netdev - Re: [PATCH v4 bpf 1/2] bpf: fix skb_do

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Date: Mon, 31 Jul 2023 15:11:27 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Yan Zhai <yan@...udflare.com>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, linux-kselftest@...r.kernel.org,
 kernel-team@...udflare.com, Jordan Griege <jgriege@...udflare.com>,
 Markus Elfring <Markus.Elfring@....de>, Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values

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