[Bridge] [PATCH net v2] net: bridge: Fix ethernet header pointer before check skb forwardable

[wangyunjian]

> -----Original Message-----
> From: Toshiaki Makita [mailto:makita.toshiaki at lab.ntt.co.jp]
> Sent: Tuesday, January 15, 2019 3:50 PM
> To: wangyunjian <wangyunjian at huawei.com>
> Cc: netdev at vger.kernel.org; xudingke <xudingke at huawei.com>;
> bridge at lists.linux-foundation.org; Nkolay Aleksandrov
> <nikolay at cumulusnetworks.com>; Roopa Prabhu
> <roopa at cumulusnetworks.com>
> Subject: Re: [PATCH net v2] net: bridge: Fix ethernet header pointer before
> check skb forwardable
> 
> On 2019/01/15 12:12, wangyunjian wrote:
> > From: Yunjian Wang <wangyunjian at huawei.com>
> >
> > The skb header should be set to ethernet header before using
> > is_skb_forwardable. Because the ethernet header length has been
> > considered in is_skb_forwardable(including dev->hard_header_len
> > length).
> >
> > To reproduce the issue:
> > 1, add 2 ports on linux bridge br using following commands:
> > $ brctl addbr br
> > $ brctl addif br eth0
> > $ brctl addif br eth1
> > 2, the mtu of eth0 and eth1 is 1500
> > 3, send a 1504 packet from eth0 to eth1
> >
> > So the expect result is packet larger than 1500 cannot pass through
> > eth0 and eth1. But currently, the packet passes through success, it
> > means eth1's mtu limit donen't take effect.
> 
> I agree with the change but this test does not look appropriate.
> In the first place is_skb_forwardable() accepts 4 bytes extra size (for VLAN
> header). Doesn't your test case fail even after the change?
> 
> --
> Toshiaki Makita

Before executing the functions is_skb_forwardable() and skb_push(), the actual
packet consist of the follwing parts, Data 1480, UDP 8, IP 20, Ethernet 14, VLAN 4.
And the value of variables are dev->mtu=1500, dev->hard_header_len=14 and
skb->len=1508. The packet cannot pass through.

I will fix the previous description of packet len.

Thanks,
	Yunjian