Re: aliasing (or renaming) kern.geom.debugflags

Hi,

On Fri, Oct 7, 2011 at 4:27 PM, Warren Block <wblock_at_wonkity.com> wrote:
> On Fri, 7 Oct 2011, Glen Barber wrote:
>
>> Hi,
>>
>> On 10/7/11 3:18 PM, Poul-Henning Kamp wrote:
>>>>
>>>> My guess is that GEOM isn't letting go of the GPT table and you have
>>>> multiple partitions in the GPT table and you're not destroying them
>>>> hierarchically in a proper manner.. but again, that's just a guess
>>>> based on hazy recollection.
>>>
>>> If none of the GPT partitions are open, you should be able to
>>> open /dev/da0.  If not, the GPT geom class is buggy.
>>>
>>
>> In my experience, without kern.geom.debugflags=16, the MBR will not be
>> written to the memstick, leaving you with what would effectively be a
>> coaster in the not-so-distant past.
>
> Tried it just now with the 9.0-BETA3 memstick image.
>
> # dd if=/tmp/FreeBSD-9.0-BETA3-amd64-memstick.img of=/dev/da0 bs=64k
> 10463+1 records in
> 10463+1 records out
> 685731840 bytes transferred in 52.614157 secs (13033219 bytes/sec)
> # mount /dev/da0p2 /mnt
> # dd if=/tmp/FreeBSD-9.0-BETA3-amd64-memstick.img of=/dev/da0 bs=64k
> dd: /dev/da0: Operation not permitted
> # sysctl kern.geom.debugflags=16
> kern.geom.debugflags: 0 -> 16
> # dd if=/tmp/FreeBSD-9.0-BETA3-amd64-memstick.img of=/dev/da0 bs=64k
> 10463+1 records in
> 10463+1 records out
> 685731840 bytes transferred in 52.915362 secs (12959031 bytes/sec)
>
in the mean time, some background application issued a sync(2),
corrupting part of what you just wrote, if not crashing the kernel.

> Followed by removing the memory stick without unmounting it to avoid
> overwriting part of the image.  No obvious problems, but no, it's not
> polite.  (I'm thinking "automounter" here.)
>
> We could make the normal procedure just the dd, followed by a note:
>
>  If this gives an 'Operation not permitted' error, make sure that the
>  target device is not in use or being automounted by some helpful
>  utility program.
>
> And maybe
>
>  If the error still appears and no other options are available,
>    sysctl kern.geomflags.debug=16
>  will override the system safety and allow writes to the device.
>
Then you can start to introduce backdoor for every single
kernel-enforced policy. `kern.geomflags.debug=16' should not be used
by the lambda user.

 - Arnaud