Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of blocks

Theodore Ts'o
2016-03-12T00:36:31Z

On Sat, Mar 12, 2016 at 09:30:47AM +1100, Dave Chinner wrote:
> It's all well and good to restrict access to the fallocate() call to
> limit who can expose stale data, but it doesn't remove the fact it
> is easy for stale data to unintentionally escape the privileged
> group once it has been exposed because there is no record of the
> fact the file contains uninitialised blocks....

Ultimately, it's up to the trusted process to make sure it never
reveals any stale data. For example, if you have the policy that all
data is encrypted at rest, and the trusted process is always going to
be decrypting any blocks it reads from disk before passing it on its
client (for example) then any stale data is going to be obscured by
the decryption step before it gets passed on.

At the end of the day it's about whether you trust the userspace
program or not. I know there's a long and venerated traition of
assuming that all application programmers are incompetent, but that
leads to file systems doing more work than what is strictly necessary,
and that has a performance tax.

And if the result is the cluster file system authors decide to create
a user space file system, and bypass the kernel file system directly,
then we have to trust them to do a competent job anyway. But if you
believe that there are still ways in which a in-kernel file system can
add value, then it's encumbent on us to to be a bit more flexible and
not assume that all userspace programmers are blithering idiots.

Cheers,

- Ted