Linux Ext2fs Undeletion mini-HOWTO
Aaron Crane, aaronc@pobox.com
v1.3, 2 February 1999
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Picture this. You've spent the last three days with no sleep, no food, not even
a shower. Your hacking compulsion has at last paid off: you've finished that
program that will bring you world-wide fame and recognition. All that you still
need to do is tar it up and put it on Metalab. Oh, and delete all those Emacs
backup files. So you say rm * ~. And too late, you notice the extra space in
that command. You've just deleted your magnum opus! But help is at hand. This
document presents a discussion of how to retrieve deleted files from a Second
Extended File System. Just maybe, you'll be able to release that program after
all...
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1. Introduction
This mini-Howto attempts to provide hints on how to retrieve deleted files from
an ext2 file system. It also contains a limited amount of discussion of how to
avoid deleting files in the first place.
I intend it to be useful certainly for people who have just had, shall we say,
a little accident with rm; however, I also hope that people read it anyway. You
never know: one day, some of the information in here could save your bacon.
The text assumes a little background knowledge about UNIX file systems in
general; however, I hope that it will be accessible to most Linux users. If you
are an outright beginner, I'm afraid that undeleting files under Linux does
require a certain amount of technical knowledge and persistence, at least for
the time being.
You will be unable to recover deleted files from an ext2 file system without at
least read access to the raw device on which the file was stored. In general,
this means that you must be root, but some distributions (such as Debian_GNU/
Linux) provide a disk group whose members have access to such devices. You also
need debugfs from the e2fsprogs package. This should have been installed by
your distribution.
Why have I written this? It stems largely from my own experiences with a
particularly foolish and disastrous rm -r command as root. I deleted about 97
JPEG files which I needed and could almost certainly not recover from other
sources. Using some helpful tips (see section Credits_and_Bibliography) and a
great deal of persistence, I recovered 91 files undamaged. I managed to
retrieve at least parts of five of the rest (enough to see what the picture was
in each case). Only one was undisplayable, and even for this one, I am fairly
sure that no more than 1024 bytes were lost (though unfortunately from the
beginning of the file; given that I know nothing about the JFIF file format I
had done as much as I could).
I shall discuss further below what sort of recovery rate you can expect for
deleted files.
1.1 Revision history
The various publicly-released revisions of this document (and their publication
dates) are as follows:
* v1.0 on 18 January 1997
* v1.1 on 23 July 1997 (see section Changes_in_version_1.1)
* v1.2 on 4 August 1997 (see section Changes_in_version_1.2)
* v1.3 on 2 February 1999 (see section Changes_in_version_1.3)
Changes in version 1.1
What changes have been made in this version? First of all, the thinko in the
example of file recovery has been fixed. Thankyou to all those who wrote to
point out my mistaek; I hope I've learned to be more careful when making up
program interaction.
Secondly, the discussion of UNIX file system layout has been rewritten to be, I
hope, more understandable. I wasn't entirely happy with it in the first place,
and some people's comments indicated that it wasn't clear.
Thirdly, the vast uuencoded gzipped tarball of fsgrab in the middle of the file
has been removed. The program is now available on my_website and on Metalab
(and mirrors).
Fourthly, the document has been translated into the Linux Documentation Project
SGML Tools content markup language. This markup language can be easily
converted to any of a number of other markup languages (including HTML and
LaTeX) for convenient display and printing. One benefit of this is that
beautiful typography in paper editions is a much more achievable goal; another
is that the document has cross-references and hyperlinks when viewed on the
Web.
Changes in version 1.2
This revision is very much an incremental change. It's here mainly to include
changes suggested by readers, one of which is particularly important.
The first change was suggested by Egil Kvaleberg egil@kvaleberg.no, who pointed
out the dump command in debugfs. Thanks again, Egil.
The second change is to mention the use of chattr for avoiding deleting
important files. Thanks to Herman Suijs H.P.M.Suijs@kub.nl for mentioning this
one.
The abstract has been revised. URLs have been added for organisations and
software. Various other minor changes have been made (including fixing typos
and so on).
Changes in version 1.3
Though it is the first release in 17 months, there is very little that is new
here. This release merely fixes a few minor errors (typos, dangling URLs, that
sort of thing -- especially the non-link to the Open Group), and updates a few
parts of the text that have become hopelessly out-of-date, such as the material
on kernel versions and on lde. Oh, and I've changed `Sunsite' to `Metalab'
throughout.
