Re: GSOC 2012 project clarification

[Matthias Drochner <M.Drochner%fz-juelich.de@localhost>]

I'm not the originator of that project, but I've recently worked on
support for SSD trim/erase commands which is quite similar technically.
If this project materializes, I do offer my help. For now just two
comments:

On Sun, 1 Apr 2012 13:04:13 -0400
Sanket Padawe <sanketp12%gmail.com@localhost> wrote:
> whenever that flag is set and a file/folder
> gets deleted i.e. at the point of unlink system call we just need to
> rewrite all the blocks of that file with some random data and then
> release those blocks.

The deletion is generally not at the time of "unlink". It happens when
the file isn't referenced by anything anymore. It needs to happen
after that point, but before the file system's allocation management
gets notified that the data blocks can be reused.
For performance, this needs to be dove asynchronously. It needs some
knowledge about kernel locking and signaling mechanisms to implement.

> So by
> generating some pseudo random numbers we can erase the previous secure
> data.

I'm not sure that pseudo-random numbers help security in the general
case, compared to just zeros. For a plain harddisk, either one is
good enough. For a SSD, both are useless. A difference would be
if the device was an encrypted disk because all-0 would be a perfect
"known plaintext". It should be configurable.

best regards
Matthias


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