svn commit: r214431 - head/bin/rm

[Robert Watson]

On Thu, 28 Oct 2010, Gary Jennejohn wrote:

>>> The -P option assumes that the underlying storage overwrites file block 
>>> when data is written on existing offset.  Several factors including the 
>>> file system and its backing store could defeat the assumption, this 
>>> includes, but is not limited to file systems that uses Copy-On-Write 
>>> strategy (e.g. ZFS or UFS when snapshot is being used), or backing 
>>> datastore that does journaling, etc.  In addition, only regular files are 
>>> overwritten, other types of files are not.
>>
>> Summary: it is very hard to tell whether -P works, even when you think you 
>> know what all the subsystems are doing.
>
> All this discussion leads me to the conclusion that we should just remove 
> the -P functionality and add a remark to the man page that that was done 
> because it isn't guaranteed to work on all file systems.
>
> Why give users a false sense of security?  If they're concerned about data 
> security then they should use geli or something similar.

I'm not sure I entirely agree with this conclusion: there are times when -P 
can be quite effective at removing data from a system, and not having it 
available when it could work might also be problematic.

Part of the problem here is that we have successfully abstracted away a lot of 
information going up the storage stack.  One possibly solution to this problem 
is to add ways for that information to propagate better -- in the same way 
that the end-to-end argument in network design is moderated by the reality 
that performance can be dramatically improved if information gathering can be 
done with an awareness of lower layers (for example, PMTU discovery), even if 
decisions about how to use that information are at the endpoints only.

For example, you could imagine a pathconf() call that asks the file system if 
overwriting a file is likely to result in the data going away -- UFS could 
answer "yes" unless it's snapshotting the file system, and ZFS could simply 
answer "no".

In FreeBSD 9, we're going to have two common file system configurations: ZFS 
and UFS+SUJ; the latter will do overwriting of files just fine on most current 
media.

Robert