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:
找磁盘superblock结构定义，hexdump -C -n 128 -s 1024 查看验证(-s 1024不用说了吧)
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:
ext4 磁盘超级块结构定义节选: ext2/3/4 magic相同，区分靠 feature; 不兼容 feature 信息对应的偏移地址/*60*/
__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EXT4_FEATURE_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))
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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 |. ... ..........|
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00000430 00 1e 4a 54 00 00 ff ff 53 ef 01 00 01 00 00 00 |..JT....S.......|
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00000440 00 1e 4a 54 00 00 00 00 00 00 00 00 01 00 00 00 |..JT............|
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00000450 00 00 00 00 0b 00 00 00 80 00 00 00 3c 00 00 00 |............<...|
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00000460 42 02 00 00 79 00 00 00 |B...y...|
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00000468
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[root@dhcp12-241 rdma.sunrpc]# hexdump -C -s 1024 ext3.image -n $((0x68))
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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 |........|
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00000468
-
[root@dhcp12-241 rdma.sunrpc]# hexdump -C -s 1024 ext2.image -n $((0x68))
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00000400 10 f4 01 00 00 d0 07 00 00 64 00 00 d6 87 07 00 |.........d......|
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00000410 05 f4 01 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
-
00000420 00 20 00 00 00 20 00 00 f0 07 00 00 00 00 00 00 |. ... ..........|
-
00000430 61 13 4a 54 00 00 ff ff 53 ef 01 00 01 00 00 00 |a.JT....S.......|
-
00000440 61 13 4a 54 00 00 00 00 00 00 00 00 01 00 00 00 |a.JT............|
-
00000450 00 00 00 00 0b 00 00 00 80 00 00 00 38 00 00 00 |............8...|
-
00000460 02 00 00 00 01 00 00 00 |........|
-
00000468
*手动改一个ext3的的superblock /*5c~60*/位置的值，再用file查看是否文件系统类型发生变化
将ext4.image superblock的
s_feature_incompat 值覆盖到ext3.image, 再 file ext3.image 查看 类型就变成ext4了!
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[root@dhcp12-241 rdma.sunrpc]# dd if=ext4.image skip=$((1024+0x60))c bs=8 count=1 iflag=skip_bytes of=ext3.image seek=$((1024+0x60))c oflag=seek_bytes
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1+0 records in
-
1+0 records out
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8 bytes (8 B) copied, 0.000174478 s, 45.9 kB/s
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[root@dhcp12-241 rdma.sunrpc]# file ext3.image
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ext3.image: Linux rev 1.0 ext4 filesystem data (extents) (huge files)
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[root@dhcp12-241 rdma.sunrpc]# file *.image
-
ext2.image: Linux rev 1.0 ext2 filesystem data, UUID=b63e742f-e5cc-4610-a702-f36d563e782c
-
ext3.image: Linux rev 1.0 ext3 filesystem data, UUID=d986c054-9cc1-4b38-bfed-09caa75ee142
-
ext4.image: Linux rev 1.0 ext4 filesystem data, UUID=f1fe77bf-ed5a-4c87-93fd-3921462b7a54 (extents) (huge files)
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[root@dhcp12-241 rdma.sunrpc]# blkid ext3.image
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ext3.image: UUID="d986c054-9cc1-4b38-bfed-09caa75ee142" SEC_TYPE="ext2" TYPE="ext3"
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[root@dhcp12-241 rdma.sunrpc]# dd if=ext4.image skip=$((1024+0x60))c bs=8 count=1 iflag=skip_bytes of=ext3.image seek=$((1024+0x60))c oflag=seek_bytes conv=notrunc
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1+0 records in
-
1+0 records out
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8 bytes (8 B) copied, 9.2809e-05 s, 86.2 kB/s
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[root@dhcp12-241 rdma.sunrpc]# file *.image
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ext2.image: Linux rev 1.0 ext2 filesystem data, UUID=b63e742f-e5cc-4610-a702-f36d563e782c
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ext3.image: Linux rev 1.0 ext4 filesystem data, UUID=d986c054-9cc1-4b38-bfed-09caa75ee142 (extents) (huge files)
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ext4.image: Linux rev 1.0 ext4 filesystem data, UUID=f1fe77bf-ed5a-4c87-93fd-3921462b7a54 (extents) (huge files)
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[root@dhcp12-241 rdma.sunrpc]# blkid ext3.image
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ext3.image: UUID="d986c054-9cc1-4b38-bfed-09caa75ee142" TYPE="ext4"
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[root@dhcp12-241 rdma.sunrpc]#
ext2 变 ext3 同理，就不演示了,,
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