Re: [PATCH] xen-blkfront: remove type check from blkfront_setup_discard

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 01/13/2014 04:30 AM, Olaf Hering wrote:
> On Fri, Jan 10, Boris Ostrovsky wrote:
>
>> I don't know discard code works but it seems to me that if you pass, for
>> example,  zero as discard_granularity (which may happen if xenbus_gather()
>> fails) then blkdev_issue_discard() in the backend will set granularity to 1
>> and continue with discard. This may not be what the the guest admin
>> requested. And he won't know about this since no error message is printed
>> anywhere.
> If I understand the code using granularity/alignment correctly, both are
> optional properties. So if the granularity is just 1 it means byte
> ranges, which is fine if the backend uses FALLOC_FL_PUNCH_HOLE. Also
> both properties are not admin controlled, for phy the blkbk drivers just
> passes on what it gets from the underlying hardware.
>
>> Similarly, if xenbug_gather("discard-secure") fails, I think the code will
>> assume that secure discard has not been requested. I don't know what
>> security implications this will have but it sounds bad to me.
> There are no security implications, if the backend does not advertise it
> then its not present.

Right. But my questions was what if the backend does advertise it and 
wants the frontent to use it but xenbus_gather() in the frontend fails. 
Do we want to silently continue without discard-secure? Is this safe?


-boris

>
> After poking around some more it seems that blkif.h is the spec, it does
> not say anything that the three properties are optional. Also the
> backend drivers in sles11sp2 and mainline create all three properties
> unconditionally. So I think a better change is to expect all three
> properties in the frontend. I will send another version of the patch.
>
>
> Olaf