This release is anticipated to be the last one before release 2.0, which will
hopefully be a full Howto. I have been working on some substantial changes
which will justify an increment of the major version number.
1.2 Canonical locations of this document
The latest public release of this document should always be available in on the
Linux_Documentation_Project_site (and mirrors).
The latest release is also kept on my_website in several formats:
* SGML_source. This is the source as I have written it, using the SGML Tools
package.
* HTML. This is HTML, automatically generated from the SGML source.
* Plain_text. This is plain text, which is also automatically generated from
the SGML source.
2. How not to delete files
It is vital to remember that Linux is unlike MS-DOS when it comes to
undeletion. For MS-DOS (and its bastard progeny Windows 95), it is generally
fairly straightforward to undelete a file - the `operating system' (I use the
term loosely) even comes with a utility which automates much of the process.
For Linux, this is not the case.
So. Rule number one (the prime directive, if you will) is:
KEEP BACKUPS
no matter what. Think of all your data. Perhaps, like me, you keep several
years' of accumulated email, contacts, programs, papers on your computer. Think
of how your life would be turned upside down if you had a catastrophic disk
failure, or if -- heaven forbid! -- a malicious cracker wiped your disks. This
is not unlikely; I have corresponded with a number of people in just such a
situation. I exhort all right-thinking Linux users to go out and buy a useful
backup device, work out a decent backup schedule, and to stick to it. Myself, I
use a spare hard disk on a second machine, and periodically mirror my home
directory onto it over the ethernet. For more information on planning a backup
schedule, read Frisch (1995) (see section Bibliography_and_Credits).
In the absence of backups, what then? (Or even in the presence of backups: belt
and braces is no bad policy where important data is concerned.)
Try to set the permissions for important files to 440 (or less): denying
yourself write access to them means that rm requires an explicit confirmation
before deleting. (I find, however, that if I'm recursively deleting a directory
with rm -r, I'll interrupt the program on the first or second confirmation
request and reissue the command as rm -rf.)
A good trick for selected files is to create a hard link to them in a hidden
directory. I heard a story once about a sysadmin who repeatedly deleted /etc/
passwd by accident (thereby half-destroying the system). One of the fixes for
this was to do something like the following (as root):
# mkdir /.backup
# ln /etc/passwd /.backup
It requires quite some effort to delete the file contents completely: if you
say
# rm /etc/passwd
then
# ln /.backup/passwd /etc
will retrieve it. Of course, this does not help in the event that you overwrite
the file, so keep backups anyway.
On an ext2 file system, it is possible to use ext2 attributes to protect
things. These attributes are manipulated with the chattr command. There is an
`append-only' attribute: a file with this attribute may be appended to, but may
not be deleted, and the existing contents of the file may not be overwritten.
If a directory has this attribute, any files or directories within it may be
modified as normal, but no files may be deleted. The `append-only' attribute is
set with
$ chattr +a FILE...
There is also an `immutable' attribute, which can only be set or cleared by
root. A file or directory with this attribute may not be modified, deleted,
renamed, or (hard) linked. It may be set as follows:
# chattr +i FILE...
The ext2fs also provides the `undeletable' attribute (+u in chattr). The
intention is that if a file with that attribute is deleted, instead of actually
being reused, it is merely moved to a `safe location' for deletion at a later
date. Unfortunately this feature has not yet been implemented in mainstream
kernels; and though in the past there has been some interest in implementing
it, it is not (to my knowledge) available for any current kernels.
Some people advocate making rm a shell alias or function for rm -i (which asks
for confirmation on every file you delete). Indeed, the Red_Hat_distribution
does this by default for all users, including root. Personally, I cannot stand
software which won't run unattended, so I don't do that. There is also the
problem that sooner or later, you'll be running in single-user mode, or using a
different shell, or even a different machine, where your rm function doesn't
exist. If you expect to be asked for confirmation, it is easy to forget where
you are and to specify too many files for deletion. Likewise, the various
scripts and programs that replace rm are, IMHO, very dangerous.
A slightly better solution is to start using a package which handles
`recyclable' deletion by providing a command not named rm. For details on
these, see Peek, et al (1993) (see section Bibliography_and_Credits). These
however still suffer from the problem that they tend to encourage the user to
have a nonchalant attitude to deletion, rather than the cautious approach that
is often required on Unix systems.
