Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment

[Wei Liu]

On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote:
> From: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Date: Wed, 30 Jul 2014 14:25:30 +0100
[...]
> Secondly, for something like UDP you can't just split the packet up
> like this, or for any other datagram protocol for that matter.
> 
> I know you're in a difficult situation, but I just can't see this
> being an acceptable approach to solving the problem right now.
> 
> Where does the MAX_SKB_FRAGS + 1 limit really come from, the size of
> the TX queue?
> 

It stems from the implicit transimit protocol since inception of
netfront / netback. Sigh.

> If you were to have a 64-slot TX queue, you ought to be able to handle
> this theoretical 51 slot SKB.
> 

There's two problems:
1. IIRC a single page ring has 256 slots, allowing 64 slots packet
   yields 4 in-flight packets in worst case.
2. Older netback could not handle this large number of slots and it's
   likely to deem the frontend malicious.

For #1, we don't actually care that much if guest screws itself by
generating 64 slot packets. #2 is more concerning.

Wei.

> And I don't think it's so theoretical, a carefully crafted sequence of
> sendfile() calls during a TCP_CORK sequence should be able to do it.
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