linux-kernel - Re: [PATCH v3 bpf 1/2] bpf: fix skb_do

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Date:   Tue, 25 Jul 2023 11:08:15 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
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>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        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,
        Jordan Griege <jgriege@...udflare.com>,
        kernel-team@...udflare.com
Subject: Re: [PATCH v3 bpf 1/2] bpf: fix skb_do_redirect return values

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 <sdf@...gle.com>
> Reported-by: Jordan Griege <jgriege@...udflare.com>
> Signed-off-by: Yan Zhai <yan@...udflare.com>
>
> ---
> 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.

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")