Re: [PATCH net-next 8/9] net: tcp: add skb drop reasons to tcp tw code path

Eric Dumazet
2022-05-16T04:22:46Z

On Sun, May 15, 2022 at 8:46 PM <menglong8.dong@gmail.com> wrote:
>
> From: Menglong Dong <imagedong@tencent.com>
>
> In order to get the reasons of skb drops, add a function argument of
> type 'enum skb_drop_reason *reason' to tcp_timewait_state_process().
>
> In the origin code, all packets to time-wait socket are treated as
> dropping with kfree_skb(), which can make users confused. Therefore,
> we use consume_skb() for the skbs that are 'good'. We can check the
> value of 'reason' to decide use kfree_skb() or consume_skb().
>
> The new reason 'TIMEWAIT' is added for the case that the skb is dropped
> as the socket in time-wait state.
>
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
> include/linux/skbuff.h | 5 +++++
> include/net/tcp.h | 7 ++++---
> net/ipv4/tcp_ipv4.c | 11 +++++++++--
> net/ipv4/tcp_minisocks.c | 24 ++++++++++++++++++++----
> net/ipv6/tcp_ipv6.c | 10 ++++++++--
> 5 files changed, 46 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 4578bbab5a3e..8d18fc5a5af6 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -560,6 +560,10 @@ struct sk_buff;
> * SKB_DROP_REASON_TCP_REQQFULLDROP
> * request queue of the listen socket is full, corresponding to
> * LINUX_MIB_TCPREQQFULLDROP
> + *
> + * SKB_DROP_REASON_TIMEWAIT
> + * socket is in time-wait state and all packet that received will
> + * be treated as 'drop', except a good 'SYN' packet
> */
> #define __DEFINE_SKB_DROP_REASON(FN) \
> FN(NOT_SPECIFIED) \
> @@ -631,6 +635,7 @@ struct sk_buff;
> FN(TCP_ABORTONDATA) \
> FN(LISTENOVERFLOWS) \
> FN(TCP_REQQFULLDROP) \
> + FN(TIMEWAIT) \
> FN(MAX)
>
> /* The reason of skb drop, which is used in kfree_skb_reason().
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 082dd0627e2e..88217b8d95ac 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -380,9 +380,10 @@ enum tcp_tw_status {
> };
>
>
> -enum tcp_tw_status tcp_timewait_state_process(struct inet_timewait_sock *tw,
> - struct sk_buff *skb,
> - const struct tcphdr *th);
> +enum tcp_tw_status
> +tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> + const struct tcphdr *th,
> + enum skb_drop_reason *reason);
> struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
> struct request_sock *req, bool fastopen,
> bool *lost_race);
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 708f92b03f42..9174ee162633 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -2134,7 +2134,8 @@ int tcp_v4_rcv(struct sk_buff *skb)
> inet_twsk_put(inet_twsk(sk));
> goto csum_error;
> }
> - switch (tcp_timewait_state_process(inet_twsk(sk), skb, th)) {
> + switch (tcp_timewait_state_process(inet_twsk(sk), skb, th,
> + &drop_reason)) {
> case TCP_TW_SYN: {
> struct sock *sk2 = inet_lookup_listener(dev_net(skb->dev),
> &tcp_hashinfo, skb,
> @@ -2150,12 +2151,18 @@ int tcp_v4_rcv(struct sk_buff *skb)
> refcounted = false;
> goto process;
> }
> + /* TCP_FLAGS or NO_SOCKET? */
> + SKB_DR_SET(drop_reason, TCP_FLAGS);
> }
> /* to ACK */
> fallthrough;
> case TCP_TW_ACK:
> tcp_v4_timewait_ack(sk, skb);
> - break;
> + refcounted = false;
> + if (drop_reason)
> + goto discard_it;
> + else
> + goto put_and_return;
> case TCP_TW_RST:
> tcp_v4_send_reset(sk, skb);
> inet_twsk_deschedule_put(inet_twsk(sk));
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index 1a21018f6f64..329724118b7f 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -83,13 +83,15 @@ tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
> */
> enum tcp_tw_status
> tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> - const struct tcphdr *th)
