diff options
author | Neal Cardwell <ncardwell@google.com> | 2024-10-01 20:05:15 +0000 |
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committer | Jakub Kicinski <kuba@kernel.org> | 2024-10-03 16:18:03 -0700 |
commit | e37ab7373696e650d3b6262a5b882aadad69bb9e (patch) | |
tree | 845f9f4371990d3d636f1bc1415aa1a9b87b4582 | |
parent | ec636707f757474c959dc4e90f58e32aa9eb060a (diff) | |
download | lwn-e37ab7373696e650d3b6262a5b882aadad69bb9e.tar.gz lwn-e37ab7373696e650d3b6262a5b882aadad69bb9e.zip |
tcp: fix to allow timestamp undo if no retransmits were sent
Fix the TCP loss recovery undo logic in tcp_packet_delayed() so that
it can trigger undo even if TSQ prevents a fast recovery episode from
reaching tcp_retransmit_skb().
Geumhwan Yu <geumhwan.yu@samsung.com> recently reported that after
this commit from 2019:
commit bc9f38c8328e ("tcp: avoid unconditional congestion window undo
on SYN retransmit")
...and before this fix we could have buggy scenarios like the
following:
+ Due to reordering, a TCP connection receives some SACKs and enters a
spurious fast recovery.
+ TSQ prevents all invocations of tcp_retransmit_skb(), because many
skbs are queued in lower layers of the sending machine's network
stack; thus tp->retrans_stamp remains 0.
+ The connection receives a TCP timestamp ECR value echoing a
timestamp before the fast recovery, indicating that the fast
recovery was spurious.
+ The connection fails to undo the spurious fast recovery because
tp->retrans_stamp is 0, and thus tcp_packet_delayed() returns false,
due to the new logic in the 2019 commit: commit bc9f38c8328e ("tcp:
avoid unconditional congestion window undo on SYN retransmit")
This fix tweaks the logic to be more similar to the
tcp_packet_delayed() logic before bc9f38c8328e, except that we take
care not to be fooled by the FLAG_SYN_ACKED code path zeroing out
tp->retrans_stamp (the bug noted and fixed by Yuchung in
bc9f38c8328e).
Note that this returns the high-level behavior of tcp_packet_delayed()
to again match the comment for the function, which says: "Nothing was
retransmitted or returned timestamp is less than timestamp of the
first retransmission." Note that this comment is in the original
2005-04-16 Linux git commit, so this is evidently long-standing
behavior.
Fixes: bc9f38c8328e ("tcp: avoid unconditional congestion window undo on SYN retransmit")
Reported-by: Geumhwan Yu <geumhwan.yu@samsung.com>
Diagnosed-by: Geumhwan Yu <geumhwan.yu@samsung.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241001200517.2756803-2-ncardwell.sw@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
-rw-r--r-- | net/ipv4/tcp_input.c | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index cc05ec1faac8..233b77890795 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2473,8 +2473,22 @@ static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp, */ static inline bool tcp_packet_delayed(const struct tcp_sock *tp) { - return tp->retrans_stamp && - tcp_tsopt_ecr_before(tp, tp->retrans_stamp); + const struct sock *sk = (const struct sock *)tp; + + if (tp->retrans_stamp && + tcp_tsopt_ecr_before(tp, tp->retrans_stamp)) + return true; /* got echoed TS before first retransmission */ + + /* Check if nothing was retransmitted (retrans_stamp==0), which may + * happen in fast recovery due to TSQ. But we ignore zero retrans_stamp + * in TCP_SYN_SENT, since when we set FLAG_SYN_ACKED we also clear + * retrans_stamp even if we had retransmitted the SYN. + */ + if (!tp->retrans_stamp && /* no record of a retransmit/SYN? */ + sk->sk_state != TCP_SYN_SENT) /* not the FLAG_SYN_ACKED case? */ + return true; /* nothing was retransmitted */ + + return false; } /* Undo procedures. */ |