3. What recovery rate can I expect?
That depends. Among the problems with recovering files on a high-quality,
multi-tasking, multi-user operating system like Linux is that you never know
when someone wants to write to the disk. So when the operating system is told
to delete a file, it assumes that the blocks used by that file are fair game
when it wants to allocate space for a new file. (This is a specific example of
a general principle for Unix-like systems: the kernel and the associated tools
assume that the users aren't idiots.) In general, the more usage your machine
gets, the less likely you are to be able to recover files successfully.
Also, disk fragmentation can affect the ease of recovering files. If the
partition containing the deleted files is very fragmented, you are unlikely to
be able to read a whole file.
If your machine, like mine, is effectively a single-user workstation, and you
weren't doing anything disk-intensive at the fatal moment of deleting those
files, I would expect a recovery rate in the same ball-park as detailed above.
I retrieved nearly 94% of the files (and these were binary files, please note)
undamaged. If you get 80% or better, you can feel pretty pleased with yourself,
I should think.
4. So, how do I undelete a file?
The procedure principally involves finding the data on the raw partition device
and making it visible again to the operating system. There are basically two
ways of doing this: one is to modify the existing file system such that the
deleted inodes have their `deleted' flag removed, and hope that the data just
magically falls back into place. The other method, which is safer but slower,
is to work out where the data lies in the partition and write it out into a new
file on another file system.
There are some steps you need to take before beginning to attempt your data
recovery; see sections Unmounting_the_file_system, Preparing_to_change_inodes
directly and Preparing_to_write_data_elsewhere for details. To find out how to
actually retrieve your files, see sections Finding_the_deleted_inodes,
Obtaining_the_details_of_the_inodes, Recovering_data_blocks and Modifying
inodes_directly.
5. Unmounting the file system
Regardless of which method you choose, the first step is to unmount the file
system containing the deleted files. I strongly discourage any urges you may
have to mess around on a mounted file system. This step should be performed as
soon as possible after you realise that the files have been deleted; the sooner
you can unmount, the smaller the chance that your data will be overwritten.
The simplest method is as follows: assuming the deleted files were in the /usr
file system, say:
# umount /usr
You may, however, want to keep some things in /usr available. So remount it
read-only:
# mount -o ro,remount /usr
If the deleted files were on the root partition, you'll need to add a -n option
to prevent mount from trying to write to /etc/mtab:
# mount -n -o ro,remount /
Regardless of all this, it is possible that there will be another process using
that file system (which will cause the unmount to fail with an error such as
`Resource busy'). There is a program which will send a signal to any process
using a given file or mount point: fuser. Try this for the /usr partition:
# fuser -v -m /usr
This lists the processes involved. Assuming none of them are vital, you can say
# fuser -k -v -m /usr
to send each process a SIGKILL (which is guaranteed to kill it), or for
example,
# fuser -k -TERM -v -m /usr
to give each one a SIGTERM (which will normally make the process exit cleanly).
6. Preparing to change inodes directly
My advice? Don't do it this way. I really don't think it's wise to play with a
file system at a low enough level for this to work. This method also has
problems in that you can only reliably recover the first 12 blocks of each
file. So if you have any long files to recover, you'll normally have to use the
other method anyway. (Although see section Will_this_get_easier_in_future? for
additional information.)
If you feel you must do it this way, my advice is to copy the raw partition
data to an image on a different partition, and then mount this using loopback:
# cp /dev/hda5 /root/working
# mount -t ext2 -o loop /root/working /mnt
(Note that obsolete versions of mount may have problems with this. If your
mount doesn't work, I strongly suggest you get the latest version, or at least
version 2.7, as some very old versions have severe security bugs.)
Using loopback means that if and when you completely destroy the file system,
all you have to do is copy the raw partition back and start over.
7. Preparing to write data elsewhere
If you chose to go this route, you need to make sure you have a rescue
partition somewhere -- a place to write out new copies of the files you
recover. Hopefully, your system has several partitions on it: perhaps a root, a
/usr, and a /home. With all these to choose from, you should have no problem:
just create a new directory on one of these.
