netdev - Re: [PATCH bpf-next] udp, bpf: Ignore connections in reuseport group after BPF sk lookup

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Date:   Mon, 27 Jul 2020 18:20:42 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <kernel-team@...udflare.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH bpf-next] udp, bpf: Ignore connections in reuseport group
 after BPF sk lookup

On Sun, Jul 26, 2020 at 02:02:28PM +0200, Jakub Sitnicki wrote:
> When BPF sk lookup invokes reuseport handling for the selected socket, it
> should ignore the fact that reuseport group can contain connected UDP
> sockets. With BPF sk lookup this is not relevant as we are not scoring
> sockets to find the best match, which might be a connected UDP socket.
> 
> Fix it by unconditionally accepting the socket selected by reuseport.
> 
> This fixes the following two failures reported by test_progs.
> 
>   # ./test_progs -t sk_lookup
>   ...
>   #73/14 UDP IPv4 redir and reuseport with conns:FAIL
>   ...
>   #73/20 UDP IPv6 redir and reuseport with conns:FAIL
>   ...
> 
> Fixes: a57066b1a019 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
> Cc: David S. Miller <davem@...emloft.net>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  net/ipv4/udp.c | 2 +-
>  net/ipv6/udp.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 7ce31beccfc2..e88efba07551 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -473,7 +473,7 @@ static struct sock *udp4_lookup_run_bpf(struct net *net,
>  		return sk;
>  
>  	reuse_sk = lookup_reuseport(net, sk, skb, saddr, sport, daddr, hnum);
> -	if (reuse_sk && !reuseport_has_conns(sk, false))
> +	if (reuse_sk)
>  		sk = reuse_sk;
>  	return sk;
>  }
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index c394e674f486..29d9691359b9 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -208,7 +208,7 @@ static inline struct sock *udp6_lookup_run_bpf(struct net *net,
>  		return sk;
>  
>  	reuse_sk = lookup_reuseport(net, sk, skb, saddr, sport, daddr, hnum);
> -	if (reuse_sk && !reuseport_has_conns(sk, false))
> +	if (reuse_sk)
>From __udp[46]_lib_lookup, 
1. The connected udp is picked by the kernel first.
   If a 4-tuple-matched connected udp is found.  It should have already
   been returned there.

2. If kernel cannot find a connected udp, the sk-lookup bpf prog can
   get a chance to pick another socket (likely bound to a different
   IP/PORT that the packet is destinated to) by bpf_sk_lookup_assign().
   However, bpf_sk_lookup_assign() does not allow TCP_ESTABLISHED.

   With the change in this patch, it then allows the reuseport-bpf-prog
   to pick a connected udp which cannot be found in step (1).  Can you
   explain a use case for this?