[dm-devel] [PATCH 4/6] block: propagate BLKROSET on the whole device to all partitions

[Hannes Reinecke]

On 1/9/21 11:42 AM, Christoph Hellwig wrote:
> Change the policy so that a BLKROSET on the whole device also affects
> partitions.  To quote Martin K. Petersen:
> 
> It's very common for database folks to twiddle the read-only state of
> block devices and partitions. I know that our users will find it very
> counter-intuitive that setting /dev/sda read-only won't prevent writes
> to /dev/sda1.
> 
> The existing behavior is inconsistent in the sense that doing:
> 
>    # blockdev --setro /dev/sda
>    # echo foo > /dev/sda1
> 
> permits writes. But:
> 
>    # blockdev --setro /dev/sda
>    <something triggers revalidate>
>    # echo foo > /dev/sda1
> 
> doesn't.
> 
> And a subsequent:
> 
>    # blockdev --setrw /dev/sda
>    # echo foo > /dev/sda1
> 
> doesn't work either since sda1's read-only policy has been inherited
> from the whole-disk device.
> 
> You need to do:
> 
>    # blockdev --rereadpt
> 
> after setting the whole-disk device rw to effectuate the same change on
> the partitions, otherwise they are stuck being read-only indefinitely.
> 
> However, setting the read-only policy on a partition does *not* require
> the revalidate step. As a matter of fact, doing the revalidate will blow
> away the policy setting you just made.
> 
> So the user needs to take different actions depending on whether they
> are trying to read-protect a whole-disk device or a partition. Despite
> using the same ioctl. That is really confusing.
> 
> I have lost count how many times our customers have had data clobbered
> because of ambiguity of the existing whole-disk device policy. The
> current behavior violates the principle of least surprise by letting the
> user think they write protected the whole disk when they actually
> didn't.
> 
> Suggested-by: Martin K. Petersen <martin.petersen at oracle.com>
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> Reviewed-by: Martin K. Petersen <martin.petersen at oracle.com>
> ---
>   block/genhd.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare at suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare at suse.de                              +49 911 74053 688
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