Re: [PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values

[Yan Zhai]

On Fri, Jul 28, 2023 at 5:02 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
>
> On 7/25/23 6:08 PM, 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. Commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel
> > infrastructure") didn't check the return code correctly, so positive
> > values are propagated back along call chain:
> >
> >    ip_finish_output2
> >      -> bpf_xmit
> >        -> run_lwt_bpf
> >          -> skb_do_redirect
>
>  From looking at skb_do_redirect, the skb_do_redirect should have consumed the
> skb except for the -EAGAIN return value. afaik, -EAGAIN could only happen by
> using the bpf_redirect_peer helper. lwt does not have the bpf_redirect_peer
> helper available, so there is no -EAGAIN case in lwt. iow, skb_do_redirect
> should have always consumed the skb in lwt. or did I miss something?
>
> If that is the case, it feels like the fix should be in run_lwt_bpf() and the
> "if (ret == 0)" test in run_lwt_bpf() is unnecessary?
>
>                         ret = skb_do_redirect(skb);
>                         if (ret == 0)
>                                 ret = BPF_REDIRECT;
>
>
Just fixing skb redirect return code won't be sufficient. I realized
there are other return paths that need to be treated, e.g. bpf reroute
path also directly returns dev_queue_xmit status. I plan to check for
LWTUNNEL_XMIT_CONTINUE (and change it to a value that does not
conflict with NET_RX_DROP and NET_XMIT_DROP) in the next revision. On
the other hand, the return value of NETDEV_TX_BUSY is another hassle.
As Dan suggested, packets might not have been freed when this is
returned from drivers. The caller of dev_queue_xmit might need to free
skb when this happens.

Yan