-ed5a-4c87-93fd-3921462b7a54 (extents) (huge files)
[root@dhcp12-241 rdma.sunrpc]# blkid ext3.image
ext3.image: UUID="d986c054-9cc1-4b38-bfed-09caa75ee142" SEC_TYPE="ext2" TYPE="ext3"
[root@dhcp12-241 rdma.sunrpc]# dd if=ext4.image skip=$((1024+0x60))c bs=8 count=1 iflag=skip_bytes of=ext3.image seek=$((1024+0x60))c oflag=seek_bytes conv=notrunc
1+0 records in
1+0 records out
8 bytes (8 B) copied, 9.2809e-05 s, 86.2 kB/s
[root@dhcp12-241 rdma.sunrpc]# file *.image
ext2.image: Linux rev 1.0 ext2 filesystem data, UUID=b63e742f-e5cc-4610-a702-f36d563e782c
ext3.image: Linux rev 1.0 ext4 filesystem data, UUID=d986c054-9cc1-4b38-bfed-09caa75ee142 (extents) (huge files)
ext4.image: Linux rev 1.0 ext4 filesystem data, UUID=f1fe77bf-ed5a-4c87-93fd-3921462b7a54 (extents) (huge files)
[root@dhcp12-241 rdma.sunrpc]# blkid ext3.image
ext3.image: UUID="d986c054-9cc1-4b38-bfed-09caa75ee142" TYPE="ext4"
[root@dhcp12-241 rdma.sunrpc]#
ext2 变 ext3 同理，就不演示了,,
dd 注意事项:
1. 修改文件记得加 conv=notrunc 不然文件会被截断;
2. skip seek 默认单位是 block, 如果是字节需要加 iflag=skip_bytes oflag=seek_bytes
Update: 2019-02-19 从 backup superblock 恢复
[student@workstation ~]$ hexdump -C -s 1024 ext4.img -n $((0x68))
00000400 00 00 01 00 00 00 04 00 33 33 00 00 65 cd 03 00 |........33..e...|
00000410 f5 ff 00 00 00 00 00 00 02 00 00 00 02 00 00 00 |................|
00000420 00 80 00 00 00 80 00 00 00 20 00 00 eb 94 6b 5c |......... ....k\|
00000430 f6 94 6b 5c 02 00 ff ff 53 ef 01 00 01 00 00 00 |..k\....S.......|
00000440 5c 8b 6b 5c 00 00 00 00 00 00 00 00 01 00 00 00 |\.k\............|
00000450 00 00 00 00 0b 00 00 00 00 01 00 00 3c 00 00 00 |............<...|
00000460 c2 02 00 00 7b 00 00 00 |....{...|
00000468
[student@workstation ~]$ dd if=/dev/zero of=ext4.img seek=1024 oflag=seek_bytes bs=256 count=1 conv=notrunc
1+0 records in
1+0 records out
256 bytes (256 B) copied, 9.8364e-05 s, 2.6 MB/s
[student@workstation ~]$ hexdump -C -s 1024 ext4.img -n $((0x68))
00000400 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000460 00 00 00 00 00 00 00 00 |........|
00000468
[student@workstation ~]$ sudo mount -t ext4 -oloop ext4.img /mnt/
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so.
[student@workstation ~]$ dd if=ext4.img skip=$((32768*4096)) iflag=skip_bytes of=ext4.img seek=1024 oflag=seek_bytes bs=256 count=1 conv=notrunc
1+0 records in
1+0 records out
256 bytes (256 B) copied, 8.3631e-05 s, 3.1 MB/s
[student@workstation ~]$ sudo mount -t ext4 -oloop ext4.img /mnt/