Re: bridge:fragmented packets dropped by bridge

[Vadim Fedorenko]

On 07.10.2022 12:21, Vyacheslav Salnikov wrote:
> Hi.
>
> I switched from kernel versions 4.9 to 5.15 and found that the MTU on
> the interfaces in the bridge does not change.
> For example:
> I have the following bridge:
> bridge      interface
> br0          sw1
>                 sw2
>                 sw3
>
> And I change with ifconfig MTU.
> I see that br0 sw1..sw3 has changed MTU from 1500 -> 1982.
>
> But if i send a ping through these interfaces, I get 1500(I added
> prints for output)
> I investigated the code and found the reason:
> The following commit came in the new kernel:
> https://github.com/torvalds/linux/commit/ac6627a28dbfb5d96736544a00c3938fa7ea6dfb
>
> And the behavior of the MTU setting has changed:
> > Kernel 4.9:
> > if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
> >     ip_mtu_locked(dst) ||
> >     !forwarding)  <--- True
> > return dst_mtu(dst) <--- 1982
> >
> >
> > / 'forwarding = true' case should always honour route mtu /
> > mtu = dst_metric_raw(dst, RTAX_MTU);
> > if (mtu)
> > return mtu;
>
>
>
> Kernel 5.15:
> > if (READ_ONCE(net->ipv4.sysctl_ip_fwd_use_pmtu) ||
> >     ip_mtu_locked(dst) ||
> >     !forwarding) { <--- True
> > mtu = rt->rt_pmtu;  <--- 0
> > if (mtu && time_before(jiffies, rt->dst.expires)) <-- False
> > goto out;
> > }
> >
> > / 'forwarding = true' case should always honour route mtu /
> > mtu = dst_metric_raw(dst, RTAX_MTU); <---- 1500
> > if (mtu) <--- True
> > goto out;
>
> As I see from the code in the end takes mtu from br_dst_default_metrics
> > static const u32 br_dst_default_metrics[RTAX_MAX] = {
> > [RTAX_MTU - 1] = 1500,
> > };
>
> Why is rt_pmtu now used instead of dst_mtu?
> Why is forwarding = False called with dst_metric_raw?
> Maybe we should add processing when mtu = rt->rt_pmtu == 0?
> Could this be an error?
If you compare ipv4_mtu code from 4.9 you will see that the very first mtu value 
is filled by rt->rt_pmtu value. I believe there were changes to the bridge code 
where rt_pmtu value got empty or cleared.

I'm still looking for the root cause of the problem, will update you once I find it.


> I found a thread discussing a similar problem. It suggested porting
> the next patch:
> Signed-off-by: Rundong Ge <rdong.ge@xxxxxxxxx>
> ---
>   include/net/ip.h | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/include/net/ip.h b/include/net/ip.h
> index 29d89de..0512de3 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -450,6 +450,8 @@ static inline unsigned int
> ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
>   static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
>      const struct sk_buff *skb)
>   {
> + if ((skb_dst(skb)->flags & DST_FAKE_RTABLE) && skb->dev)
> + return min(skb->dev->mtu, IP_MAX_MTU);
>    if (!sk || !sk_fullsock(sk) || ip_sk_use_pmtu(sk)) {
>    bool forwarding = IPCB(skb)->flags & IPSKB_FORWARDED;
>
>
> Why was this patch not accepted in the end?