> + const struct tcphdr *th,
> + enum skb_drop_reason *reason)
> {
> struct tcp_options_received tmp_opt;
> struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
> bool paws_reject = false;
>
> tmp_opt.saw_tstamp = 0;
> + *reason = SKB_DROP_REASON_NOT_SPECIFIED;
> if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
> tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
>
> @@ -113,11 +115,16 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> return tcp_timewait_check_oow_rate_limit(
> tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
>
> - if (th->rst)
> + if (th->rst) {
> + SKB_DR_SET(*reason, TCP_RESET);
> goto kill;
> + }
>
> - if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
> + if (th->syn && !before(TCP_SKB_CB(skb)->seq,
> + tcptw->tw_rcv_nxt)) {
> + SKB_DR_SET(*reason, TCP_FLAGS);
> return TCP_TW_RST;
> + }
>
> /* Dup ACK? */
> if (!th->ack ||
> @@ -143,6 +150,9 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> }
>
> inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
> +
> + /* skb should be free normally on this case. */
> + *reason = SKB_NOT_DROPPED_YET;
> return TCP_TW_ACK;
> }
>
> @@ -174,6 +184,7 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> * protocol bug yet.
> */
> if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
> + SKB_DR_SET(*reason, TCP_RESET);
> kill:
> inet_twsk_deschedule_put(tw);
> return TCP_TW_SUCCESS;
> @@ -216,11 +227,14 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> if (isn == 0)
> isn++;
> TCP_SKB_CB(skb)->tcp_tw_isn = isn;
> + *reason = SKB_NOT_DROPPED_YET;
> return TCP_TW_SYN;
> }
>
> - if (paws_reject)
> + if (paws_reject) {
> + SKB_DR_SET(*reason, TCP_RFC7323_PAWS);
> __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
> + }
>
> if (!th->rst) {
> /* In this case we must reset the TIMEWAIT timer.
> @@ -232,9 +246,11 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
> if (paws_reject || th->ack)
> inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
>
> + SKB_DR_OR(*reason, TIMEWAIT);
> return tcp_timewait_check_oow_rate_limit(
> tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
> }
> + SKB_DR_SET(*reason, TCP_RESET);
> inet_twsk_put(tw);
> return TCP_TW_SUCCESS;
> }
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 27c51991bd54..5c777006de3d 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1795,7 +1795,8 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
> goto csum_error;
> }
>
> - switch (tcp_timewait_state_process(inet_twsk(sk), skb, th)) {
> + switch (tcp_timewait_state_process(inet_twsk(sk), skb, th,
> + &drop_reason)) {
> case TCP_TW_SYN:
> {
> struct sock *sk2;
> @@ -1815,12 +1816,17 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
> refcounted = false;
> goto process;
> }
> + SKB_DR_SET(drop_reason, TCP_FLAGS);
> }
> /* to ACK */
> fallthrough;
> case TCP_TW_ACK:
> tcp_v6_timewait_ack(sk, skb);
> - break;
> + refcounted = false;
> + if (drop_reason)
> + goto discard_it;
> + else
> + goto put_and_return;

My brain exploded.
I guarantee you that whoever is going to look at this code in one year
will be completely lost.
Adding so much complexity is crazy.

About: refcounted = false ;

This is not going to be used id "goto discard_it;" is taken.

If the "goto put_and_return;" is taken, this boils down to:

return ret ? -1 : 0;

Are you sure ret is even initialized at this point ?

Why are we doing this ? We were doing "return 0;"

Also, where is skb freed ?

Are you calling tcp_v6_timewait_ack(sk, skb) after skb has been freed ???

> case TCP_TW_RST:
> tcp_v6_send_reset(sk, skb);
> inet_twsk_deschedule_put(inet_twsk(sk));
> --
> 2.36.1
>