netdev - Re: [PATCH] net: Expand headroom to send fragmented packets in bridge fragment forward

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Message-ID: <20250409091821.GA17911@breakpoint.cc>
Date: Wed, 9 Apr 2025 11:18:21 +0200
From: Florian Westphal <fw@...len.de>
To: Huajian Yang <huajianyang@...micro.com>
Cc: pablo@...filter.org, kadlec@...filter.org, razor@...ckwall.org,
	idosch@...dia.com, davem@...emloft.net, dsahern@...nel.org,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	horms@...nel.org, netfilter-devel@...r.kernel.org,
	coreteam@...filter.org, bridge@...ts.linux.dev,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: Expand headroom to send fragmented packets in
 bridge fragment forward

Huajian Yang <huajianyang@...micro.com> wrote:
> The config NF_CONNTRACK_BRIDGE will change the way fragments are processed.
> Bridge does not know that it is a fragmented packet and forwards it
> directly, after NF_CONNTRACK_BRIDGE is enabled, function nf_br_ip_fragment
> will check and fraglist this packet.
> 
> Some network devices that would not able to ping large packet under bridge,
> but large packet ping is successful if not enable NF_CONNTRACK_BRIDGE.

Can you add a new test to tools/testing/selftests/net/netfilter/ that
demonstrates this problem?

> In function nf_br_ip_fragment, checking the headroom before sending is
> undoubted, but it is unreasonable to directly drop skb with insufficient
> headroom.

Are we talking about
if (first_len - hlen > mtu
  or
skb_headroom(skb) < ll_rs)

?

>  
>  		if (first_len - hlen > mtu ||
>  		    skb_headroom(skb) < ll_rs)
> -			goto blackhole;
> +			goto expand_headroom;

I guess this should be

if (first_len - hlen > mtu)
	goto blackhole;
if (skb_headroom(skb) < ll_rs)
	goto expand_headroom;

... but I'm not sure what the actual problem is.

> +expand_headroom:
> +	struct sk_buff *expand_skb;
> +
> +	expand_skb = skb_copy_expand(skb, ll_rs, skb_tailroom(skb), GFP_ATOMIC);
> +	if (unlikely(!expand_skb))
> +		goto blackhole;

Why does this need to make a full skb copy?
Should that be using skb_expand_head()?

>  slow_path:

Actually, can't you just (re)use the slowpath for the skb_headroom < ll_rs
case instead of adding headroom expansion?