If you have only a root partition, and store everything on that, things are
slightly more awkward. Perhaps you have an MS-DOS or Windows partition you
could use? Or you have the ramdisk driver in your kernel, maybe as a module? To
use the ramdisk (assuming a kernel more recent than 1.3.48), say the following:
# dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
# mke2fs -v -m 0 /dev/ram0 2048
# mount -t ext2 /dev/ram0 /mnt
This creates a 2MB ramdisk volume, and mounts it on /mnt.
A short word of warning: if you use kerneld (or its replacement kmod in 2.2.x
and later 2.1.x kernels) to automatically load and unload kernel modules, then
don't unmount the ramdisk until you've copied any files from it onto non-
volatile storage. Once you unmount it, kerneld assumes it can unload the module
(after the usual waiting period), and once this happens, the memory gets re-
used by other parts of the kernel, losing all the painstaking hours you just
spent recovering your data.
If you have a Zip, Jaz, or LS-120 drive, or something similar, it would
probably be a good choice for a rescue partition location. Otherwise, you'll
just have to stick with floppies.
The other thing you're likely to need is a program which can read the necessary
data from the middle of the partition device. At a pinch, dd will do the job,
but to read from, say, 600 MB into an 800 MB partition, dd insists on reading
but ignoring the first 600 MB. This takes a not inconsiderable amount of time,
even on fast disks. My way round this was to write a program which will seek to
the middle of the partition. It's called fsgrab; you can find the source
package on my_website or on Metalab (and mirrors). If you want to use this
method, the rest of this mini-Howto assumes that you have fsgrab.
If none of the files you are trying to recover were more than 12 blocks long
(where a block is usually one kilobyte), then you won't need fsgrab.
If you need to use fsgrab but don't want to download and build it, it is fairly
straightforward to translate an fsgrab command-line to one for dd. If we have
fsgrab -c count -s skip device
then the corresponding (but typically much slower) dd command is
dd bs=1k if=device count=count skip=skip
I must warn you that, although fsgrab functioned perfectly for me, I can take
no responsibility for how it performs. It was really a very quick and dirty
kludge just to get things to work. For more details on the lack of warranty,
see the `No Warranty' section in the COPYING file included with it (the GNU
General Public Licence).
8. Finding the deleted inodes
The next step is to ask the file system which inodes have recently been freed.
This is a task you can accomplish with debugfs. Start debugfs with the name of
the device on which the file system is stored:
# debugfs /dev/hda5
If you want to modify the inodes directly, add a -w option to enable writing to
the file system:
# debugfs -w /dev/hda5
The debugfs command to find the deleted inodes is lsdel. So, type the command
at the prompt:
debugfs: lsdel
After much wailing and grinding of disk mechanisms, a long list is piped into
your favourite pager (the value of $PAGER). Now you'll want to save a copy of
this somewhere else. If you have less, you can type -o followed by the name of
an output file. Otherwise, you'll have to arrange to send the output elsewhere.
Try this:
debugfs: quit
# echo lsdel | debugfs /dev/hda5 > lsdel.out
Now, based only on the deletion time, the size, the type, and the numerical
permissions and owner, you must work out which of these deleted inodes are the
ones you want. With luck, you'll be able to spot them because they're the big
bunch you deleted about five minutes ago. Otherwise, trawl through that list
carefully.
I suggest that if possible, you print out the list of the inodes you want to
recover. It will make life a lot easier.
9. Obtaining the details of the inodes
debugfs has a stat command which prints details about an inode. Issue the
command for each inode in your recovery list. For example, if you're interested
in inode number 148003, try this:
debugfs: stat <148003>
Inode: 148003 Type: regular Mode: 0644 Flags: 0x0
Version: 1
User: 503 Group: 100 Size: 6065
File ACL: 0 Directory ACL: 0
Links: 0 Blockcount: 12
Fragment: Address: 0 Number: 0 Size: 0
ctime: 0x31a9a574 -- Mon May 27 13:52:04 1996
atime: 0x31a21dd1 -- Tue May 21 20:47:29 1996
mtime: 0x313bf4d7 -- Tue Mar 5 08:01:27 1996
dtime: 0x31a9a574 -- Mon May 27 13:52:04 1996
BLOCKS:
594810 594811 594814 594815 594816 594817
TOTAL: 6
If you have a lot of files to recover, you'll want to automate this. Assuming
that your lsdel list of inodes to recover in is in lsdel.out, try this:
# cut -c1-6 lsdel.out | grep "[0-9]" | tr -d " " > inodes
This new file inodes contains just the numbers of the inodes to recover, one
per line. We save it because it will very likely come in handy later on. Then
you just say:
# sed 's/^.*$/stat <\0>/' inodes | debugfs /dev/hda5 > stats
and stats contains the output of all the stat commands.
