confusion about multipath_prepare_ioctl

confusion about multipath_prepare_ioctl

[Wang Sheng-Hui]

Dear,

Sorry to trouble you.

I noticed some code in dm-*.c like:
"
static int multipath_prepare_ioctl(struct dm_target *ti,
		struct block_device **bdev, fmode_t *mode)
{
...
	/*
	 * Only pass ioctls through if the device sizes match exactly.
	 */
	if (!r && ti->len != i_size_read((*bdev)->bd_inode) >> SECTOR_SHIFT)
		return 1;
...
}
"
Here, return value 1 means 
"ioctl is being issued against a subset of the parent bdev; require extra privileges."
(comment in dm_blk_ioctl)

I'm confused by the comment and '!=' test for multipath. 
In which cases, the size of low level single device is not equal to the parent 
size of multipath device?


Regards,
shenghui

Re: confusion about multipath_prepare_ioctl

[Mike Snitzer]

On Mon, Mar 05 2018 at 10:35pm -0500,
Wang Sheng-Hui <shhuiw@foxmail.com> wrote:

> Dear,
> 
> Sorry to trouble you.
> 
> I noticed some code in dm-*.c like:
> "
> static int multipath_prepare_ioctl(struct dm_target *ti,
> 		struct block_device **bdev, fmode_t *mode)
> {
> ...
> 	/*
> 	 * Only pass ioctls through if the device sizes match exactly.
> 	 */
> 	if (!r && ti->len != i_size_read((*bdev)->bd_inode) >> SECTOR_SHIFT)
> 		return 1;
> ...
> }
> "
> Here, return value 1 means 
> "ioctl is being issued against a subset of the parent bdev; require extra privileges."
> (comment in dm_blk_ioctl)
> 
> I'm confused by the comment and '!=' test for multipath. 
> In which cases, the size of low level single device is not equal to the parent 
> size of multipath device?

Given that ti->len is sent down from userspace, the DM multipath
target's ti->len _could_ be smaller than the underlying path(s).  But in
practice that doesn't occur with multipathd.. a partitioned multipath
device is generally done, via kpartx, in terms of linear mappings ontop
of the multipath device.

The same != test is done in the dm linear target and is much more
relevant to concerns about ioctls being sent to partition.

Mike

Re: confusion about multipath_prepare_ioctl

[shhuiw]

On 2018年03月06日 13:44, Mike Snitzer wrote:
> On Mon, Mar 05 2018 at 10:35pm -0500,
> Wang Sheng-Hui <shhuiw@foxmail.com> wrote:
>
>> Dear,
>>
>> Sorry to trouble you.
>>
>> I noticed some code in dm-*.c like:
>> "
>> static int multipath_prepare_ioctl(struct dm_target *ti,
>> 		struct block_device **bdev, fmode_t *mode)
>> {
>> ...
>> 	/*
>> 	 * Only pass ioctls through if the device sizes match exactly.
>> 	 */
>> 	if (!r && ti->len != i_size_read((*bdev)->bd_inode) >> SECTOR_SHIFT)
>> 		return 1;
>> ...
>> }
>> "
>> Here, return value 1 means 
>> "ioctl is being issued against a subset of the parent bdev; require extra privileges."
>> (comment in dm_blk_ioctl)
>>
>> I'm confused by the comment and '!=' test for multipath. 
>> In which cases, the size of low level single device is not equal to the parent 
>> size of multipath device?
> Given that ti->len is sent down from userspace, the DM multipath
> target's ti->len _could_ be smaller than the underlying path(s).  But in
> practice that doesn't occur with multipathd.. a partitioned multipath
> device is generally done, via kpartx, in terms of linear mappings ontop
> of the multipath device.
>
> The same != test is done in the dm linear target and is much more
> relevant to concerns about ioctls being sent to partition.
>
> Mike
>
>
Thanks for your explanation, Mike!

Regards,
shenghui




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