Shred: Doesn't work on Journaled FS?

Question

Shred documentation says shred is "not guaranteed to be effective" (See bottom). So if I shred a document on my Ext3 filesystem or on a Raid, what happens? Do I shred part of the file? Does it sometimes shred the whole thing and sometimes not? Can it shred other stuff? Does it only shred the file header?

CAUTION: Note that shred relies on a very important assumption: that the file system overwrites data in place. This is the traditional way to do things, but many modern file system designs do not satisfy this assumption. The following are examples of file systems on which shred is not effective, or is not guaranteed to be effective in all file sys‐ tem modes:

• log-structured or journaled file systems, such as those supplied with AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)

• file systems that write redundant data and carry on even if some writes fail, such as RAID-based file systems

• file systems that make snapshots, such as Network Appliance’s NFS server

• file systems that cache in temporary locations, such as NFS version 3 clients

• compressed file systems

In the case of ext3 file systems, the above disclaimer applies (and shred is thus of limited effectiveness) only in data=journal mode, which journals file data in addition to just metadata. In both the data=ordered (default) and data=writeback modes, shred works as usual. Ext3 journaling modes can be changed by adding the data=something option to the mount options for a particular file system in the /etc/fstab file, as documented in the mount man page (man mount).

Cataphoresis

Answer 1

All shred does is overwrite, flush, check success, and repeat. It does absolutely nothing to find out whether overwriting a file actually results in the blocks which contained the original data being overwritten. This is because without knowing non-standard things about the underlying filesystem, it can't.

So, journaling filesystems won't overwrite the original blocks in place, because that would stop them recovering cleanly from errors where the change is half-written. If data is journaled, then each pass of shred might be written to a new location on disk, in which case nothing is shredded.

RAID filesystems (depending on the RAID mode) might not overwrite all of the copies of the original blocks. If there's redundancy, you might shred one disk but not the other(s), or you might find that different passes have affected different disks such that each disk is partly shredded.

On any filesystem, the disk hardware itself might just so happen to detect an error (or, in the case of flash, apply wear-leveling even without an error) and remap the logical block to a different physical block, such that the original is marked faulty (or unused) but never overwritten.

Compressed filesystems might not overwrite the original blocks, because the data with which shred overwrites is either random or extremely compressible on each pass, and either one might cause the file to radically change its compressed size and hence be relocated. NTFS stores small files in the MFT, and when shred rounds up the filesize to a multiple of one block, its first "overwrite" will typically cause the file to be relocated out to a new location, which will then be pointlessly shredded leaving the little MFT slot untouched.

Shred can't detect any of these conditions (unless you have a special implementation which directly addresses your fs and block driver - I don't know whether any such things actually exist). That's why it's more reliable when used on a whole disk than on a filesystem.

Shred never shreds "other stuff" in the sense of other files. In some of the cases above it shreds previously-unallocated blocks instead of the blocks which contain your data. It also doesn't shred any metadata in the filesystem (which I guess is what you mean by "file header"). The -u option does attempt to overwrite the file name, by renaming to a new name of the same length and then shortening that one character at a time down to 1 char, prior to deleting the file. You can see this in action if you specify -v too.

Recitation

Answer 2

The other answers have already done a good job of explaining why shred may not be able to do its job properly.

This can be summarised as:

shred only works on partitions, not individual files

As explained in the other answers, if you shred a single file:

• there is no guarantee the actual data is really overwritten, because the filesystem may send writes to the same file to different locations on disk
• there is no guarantee the fs did not create copies of the data elsewhere
• the fs might even decide to "optimize away" your writes, because you are writing the same file repeatedly (syncing is supposed to prevent this, but again: no guarantee)

But even if you know that your filesystem does not do any of the nasty things above, you also have to consider that many applications will automatically create copies of file data:

• crash recovery files which word processors, editors (such as vim) etc. will write periodically
• thumbnail/preview files in file managers (sometimes even for non-imagefiles)
• temporary files that many applications use

So, short of checking every single binary you use to work with your data, it might have been copied right, left & center without you knowing. The only realistic way is to always shred complete partitions (or disks).

Antimonous

Answer 3

The concern is that data might exist on more than one place on the disk. When the data exists in exactly one location, then shred can deterministically "erase" that information. However, file systems that journal or other advanced file systems may write your file's data in multiple locations, temporarily, on the disk. Shred -- after the fact -- has no way of knowing about this and has no way of knowing where the data may have been temporarily written to disk. Thus, it has no way of erasing or overwriting those disk sectors.

Imagine this: You write a file to disk on a journaled file system that journals not just metadata but also the file data. The file data is temporarily written to the journal, and then written to its final location. Now you use shred on the file. The final location where the data was written can be safely overwritten with shred. However, shred would have to have some way of guaranteeing that the sectors in the journal that temporarily contained your file's contents are also overwritten to be able to promise that your file is truly not recoverable. Imagine a file system where the journal is not even in a fixed location or of a fixed length.

If you are using shred, then you're trying to ensure that there is no possible way your data could be reconstructed. The authors of shred are being honest that there are some conditions beyond their control where they cannot make this guarantee.

Kagu