Subject: Re: [PATCH v3 bpf 1/2] bpf: fix skb_do_redirect return values

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 25 Jul 2023 10:07:37 -0700

On 07/25, Yan Zhai wrote:
> On Tue, Jul 25, 2023 at 4:14 AM Jakub Sitnicki <[email-masked]> wrote:
> >
> > On Mon, Jul 24, 2023 at 09:13 PM -07, Yan Zhai wrote:
> > > skb_do_redirect returns various of values: error code (negative), 0
> > > (success), and some positive status code, e.g. NET_XMIT_CN, NET_RX_DROP.
> > > Such code are not handled at lwt xmit hook in function ip_finish_output2
> > > and ip6_finish_output, which can cause unexpected problems. This change
> > > converts the positive status code to proper error code.
> > >
> > > Suggested-by: Stanislav Fomichev <[email-masked]>
> > > Reported-by: Jordan Griege <[email-masked]>
> > > Signed-off-by: Yan Zhai <[email-masked]>
> > >
> > > ---
> > > v3: converts also RX side return value in addition to TX values
> > > v2: code style change suggested by Stanislav Fomichev
> > > ---
> > >  net/core/filter.c | 12 +++++++++++-
> > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 06ba0e56e369..3e232ce11ca0 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -2095,7 +2095,12 @@ static const struct bpf_func_proto bpf_csum_level_proto = {
> > >
> > >  static inline int __bpf_rx_skb(struct net_device *dev, struct sk_buff *skb)
> > >  {
> > > -     return dev_forward_skb_nomtu(dev, skb);
> > > +     int ret = dev_forward_skb_nomtu(dev, skb);
> > > +
> > > +     if (unlikely(ret > 0))
> > > +             return -ENETDOWN;
> > > +
> > > +     return 0;
> > >  }
> > >
> > >  static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
> > > @@ -2106,6 +2111,8 @@ static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
> > >       if (likely(!ret)) {
> > >               skb->dev = dev;
> > >               ret = netif_rx(skb);
> > > +     } else if (ret > 0) {
> > > +             return -ENETDOWN;
> > >       }
> > >
> > >       return ret;
> > > @@ -2129,6 +2136,9 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
> > >       ret = dev_queue_xmit(skb);
> > >       dev_xmit_recursion_dec();
> > >
> > > +     if (unlikely(ret > 0))
> > > +             ret = net_xmit_errno(ret);
> > > +
> > >       return ret;
> > >  }
> >
> > net_xmit_errno maps NET_XMIT_DROP to -ENOBUFS. It would make sense to me
> > to map NET_RX_DROP to -ENOBUFS as well, instead of -ENETDOWN, to be
> > consistent.
> >
> In fact I looked at all those errno, but found none actually covers
> this situation completely. For the redirect ingress case, there are
> three reasons to fail: backlog full, dev down, and MTU issue. This
> won't be a problem for typical RX paths, since the error code is
> usually discarded by call sites of deliver_skb. But redirect ingress
> opens a call chain that would propagate this error to local sendmsg,
> which may be very confusing to troubleshoot in a complex environment
> (especially when backlog fills).
> 
> That said I agree ENOBUF covers the most likely reason to fail
> (backlog). Let me change to that one in the next version if there are
> no new suggestions.

nit: also maybe wrap these rx paths into some new net_rx_errno ?
To mirror the tx side.
 
> > It looks like the Fixes tag for this should point to the change that
> > introduced BPF for LWT:
> >
> > Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
> >
> Thanks for finding the tag. I was debating if it should be LWT commit
> or bpf_redirect commit: the error is not handled at LWT, but it seems
> actually innocent. The actual fix is the return value from the bpf
> redirect code. Let me incorporate both in the next one to justify
> better.
> 
> --
> Yan