10. Recovering data blocks
This part is either very easy or distinctly less so, depending on whether the
file you are trying to recover is more than 12 blocks long.
10.1 Short files
If the file was no more than 12 blocks long, then the block numbers of all its
data are stored in the inode: you can read them directly out of the stat output
for the inode. Moreover, debugfs has a command which performs this task
automatically. To take the example we had before, repeated here:
debugfs: stat <148003>
Inode: 148003 Type: regular Mode: 0644 Flags: 0x0
Version: 1
User: 503 Group: 100 Size: 6065
File ACL: 0 Directory ACL: 0
Links: 0 Blockcount: 12
Fragment: Address: 0 Number: 0 Size: 0
ctime: 0x31a9a574 -- Mon May 27 13:52:04 1996
atime: 0x31a21dd1 -- Tue May 21 20:47:29 1996
mtime: 0x313bf4d7 -- Tue Mar 5 08:01:27 1996
dtime: 0x31a9a574 -- Mon May 27 13:52:04 1996
BLOCKS:
594810 594811 594814 594815 594816 594817
TOTAL: 6
This file has six blocks. Since this is less than the limit of 12, we get
debugfs to write the file into a new location, such as /mnt/recovered.000:
debugfs: dump <148003> /mnt/recovered.000
Of course, this can also be done with fsgrab; I'll present it here as an
example of using it:
# fsgrab -c 2 -s 594810 /dev/hda5 > /mnt/recovered.000
# fsgrab -c 4 -s 594814 /dev/hda5 >> /mnt/recovered.000
With either debugfs or fsgrab, there will be some garbage at the end of /mnt/
recovered.000, but that's fairly unimportant. If you want to get rid of it, the
simplest method is to take the Size field from the inode, and plug it into the
bs option in a dd command line:
# dd count=1 if=/mnt/recovered.000 of=/mnt/resized.000 bs=6065
Of course, it is possible that one or more of the blocks that made up your file
has been overwritten. If so, then you're out of luck: that block is gone
forever. (But just imagine if you'd unmounted sooner!)
10.2 Longer files
The problems appear when the file has more than 12 data blocks. It pays here to
know a little of how UNIX file systems are structured. The file's data is
stored in units called `blocks'. These blocks may be numbered sequentially. A
file also has an `inode', which is the place where information such as owner,
permissions, and type are kept. Like blocks, inodes are numbered sequentially,
although they have a different sequence. A directory entry consists of the name
of the file and an inode number.
But with this state of affairs, it is still impossible for the kernel to find
the data corresponding to a directory entry. So the inode also stores the
location of the file's data blocks, as follows:
* The block numbers of the first 12 data blocks are stored directly in the
inode; these are sometimes referred to as the direct blocks.
* The inode contains the block number of an indirect block. An indirect block
contains the block numbers of 256 additional data blocks.
* The inode contains the block number of a doubly indirect block. A doubly
indirect block contains the block numbers of 256 additional indirect blocks.
* The inode contains the block number of a triply indirect block. A triply
indirect block contains the block numbers of 256 additional doubly indirect
blocks.
Read that again: I know it's complex, but it's also important.
Now, the kernel implementation for all versions up to and including 2.0.36
unfortunately zeroes all indirect blocks (and doubly indirect blocks, and so
on) when deleting a file. So if your file was longer than 12 blocks, you have
no guarantee of being able to find even the numbers of all the blocks you need,
let alone their contents.
The only method I have been able to find thus far is to assume that the file
was not fragmented: if it was, then you're in trouble. Assuming that the file
was not fragmented, there are several layouts of data blocks, according to how
many data blocks the file used:
0 to 12
The block numbers are stored in the inode, as described above.
13 to 268
After the direct blocks, count one for the indirect block, and then there
are 256 data blocks.
269 to 65804
As before, there are 12 direct blocks, a (useless) indirect block, and
256 blocks. These are followed by one (useless) doubly indirect block,
and 256 repetitions of one (useless) indirect block and 256 data blocks.
