RE: [RFC PATCH v9 12/16] Add mp(mediate passthru) device.

[Xin, Xiaohui]

>From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx]
>Sent: Sunday, September 12, 2010 9:37 PM
>To: Xin, Xiaohui
>Cc: netdev@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>mingo@xxxxxxx; davem@xxxxxxxxxxxxx; herbert@xxxxxxxxxxxxxxxxxxxx;
>jdike@xxxxxxxxxxxxxxx
>Subject: Re: [RFC PATCH v9 12/16] Add mp(mediate passthru) device.
>
>On Sat, Sep 11, 2010 at 03:41:14PM +0800, Xin, Xiaohui wrote:
>> >>Playing with rlimit on data path, transparently to the application in this way
>> >>looks strange to me, I suspect this has unexpected security implications.
>> >>Further, applications may have other uses for locked memory
>> >>besides mpassthru - you should not just take it because it's there.
>> >>
>> >>Can we have an ioctl that lets userspace configure how much
>> >>memory to lock? This ioctl will decrement the rlimit and store
>> >>the data in the device structure so we can do accounting
>> >>internally. Put it back on close or on another ioctl.
>> >Yes, we can decrement the rlimit in ioctl in one time to avoid
>> >data path.
>> >
>> >>Need to be careful for when this operation gets called
>> >>again with 0 or another small value while we have locked memory -
>> >>maybe just fail with EBUSY?  or wait until it gets unlocked?
>> >>Maybe 0 can be special-cased and deactivate zero-copy?.
>> >>
>>
>> How about we don't use a new ioctl, but just check the rlimit
>> in one MPASSTHRU_BINDDEV ioctl? If we find mp device
>> break the rlimit, then we fail the bind ioctl, and thus can't do
>> zero copy any more.
>
>Yes, and not just check, but decrement as well.
>I think we should give userspace control over
>how much memory we can lock and subtract from the rlimit.
>It's OK to add this as a parameter to MPASSTHRU_BINDDEV.
>Then increment the rlimit back on unbind and on close?
>
>This opens up an interesting condition: process 1
>calls bind, process 2 calls unbind or close.
>This will increment rlimit for process 2.
>Not sure how to fix this properly.
>
I can't too, can we do any synchronous operations on rlimit stuff?
I quite suspect in it.
 
>--
>MST
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