Re: free space wiper...

From: Kasper Dupont (kasperd@daimi.au.dk)
Date: 01/05/02 

From: Kasper Dupont <kasperd@daimi.au.dk>
Date: Sat, 05 Jan 2002 22:38:08 +0100

Michael Erskine wrote:
>
> dtk@berlin.com (donald) wrote in message news:<e43f4ba.0112220500.579d3cd7@posting.google.com>...
> > I'm looking for some utility that will wipe (overwrite with random
> > data)unused disk space. I tried making a big file that fills my fs but
> > it doesn't seem to work, i can still find old erased data in some
> > blocks. I have standard ext2fs.
> >
> > Thanks for all answers...
>
> This snippet will clear swap by allocating swap one int at a time and
> writing a random value to it until swap is full. The kernel will then
> kill it and swap will be full of randmo values (except that which was
> already allocated at the beginning of the run).
>
> #include <stdio.h>
> #include <stdlib.h>
> int main() {
> int *p;
> while (1) {
> p = calloc(1, sizeof(int));
> *p = rand();
> }
> return 0;
> }
>
> -m-

Probably not the best way to do it.
- You can expect some memory management overhead, so not all of
  the memory is overwritten with random data. Most of it is
  actually overwritten with management data and possibly also
  some zero padding. It would be better to mmap large chunks of
  memory and fill them with random bytes.
- The quality of the random bytes produced by rand is not very
  good. If you want good quality random bytes /dev/urandom would
  be an option.
- You are going to use all free physical memory before you start
  swapping. That is probably not desired. I don't know a good
  solution to that problem. And when the swap is full you should
  not start using physical memory.
- If you have large amounts of swapspace you might run out of
  virtual addressspace before you run out of swapspace. That
  problem can be solved by having multiple processes filling
  buffers at the same time.

And actually sensitive data on swap partitions should be avoided
in the first place. The ml*** syscall could help a little bit
here. 

-- 
Kasper Dupont

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