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[79.191.181.173]) by smtp.gmail.com with ESMTPSA id s7-20020a170906a18700b00929fc8d264dsm7197146ejy.17.2023.04.12.03.28.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Apr 2023 03:28:10 -0700 (PDT) References: <20230407171654.107311-1-john.fastabend@gmail.com> <20230407171654.107311-6-john.fastabend@gmail.com> User-agent: mu4e 1.6.10; emacs 28.2 From: Jakub Sitnicki To: John Fastabend Cc: daniel@iogearbox.net, lmb@isovalent.com, edumazet@google.com, bpf@vger.kernel.org, netdev@vger.kernel.org, ast@kernel.org, andrii@kernel.org, will@isovalent.com Subject: Re: [PATCH bpf v6 05/12] bpf: sockmap, TCP data stall on recv before accept Date: Wed, 12 Apr 2023 12:26:04 +0200 In-reply-to: <20230407171654.107311-6-john.fastabend@gmail.com> Message-ID: <87h6tlgzt3.fsf@cloudflare.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Apr 07, 2023 at 10:16 AM -07, John Fastabend wrote: > A common mechanism to put a TCP socket into the sockmap is to hook the > BPF_SOCK_OPS_{ACTIVE_PASSIVE}_ESTABLISHED_CB event with a BPF program > that can map the socket info to the correct BPF verdict parser. When > the user adds the socket to the map the psock is created and the new > ops are assigned to ensure the verdict program will 'see' the sk_buffs > as they arrive. > > Part of this process hooks the sk_data_ready op with a BPF specific > handler to wake up the BPF verdict program when data is ready to read. > The logic is simple enough (posted here for easy reading) > > static void sk_psock_verdict_data_ready(struct sock *sk) > { > struct socket *sock = sk->sk_socket; > > if (unlikely(!sock || !sock->ops || !sock->ops->read_skb)) > return; > sock->ops->read_skb(sk, sk_psock_verdict_recv); > } > > The oversight here is sk->sk_socket is not assigned until the application > accepts() the new socket. However, its entirely ok for the peer application > to do a connect() followed immediately by sends. The socket on the receiver > is sitting on the backlog queue of the listening socket until its accepted > and the data is queued up. If the peer never accepts the socket or is slow > it will eventually hit data limits and rate limit the session. But, > important for BPF sockmap hooks when this data is received TCP stack does > the sk_data_ready() call but the read_skb() for this data is never called > because sk_socket is missing. The data sits on the sk_receive_queue. > > Then once the socket is accepted if we never receive more data from the > peer there will be no further sk_data_ready calls and all the data > is still on the sk_receive_queue(). Then user calls recvmsg after accept() > and for TCP sockets in sockmap we use the tcp_bpf_recvmsg_parser() handler. > The handler checks for data in the sk_msg ingress queue expecting that > the BPF program has already run from the sk_data_ready hook and enqueued > the data as needed. So we are stuck. > > To fix do an unlikely check in recvmsg handler for data on the > sk_receive_queue and if it exists wake up data_ready. We have the sock > locked in both read_skb and recvmsg so should avoid having multiple > runners. > > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()") > Signed-off-by: John Fastabend > --- > net/ipv4/tcp_bpf.c | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c > index 804bd0c247d0..ae6c7130551c 100644 > --- a/net/ipv4/tcp_bpf.c > +++ b/net/ipv4/tcp_bpf.c > @@ -212,6 +212,26 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > return tcp_recvmsg(sk, msg, len, flags, addr_len); > > lock_sock(sk); > + > + /* We may have received data on the sk_receive_queue pre-accept and > + * then we can not use read_skb in this context because we haven't > + * assigned a sk_socket yet so have no link to the ops. The work-around > + * is to check the sk_receive_queue and in these cases read skbs off > + * queue again. The read_skb hook is not running at this point because > + * of lock_sock so we avoid having multiple runners in read_skb. > + */ > + if (unlikely(!skb_queue_empty(&sk->sk_receive_queue))) { > + tcp_data_ready(sk); > + /* This handles the ENOMEM errors if we both receive data > + * pre accept and are already under memory pressure. At least > + * let user no to retry. Nit: s/no/know/ > + */ > + if (unlikely(!skb_queue_empty(&sk->sk_receive_queue))) { > + copied = -EAGAIN; > + goto out; > + } > + } > + > msg_bytes_ready: > copied = sk_msg_recvmsg(sk, psock, msg, len, flags); > /* The typical case for EFAULT is the socket was gracefully Similar to patch 04/12, we will need this corner case fix in tcp_bpf_recvmsg as well. Reviewed-by: Jakub Sitnicki