答复: [PATCH] net: Expand headroom to send fragmented packets in bridge fragment forward

["Yang Huajian（杨华健）" <huajianyang@asrmicro.com>]

Thank you for your reply!

> 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?

Maybe I can't demonstrate this problem with a shell script,
I actually discovered this problem while debugging a wifi network device.
This netdevice is set a large needed_headroom(80), so ll_rs is oversize and goto blackhole.

We can easily to reproduce it by configing needed_headroom in a netdevice,
then add this netdevice to a bridge, and test bridge forwarding.

ping large packet could reproduce this appearance.(successful if not enable NF_CONNTRACK_BRIDGE)

> 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.

Yes, your guess is correct!

Actual problem: I think it is unreasonable to directly drop skb with insufficient headroom.

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

Using skb_expand_head has the same effect.

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

I tested it just now, reuse the slowpath will successed.
But maybe this change cannot resolve all cases if the netdevice really needs this headroom.

Best Regards,
Huajian

-----邮件原件-----
发件人: Florian Westphal [mailto:fw@strlen.de] 
发送时间: 2025年4月9日 17:18
收件人: Yang Huajian（杨华健） <huajianyang@asrmicro.com>
抄送: pablo@netfilter.org; kadlec@netfilter.org; razor@blackwall.org; idosch@nvidia.com; davem@davemloft.net; dsahern@kernel.org; edumazet@google.com; kuba@kernel.org; pabeni@redhat.com; horms@kernel.org; netfilter-devel@vger.kernel.org; coreteam@netfilter.org; bridge@lists.linux.dev; netdev@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: [PATCH] net: Expand headroom to send fragmented packets in bridge fragment forward

Huajian Yang <huajianyang@asrmicro.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?