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EXT
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Ext - Extended Filesystem

Ext2 is the most common filesystem for not journaling partitions (partitions that don't change much) like the boot partition. Ext3/4 are journaling and are used usually for the rest partitions.
All block groups in the filesystem have the same size and are stored sequentially. This allows the kernel to easily derive the location of a block group in a disk from its integer index.
Every block group contains the following pieces of information:
  • A copy of the filesystemโ€™s superblock
  • A copy of the block group descriptors
  • A data block bitmap which is used to identify the free blocks inside the group
  • An inode bitmap, which is used to identify the free inodes inside the group
  • inode table: it consists of a series of consecutive blocks, each of which contains a predefined Figure 1 Ext2 inode number of inodes. All inodes have the same size: 128 bytes. A 1,024 byte block contains 8 inodes, while a 4,096-byte block contains 32 inodes. Note that in Ext2, there is no need to store on disk a mapping between an inode number and the corresponding block number because the latter value can be derived from the block group number and the relative position inside the inode table. For example, suppose that each block group contains 4,096 inodes and that we want to know the address on disk of inode 13,021. In this case, the inode belongs to the third block group and its disk address is stored in the 733rd entry of the corresponding inode table. As you can see, the inode number is just a key used by the Ext2 routines to retrieve the proper inode descriptor on disk quickly
  • data blocks, containing files. Any block which does not contain any meaningful information, it is said to be free.

Ext Optional Features

Features affect where the data is located, how the data is stored in inodes and some of them might supply additional metadata for analysis, therefore features are important in Ext.
Ext has optional features that your OS may or may not support, there are 3 possibilities:
  • Compatible
  • Incompatible
  • Compatible Read Only: It can be mounted but not for writing
If there are incompatible features you won't be able to mount the filesystem as the OS won't know how the access the data.
Suspected attacker might have non-standard extensions
Any utility that reads the superblock will be able to indicate the features of a Ext filesystem, but you could also use file -sL /dev/sd*

Superblock

The superblock is the first 1024 bytes from the start, it's repeated in the first block of each group and contains:
  • Block size
  • Total blocks
  • Blocks per block group
  • Reserved blocks before the first block group
  • Total inodes
  • Inodes per block group
  • Volume name
  • Last write time
  • Last mount time
  • Path where the file system was last mounted
  • Filesystem status (clean?)
It's possible to obtain this information from an Ext filesystem file using:
1
fsstat -o <offsetstart> /pat/to/filesystem-file.ext
2
#You can get the <offsetstart> with the "p" command inside fdisk
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You can also use the free gui application: https://www.disk-editor.org/index.html Or you can also use python to obtain the superblock information: https://pypi.org/project/superblock/โ€‹