65805 or more
The layout of the first 65804 blocks is as above. Then follow one
(useless) triply indirect block and 256 repetitions of a `doubly indirect
sequence'. Each doubly indirect sequence consists of a (useless) doubly
indirect block, followed by 256 repetitions of one (useless) indirect
block and 256 data blocks.
Of course, even if these assumed data block numbers are correct, there is no
guarantee that the data in them is intact. In addition, the longer the file
was, the less chance there is that it was written to the file system without
appreciable fragmentation (except in special circumstances).
You should note that I assume throughout that your blocksize is 1024 bytes, as
this is the standard value. If your blocks are bigger, some of the numbers
above will change. Specifically: since each block number is 4 bytes long,
blocksize/4 is the number of block numbers that can be stored in each indirect
block. So every time the number 256 appears in the discussion above, replace it
with blocksize/4. The `number of blocks required' boundaries will also have to
be changed.
Let's look at an example of recovering a longer file.
debugfs: stat <1387>
Inode: 148004 Type: regular Mode: 0644 Flags: 0x0
Version: 1
User: 503 Group: 100 Size: 1851347
File ACL: 0 Directory ACL: 0
Links: 0 Blockcount: 3616
Fragment: Address: 0 Number: 0 Size: 0
ctime: 0x31a9a574 -- Mon May 27 13:52:04 1996
atime: 0x31a21dd1 -- Tue May 21 20:47:29 1996
mtime: 0x313bf4d7 -- Tue Mar 5 08:01:27 1996
dtime: 0x31a9a574 -- Mon May 27 13:52:04 1996
BLOCKS:
8314 8315 8316 8317 8318 8319 8320 8321 8322 8323 8324 8325 8326
8583
TOTAL: 14
There seems to be a reasonable chance that this file is not fragmented:
certainly, the first 12 blocks listed in the inode (which are all data blocks)
are contiguous. So, we can start by retrieving those blocks:
# fsgrab -c 12 -s 8314 /dev/hda5 > /mnt/recovered.001
Now, the next block listed in the inode, 8326, is an indirect block, which we
can ignore. But we trust that it will be followed by 256 data blocks (numbers
8327 through 8582).
# fsgrab -c 256 -s 8327 /dev/hda5 >> /mnt/recovered.001
The final block listed in the inode is 8583. Note that we're still looking good
in terms of the file being contiguous: the last data block we wrote out was
number 8582, which is 8327 + 255. This block 8583 is a doubly indirect block,
which we can ignore. It is followed by up to 256 repetitions of an indirect
block (which is ignored) followed by 256 data blocks. So doing the arithmetic
quickly, we issue the following commands. Notice that we skip the doubly
indirect block 8583, and the indirect block 8584 immediately (we hope)
following it, and start at block 8585 for data.
# fsgrab -c 256 -s 8585 /dev/hda5 >> /mnt/recovered.001
# fsgrab -c 256 -s 8842 /dev/hda5 >> /mnt/recovered.001
# fsgrab -c 256 -s 9099 /dev/hda5 >> /mnt/recovered.001
# fsgrab -c 256 -s 9356 /dev/hda5 >> /mnt/recovered.001
# fsgrab -c 256 -s 9613 /dev/hda5 >> /mnt/recovered.001
# fsgrab -c 256 -s 9870 /dev/hda5 >> /mnt/recovered.001
Adding up, we see that so far we've written 12 + (7 * 256) blocks, which is
1804. The `stat' results for the inode gave us a `blockcount' of 3616;
unfortunately these blocks are 512 bytes long (as a hangover from UNIX), so we
really want 3616/2 = 1808 blocks of 1024 bytes. That means we need only four
more blocks. The last data block written was number 10125. As we've been doing
so far, we skip an indirect block (number 10126); we can then write those last
four blocks.
# fsgrab -c 4 -s 10127 /dev/hda5 >> /mnt/recovered.001
Now, with some luck the entire file has been recovered successfully.
11. Modifying inodes directly
This method is, on the surface, much easier. However, as mentioned above, it
cannot yet cope with files longer than 12 blocks.
