Re: Wipe2fs - utility to wipe unused space in ext2/3

From: /dev/rob0 (rob0_at_gmx.co.uk)
Date: 08/20/03


Date: Wed, 20 Aug 2003 09:39:35 -0700

In article <bhumuu$9to$1@gemini.ntu.edu.tw>, Chuan-kai Lin wrote:
> I discovered a surprising fact: that under normal conditions no data is
> actually left in the slack space after the end of files!

Interesting!

> all the previous data in in slack space is overwritten. It is more work
> for the kernel to preserve the data in the slack space, so the kernel
> generally does not bother with these things. In the case of recent
> Linux kernels, the slack space is overwritten by 0.

So, it still can be worthwhile to use some random overwrites.

>> And in so doing, one should become aware of the need for entropy and of
>> the limitations of pseudo-random number generators (PRNG's). In theory
>> at least, if a PRNG lacks entropy, its output could be predicted, and
>> you might as well have used /dev/zero and /dev/ff.
>
> I never quite understood this: suppose I overwrite the entire disk with
> random numbers 4 times to wipe the original data on the disk. How would
> knowing the random numbers used help physical media-based recovery?

IIUC: if the pattern can be recreated it will greatly ease the process
of physical examination of the madia.

Oh, and a minor point: the use of the word "recovery" in this context is
inaccurate, a "pet peeve" (annoyance) to me. If I write data to my
media, it belongs to me. If I determine that no one should have access
to it, and then wipe it, "recovery" is in fact "theft".

Yes, in some cases, such as if the media belongs to a company I worked
for, it is true recovery. But I do not recognise the right of
governments to access private data, no matter the circumstances. I *do*
recognise their power to do anything they want, and that the majority of
the people in this world probably do not agree with me. I'm not wishing
to trigger a flame war, but I think it's useful to cut through statist
euphemisms.

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