Subject: Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy" series.

From: Zoltan Kiss <[email-masked]>
Date: Fri, 2 May 2014 16:26:33 +0100

On 02/05/14 16:21, Eric Dumazet wrote:
> On Fri, 2014-05-02 at 15:47 +0100, Zoltan Kiss wrote:
>
>> Sorry, I was misleading and wrong. Can you try out this scenario with
>> the attached patch?
>
> Guys, I already told you skb->truesize 'mismatch' could not explain
> packet corruptions. This comes from an expert in this matter, you can
> trust me.
>
> What could happens here is that TCP stack merges skbs (TCP coalescing)
These packets shouldn't reach Dom0's TCP stack at all, 
bridge/openvswitch grabs them before. And in the sending/receiving guest 
these skbs don't have this flag.
However generally it is possible that a guest talks directly to Dom0, in 
which case your proposed fix could be valid.
>
> Problem is that SKBTX_DEV_ZEROCOPY addition did not take care of this.
>
> We have to forbid these merges from happening, because one skb has a
> single destructor_arg.
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1b62343f5837..85995a14aafc 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3838,7 +3839,10 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>   		return true;
>   	}
>
> -	if (skb_has_frag_list(to) || skb_has_frag_list(from))
> +	if (skb_has_frag_list(to) ||
> +	    skb_has_frag_list(from) ||
> +	    (skb_shinfo(to)->tx_flags & SKBTX_DEV_ZEROCOPY) ||
> +	    (skb_shinfo(from)->tx_flags & SKBTX_DEV_ZEROCOPY))
>   		return false;
>
>   	if (skb_headlen(from) != 0) {
>
>
>
>
>
>