For each inode you want to recover, you must set the usage count to one, and
set the deletion time to zero. This is done with the mi (modify inode) command
in debugfs. Some sample output, modifying inode 148003 from above:
debugfs: mi <148003>
Mode [0100644]
User ID [503]
Group ID [100]
Size [6065]
Creation time [833201524]
Modification time [832708049]
Access time [826012887]
Deletion time [833201524] 0
Link count [0] 1
Block count [12]
File flags [0x0]
Reserved1 [0]
File acl [0]
Directory acl [0]
Fragment address [0]
Fragment number [0]
Fragment size [0]
Direct Block #0 [594810]
Direct Block #1 [594811]
Direct Block #2 [594814]
Direct Block #3 [594815]
Direct Block #4 [594816]
Direct Block #5 [594817]
Direct Block #6 [0]
Direct Block #7 [0]
Direct Block #8 [0]
Direct Block #9 [0]
Direct Block #10 [0]
Direct Block #11 [0]
Indirect Block [0]
Double Indirect Block [0]
Triple Indirect Block [0]
That is, I set the deletion time to 0 and the link count to 1 and just pressed
return for each of the other fields. Granted, this is a little unwieldy if you
have a lot of files to recover, but I think you can cope. If you'd wanted
chrome, you'd have used a graphical `operating system' with a pretty `Recycle
Bin'.
By the way: the mi output refers to a `Creation time' field in the inode. This
is a lie! (Or misleading, anyway.) The fact of the matter is that you cannot
tell on a UNIX file system when a file was created. The st_ctime member of a
struct stat refers to the `inode change time', that is, the last time when any
inode details were changed. Here endeth today's lesson.
Note that more recent versions of debugfs than the one I'm using probably do
not include some of the fields in the listing above (specifically, Reserved1
and (some of?) the fragment fields).
Once you've modified the inodes, you can quit debugfs and say:
# e2fsck -f /dev/hda5
The idea is that each of the deleted files has been literally undeleted, but
none of them appear in any directory entries. The e2fsck program can detect
this, and will add a directory entry for each file in the /lost+found directory
of the file system. (So if the partition is normally mounted on /usr, the files
will now appear in /usr/lost+found when you next mount it.) All that still
remains to be done is to work out the name of each file from its contents, and
return it to its correct place in the file system tree.
When you run e2fsck, you will get some informative output, and some questions
about what damage to repair. Answer `yes' to everything that refers to `summary
information' or to the inodes you've changed. Anything else I leave up to you,
although it's usually a good idea to say `yes' to all the questions. When
e2fsck finishes, you can remount the file system.
Actually, there's an alternative to having e2fsck leave the files in /
lost+found: you can use debugfs to create a link in the file system to the
inode. Use the link command in debugfs after you've modified the inode:
debugfs: link <148003> foo.txt
This creates a file called foo.txt in what debugfs thinks is the current
directory; foo.txt will be your file. You'll still need to run e2fsck to fix
the summary information and block counts and so on.
12. Will this get easier in future?
Yes. In fact, I believe it already has. Although as of this writing, current
stable kernels (in the 2.0.x series) zero indirect blocks, this does not apply
to development kernels in the 2.1.x series, nor to the stable 2.2.x series. As
I write this on 2 February 1999, kernel 2.2.1 was released a few days ago;
Linux vendors are likely to start producing distributions containing and
supporting 2.2.x kernels a month or two from now.
Once the indirect-zeroing limitation has been overcome in the production
kernels, a lot of my objections to the technique of modifying inodes by hand
will disappear. At the same time, it will also become possible to use the dump
command in debugfs on long files, and to conveniently use other undeletion
tools.
13. Are there any tools to automate this process?
As it happens, there are. Unfortunately, I believe that they currently suffer
from the same problem as the manual inode modification technique: indirect
blocks are unrecoverable. However, given the likelihood that this will shortly
no longer be a problem, it's well worth looking these programs out now.
I have written a tool called e2recover, which is essentially a Perl wrapper
around fsgrab. It makes a reasonable amount of effort to deal with zeroed
indirect blocks, and seems to work fairly well as long as there was no
fragmentation. It also correctly sets the permissions (and when possible the
ownership) of recovered files, and even makes sure that recovered files have
the correct length.
I originally wrote e2recover for the forthcoming major update to this Howto;
unfortunately this means that much of the useful documentation for e2recover is
scheduled for inclusion in that update. Be that as it may, it should be useful
now; it can be downloaded from my_web_site, and soon from Metalab.
