Blocking the access to the device files.

[greg.freemyer@gmail.com (Greg Freemyer)]

On Wed, Dec 29, 2010 at 1:54 PM, mindentropy <mindentropy@gmail.com> wrote:
> On Wednesday 29 Dec 2010 10:31:37 pm Greg Freemyer wrote:
>> On Wed, Dec 29, 2010 at 11:12 AM, Mulyadi Santosa
>>
>> <mulyadi.santosa@gmail.com> wrote:
>> > On Wed, Dec 29, 2010 at 20:06, Prasad Joshi <prasadjoshi124@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
>>
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>
> Greg,
>> So doing raw
>> reads of underlying device while it was mounted could cause the caches
>> to get out of sync.
>
> ? ?So doing a 'dd' would cause the kernel to oops?
>
> Thanks.

I really don't remember what the problem was.  It was about 2003 or
2004 I think that I came across the issue.  I don't recall if it was
XFS or ext2/3 or what.

I just know I came to the conclusion that dd if=/dev/sda of=/dev/null
was not always safe if the drive had a mounted filesystem.  Hopefully
its been fixed by now.

Greg