inodes

The inodes contain the list of blocks that contains the actual data of a file. If the file is big, and inode may contain pointers to other inodes that points to the blocks/more inodes containing the file data.
In Ext2 and Ext3 inodes are of size 128B, Ext4 currently uses 156B but allocates 256B on disk to allow a future expansion.
Inode structure:
Offset
Size
Name
DescriptionF
0x0
2
File Mode
File mode and type
0x2
2
UID
Lower 16 bits of owner ID
0x4
4
Size Il
Lower 32 bits of file size
0x8
4
Atime
Access time in seconds since epoch
0xC
4
Ctime
Change time in seconds since epoch
0x10
4
Mtime
Modify time in seconds since epoch
0x14
4
Dtime
Delete time in seconds since epoch
0x18
2
GID
Lower 16 bits of group ID
0x1A
2
Hlink count
Hard link count
0xC
4
Blocks Io
Lower 32 bits of block count
0x20
4
Flags
Flags
0x24
4
Union osd1
Linux: I version
0x28
69
Block[15]
15 pointes to data block
0x64
4
Version
File version for NFS
0x68
4
File ACL low
Lower 32 bits of extended attributes (ACL, etc)
0x6C
4
File size hi
Upper 32 bits of file size (ext4 only)
0x70
4
Obsolete fragment
An obsoleted fragment address
0x74
12
Osd 2
Second operating system dependent union
0x74
2
Blocks hi
Upper 16 bits of block count
0x76
2
File ACL hi
Upper 16 bits of extended attributes (ACL, etc.)
"Modify" is the timestamp of the last time the file's content has been mofified. This is often called "mtime". "Change" is the timestamp of the last time the file's inode has been changed, like by changing permissions, ownership, file name, number of hard links. It's often called "ctime".
Inode structure extended (Ext4):
Offset
Size
Name
Description
0x80
2
Extra size
How many bytes beyond standard 128 are used
0x82
2
Checksum hi
Upper 16 bits of inode checksum
0x84
4
Ctime extra
Change time extra bits
0x88
4
Mtime extra
Modify time extra bits
0x8C
4
Atime extra
Access time extra bits
0x90
4
Crtime
File create time (seconds since epoch)
0x94
4
Crtime extra
File create time extra bits
0x98
4
Version hi
Upper 32 bits of version
0x9C
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Unused
Reserved space for future expansions
Special inodes:
Inode
Special Purpose
0
No such inode, numberings starts at 1
1
Defective block list
2
Root directory
3
User quotas
4
Group quotas
5
Boot loader
6
Undelete directory
7
Reserved group descriptors (for resizing filesystem)
8
Journal
9
Exclude inode (for snapshots)
10
Replica inode
11
First non-reserved inode (often lost + found)
Not that the creation time only appears in Ext4.
Knowing the inode number you can easily find it's index:
  • Block group where an inode belongs: (Inode number - 1) / (Inodes per group)
  • Index inside it's group: (Inode number - 1) mod(Inodes/groups)
  • Offset into inode table: Inode number * (Inode size)
  • The "-1" is because the inode 0 is undefined (not used)
1
ls -ali /bin | sort -n #Get all inode numbers and sort by them
2
stat /bin/ls #Get the inode information of a file
3
istat -o <start offset> /path/to/image.ext 657103 #Get information of that inode inside the given ext file
4
icat -o <start offset> /path/to/image.ext 657103 #Cat the file
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File Mode
Number
Description
15
Reg/Slink-13/Socket-14
14
Directory/Block Bit 13
13
Char Device/Block Bit 14
12
FIFO
11
Set UID
10
Set GID
9
Sticky Bit (without it, anyone with Write & exec perms on a directory can delete and rename files)
8
Owner Read
7
Owner Write
6
Owner Exec
5
Group Read
4
Group Write
3
Group Exec
2
Others Read
1
Others Write
0
Others Exec
The bold bits (12, 13, 14, 15) indicate the type of file the file is (a directory, socket...) only one of the options in bold may exit.
Directories
Offset
Size
Name
Description
0x0
4
Inode
โ€‹
0x4
2
Rec len
Record length
0x6
1
Name len
Name length
0x7
1
File type
0x00 Unknown 0x01 Regular
0x02 Director
0x03 Char device
0x04 Block device
0x05 FIFO
0x06 Socket
0x07 Sym link
0x8
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Name
Name string (up to 255 characters)
In order to increase the performance, Root hash Directory blocks may be used.
Extended Attributes
Can be stored in
  • Extra space between inodes (256 - inode size, usually = 100)
  • A data block pointed to by file_acl in inode
Can be used to store anything as a users attribute if name starts with "user".
Data can ne hidden this way.
Extended Attributes Entries
Offset
Size
Name
Description
0x0
1
Name len
Length of attribute name
0x1
1
Name index
0x0 = no prefix
0x1 = user. Prefix
0x2 = system.posix_acl_access
0x3 = system.posix_acl_default
0x4 = trusted.
0x6 = security.
0x7 = system.
0x8 = system.richacl
0x2
2
Value offs
Offset from first inode entry or start of block
0x4
4
Value blocks
Disk block where value stored or zero for this block
0x8
4
Value size
Length of value
0xC
4
Hash
Hash for attribs in block or zero if in inode
0x10
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Name
Attribute name w/o trailing NULL
1
setfattr -n 'user.secret' -v 'This is a secret' file.txt #Save a secret using extended attributes
2
getfattr file.txt #Get extended attribute names of a file
3
getdattr -n 'user.secret' file.txt #Get extended attribute called "user.secret"
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Filesystem View

In order to see the contents of the file system you can use the free tool: https://www.disk-editor.org/index.html Or you can mount it in your linux using mount command.
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