Scott D. Heavner is the author of lde, the Linux Disk Editor. It can be used as
both a binary disk editor, and as an equivalent to debugfs for ext2 and minix
file systems, and even for xia file systems (though xia support is no longer
available in 2.1.x and 2.2.x kernels). It has some features for assisting
undeletion, both by walking the block list for a file, and by grepping through
disk contents. It also has some fairly useful documentation on basic file
system concepts, as well as a document on how to use it for undeletion. Version
2.4 of lde is available on Metalab and mirrors, or on the_author's_web_site.
Another possibility is offered by the GNU Midnight Commander, mc. This is a
full-screen file management tool, based AFAIK on a certain MS-DOS program
commonly known as `NC'. mc supports the mouse on the Linux console and in an
xterm, and provides virtual file systems which allow tricks like cd-ing to a
tarfile. Among its virtual file systems is one for ext2 undeletion. It all
sounds very handy, although I must admit I don't use the program myself -- I
prefer good old-fashioned shell commands.
To use the undeletion feature, you have to configure the program with the --
with-ext2undel option; you'll also need the development libraries and include
files that come with the e2fsprogs package. The version provided in Debian_GNU/
Linux is built in this way; the same may apply to packages for other Linux
distributions. Once the program is built, you can tell it to cd undel:/dev/
hda5, and get a `directory listing' of deleted files. Like many current
undeletion tools, it handles zeroed indirect blocks poorly -- it typically just
recovers the first 12k of long files.
The current version may be downloaded from the_Midnight_Commander_ftp_site.
14. Colophon
I intend to produce regular updates to this document as long as I have both
enough time to do it, and something interesting to say. This means that I am
eager to hear comments from readers. Could my writing be clearer? Can you think
of something that would make matters easier? Is there some new tool that does
it all automatically? Whatever. If you have something to say about this
document or about the fsgrab or e2recover tools, drop me a line on
aaronc@pobox.com.
15. Credits and Bibliography
`If I have seen farther than others, it is because I was standing on
the shoulders of giants.' (Isaac Newton)
This mini-Howto was originally derived from a posting in the comp.os.linux.misc
newsgroup by Robin Glover swrglovr@met.rdg.ac.uk. I would like to thank Robin
for graciously allowing me to rework his ideas into this mini-Howto.
I would also like to take this opportunity to thank once again all the people
who've written to me about the Howto. Receiving grateful comments makes the
effort worth while.
Some bibliographic references:
* Frisch, Æleen (1995), Essential System Administration, second edition,
O'Reilly and Associates, Inc., ISBN: 1-56592-127-5.
* Garfinkel, Simson, Daniel Weise and Steven Strassmann (1994), The Unix-Haters
Handbook, IDG Books, ISBN: 1-56884-203-1. Much of this book is merely the
adolescent whinings of people who think that their operating system was so
much better than Unix, and much of the rest simply doesn't apply if you have
a well-written user-space such as GNU. But there is some wheat among the
chaff; for example, the discussion of how easy it is to delete files under
Unix is well worth reading.
* Glover, Robin (31 Jan 1996), HOW-TO : undelete linux files (ext2fs/debugfs),
comp.os.linux.misc Usenet posting.
* Peek, Jerry, Tim O'Reilly, Mike Loukides et al (1993), UNIX Power Tools,
O'Reilly and Associates, Inc./Random House, Inc., ISBN: 0-679-79073-X. Second
edition, 1998.
16. Legalities
All trademarks are the property of their respective owners. Specifically:
* MS-DOS and Windows are trademarks of Microsoft.
* UNIX is a trademark of the_Open_Group.
* Linux is a trademark of Linus Torvalds in the USA and some other countries.
This document is Copyright © 1997, 1999 Aaron Crane aaronc@pobox.com. It may be
freely redistributed in its entirety, including the whole of this copyright
notice, but may not be changed without permission from either the author or the
Linux Documentation Project HOWTO Coordinator. Dispensation is granted for
copying small verbatim portions for the purposes of reviews or for quoting; in
these circumstances, sections may be reproduced in the presence of an
appropriate citation but without this copyright notice.
The author requests but does not require that parties intending to sell copies
of this document, whether on computer-readable or human-readable media, inform
either him or the Linux HOWTO Coordinator of their intentions.
The Linux HOWTO Coordinator is currently Tim Bynum linux-howto@metalab.unc.edu.