Blocking the access to the device files.

[Greg Freemyer]

On Wed, Dec 29, 2010 at 11:12 AM, Mulyadi Santosa
<mulyadi.santosa at gmail.com> wrote:
> On Wed, Dec 29, 2010 at 20:06, Prasad Joshi <prasadjoshi124 at gmail.com> wrote:
>> Hello All,
>>
>> ZFS file system has a property called devices. If turned off, ZFS
>> would not allow access to the device files (block/character) present
>> on the file system. I want to implement the same behavior on the a
>> Linux File System.
>
> I don't know about ZFS, so could you please elaborate on what you mean
> by "ZFS could disallow access"?
>
> IMHO, (untested), you could simply do it using usual Linux
> file/directory permission up to SELinux/AppArmor....so, is that what
> you mean?
>
> --
> regards,
>
> Mulyadi Santosa

Mulyadi,

My guess is that it is more complex than that.

Some filesystems have issues if the raw drive is read while the
filesystem is mounted.  I think it is caused by inconsistencies in the
various cache's.  ie. iirc, At least in the 2.4 kernel there was not a
single unified cache for block layer and filesystems.  So doing raw
reads of underlying device while it was mounted could cause the caches
to get out of sync.

I don't recall the details, but either the kernel would oops or the
filesystem would become corrupt.  I don't know if any 2.6 filesystems
still have that issue.  Anyway ZFS must have a similar issue.

So a ZFS filesystem developer knowing this was a conflict could add a
check in the /dev/sda open() that would fail the open if there was a
mounted filesystem of type ZFS on the drive.

And the mount should fail if /dev/sda is already open.

I'm not aware of the 2.6.x linux kernel offering any infrastructure to
help with that issue.

Greg