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

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/31/23 2:35 PM, Yan Zhai wrote:
> On Fri, Jul 28, 2023 at 5:02 PM Martin KaFai Lau <martin.lau@linux.dev> 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.

I suspect we are talking about different things or I am still missing something.

I was thinking skb_do_redirect() should have always consumed the skb and 
bpf_xmit should always return LWTUNNEL_XMIT_DONE also (instead of 
LWTUNNEL_XMIT_CONTINUE described in the this patch commit message). It is what 
sch_handle_egress() is doing also. Could you explain how is it different from 
the skb_do_redirect usage in sch_handle_egress() or you are suggesting the 
current sch_handle_egress() has the issue too also?


> 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