Re: [PATCH net-next v3 3/3] net: tcp: fix unexcepted socket die when snd_wnd is 0
From: Eric Dumazet <edumazet@google.com>
Date: 2023-08-08 16:47:49
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On Tue, Aug 8, 2023 at 1:59 PM [off-list ref] wrote:
quoted hunk ↗ jump to hunk
From: Menglong Dong <redacted> In tcp_retransmit_timer(), a window shrunk connection will be regarded as timeout if 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX'. This is not right all the time. The retransmits will become zero-window probes in tcp_retransmit_timer() if the 'snd_wnd==0'. Therefore, the icsk->icsk_rto will come up to TCP_RTO_MAX sooner or later. However, the timer can be delayed and be triggered after 122877ms, not TCP_RTO_MAX, as I tested. Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' is always true once the RTO come up to TCP_RTO_MAX, and the socket will die. Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->icsk_timeout', which is exact the timestamp of the timeout. Meanwhile, using the later one of tp->retrans_stamp and tp->rcv_tstamp as the last updated timestamp in the receiving path, as "tp->rcv_tstamp" can restart from idle, then tp->rcv_tstamp could already be a long time (minutes or hours) in the past even on the first RTO. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Link: https://lore.kernel.org/netdev/CADxym3YyMiO+zMD4zj03YPM3FBi-1LHi6gSD2XT8pyAMM096pg@mail.gmail.com/ (local) Signed-off-by: Menglong Dong <redacted> --- v3: - use after() instead of max() in tcp_rtx_probe0_timed_out() v2: - consider the case of the connection restart from idle, as Neal comment --- net/ipv4/tcp_timer.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c index d45c96c7f5a4..f30d1467771c 100644 --- a/net/ipv4/tcp_timer.c +++ b/net/ipv4/tcp_timer.c@@ -454,6 +454,18 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req) req->timeout << req->num_timeout, TCP_RTO_MAX); } +static bool tcp_rtx_probe0_timed_out(struct sock *sk)
const struct sock *sk
+{
+ struct tcp_sock *tp = tcp_sk(sk);const struct tcp_sock *tp = tcp_sk(sk);
+ u32 timeout_ts, rtx_ts, rcv_ts; + + rtx_ts = tp->retrans_stamp; + rcv_ts = tp->rcv_tstamp; + timeout_ts = after(rtx_ts, rcv_ts) ? rtx_ts : rcv_ts; + timeout_ts += TCP_RTO_MAX;
If we are concerned with a socket dying too soon, I would suggest
adding 2*TCP_RTO_MAX instead of TCP_RTO_MAX
When a receiver is OOMing, it is possible the ACK RWIN 0 can not be sent all,
so tp->rcv_tstamp will not be refreshed. Or ACK could be lost in the
network path.
This also suggests the net_dbg_ratelimited("Peer %pI4:%u/%u
unexpectedly shrunk window %u:%u (repaired)\n"...) messages
are slightly wrong, because they could be printed even if we did not
receive a new ACK packet from the remote peer.
Perhaps we should change them to include delays (how long @skb stayed
in rtx queue, how old is the last ACK we received)
quoted hunk ↗ jump to hunk
+ + return after(inet_csk(sk)->icsk_timeout, timeout_ts); +} /** * tcp_retransmit_timer() - The TCP retransmit timeout handler@@ -519,7 +531,7 @@ void tcp_retransmit_timer(struct sock *sk) tp->snd_una, tp->snd_nxt); } #endif - if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) { + if (tcp_rtx_probe0_timed_out(sk)) { tcp_write_err(sk); goto out; } --2.40.1