6.6-stable review patch. If anyone has any objections, please let me know.
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From: Neal Cardwell ncardwell@google.com
[ Upstream commit b41b4cbd9655bcebcce941bef3601db8110335be ]
Fix tcp_enter_recovery() so that if there are no retransmits out then we zero retrans_stamp when entering fast recovery. This is necessary to fix two buggy behaviors.
Currently a non-zero retrans_stamp value can persist across multiple back-to-back loss recovery episodes. This is because we generally only clears retrans_stamp if we are completely done with loss recoveries, and get to tcp_try_to_open() and find !tcp_any_retrans_done(sk). This behavior causes two bugs:
(1) When a loss recovery episode (CA_Loss or CA_Recovery) is followed immediately by a new CA_Recovery, the retrans_stamp value can persist and can be a time before this new CA_Recovery episode starts. That means that timestamp-based undo will be using the wrong retrans_stamp (a value that is too old) when comparing incoming TS ecr values to retrans_stamp to see if the current fast recovery episode can be undone.
(2) If there is a roughly minutes-long sequence of back-to-back fast recovery episodes, one after another (e.g. in a shallow-buffered or policed bottleneck), where each fast recovery successfully makes forward progress and recovers one window of sequence space (but leaves at least one retransmit in flight at the end of the recovery), followed by several RTOs, then the ETIMEDOUT check may be using the wrong retrans_stamp (a value set at the start of the first fast recovery in the sequence). This can cause a very premature ETIMEDOUT, killing the connection prematurely.
This commit changes the code to zero retrans_stamp when entering fast recovery, when this is known to be safe (no retransmits are out in the network). That ensures that when starting a fast recovery episode, and it is safe to do so, retrans_stamp is set when we send the fast retransmit packet. That addresses both bug (1) and bug (2) by ensuring that (if no retransmits are out when we start a fast recovery) we use the initial fast retransmit of this fast recovery as the time value for undo and ETIMEDOUT calculations.
This makes intuitive sense, since the start of a new fast recovery episode (in a scenario where no lost packets are out in the network) means that the connection has made forward progress since the last RTO or fast recovery, and we should thus "restart the clock" used for both undo and ETIMEDOUT logic.
Note that if when we start fast recovery there *are* retransmits out in the network, there can still be undesirable (1)/(2) issues. For example, after this patch we can still have the (1) and (2) problems in cases like this:
+ round 1: sender sends flight 1
+ round 2: sender receives SACKs and enters fast recovery 1, retransmits some packets in flight 1 and then sends some new data as flight 2
+ round 3: sender receives some SACKs for flight 2, notes losses, and retransmits some packets to fill the holes in flight 2
+ fast recovery has some lost retransmits in flight 1 and continues for one or more rounds sending retransmits for flight 1 and flight 2
+ fast recovery 1 completes when snd_una reaches high_seq at end of flight 1
+ there are still holes in the SACK scoreboard in flight 2, so we enter fast recovery 2, but some retransmits in the flight 2 sequence range are still in flight (retrans_out > 0), so we can't execute the new retrans_stamp=0 added here to clear retrans_stamp
It's not yet clear how to fix these remaining (1)/(2) issues in an efficient way without breaking undo behavior, given that retrans_stamp is currently used for undo and ETIMEDOUT. Perhaps the optimal (but expensive) strategy would be to set retrans_stamp to the timestamp of the earliest outstanding retransmit when entering fast recovery. But at least this commit makes things better.
Note that this does not change the semantics of retrans_stamp; it simply makes retrans_stamp accurate in some cases where it was not before:
(1) Some loss recovery, followed by an immediate entry into a fast recovery, where there are no retransmits out when entering the fast recovery.
(2) When a TFO server has a SYNACK retransmit that sets retrans_stamp, and then the ACK that completes the 3-way handshake has SACK blocks that trigger a fast recovery. In this case when entering fast recovery we want to zero out the retrans_stamp from the TFO SYNACK retransmit, and set the retrans_stamp based on the timestamp of the fast recovery.
We introduce a tcp_retrans_stamp_cleanup() helper, because this two-line sequence already appears in 3 places and is about to appear in 2 more as a result of this bug fix patch series. Once this bug fix patches series in the net branch makes it into the net-next branch we'll update the 3 other call sites to use the new helper.
This is a long-standing issue. The Fixes tag below is chosen to be the oldest commit at which the patch will apply cleanly, which is from Linux v3.5 in 2012.
Fixes: 1fbc340514fc ("tcp: early retransmit: tcp_enter_recovery()") 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/20241001200517.2756803-3-ncardwell.sw@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 31e4f7bce00ce..057d39248bec7 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2501,6 +2501,16 @@ static bool tcp_any_retrans_done(const struct sock *sk) return false; }
+/* If loss recovery is finished and there are no retransmits out in the + * network, then we clear retrans_stamp so that upon the next loss recovery + * retransmits_timed_out() and timestamp-undo are using the correct value. + */ +static void tcp_retrans_stamp_cleanup(struct sock *sk) +{ + if (!tcp_any_retrans_done(sk)) + tcp_sk(sk)->retrans_stamp = 0; +} + static void DBGUNDO(struct sock *sk, const char *msg) { #if FASTRETRANS_DEBUG > 1 @@ -2868,6 +2878,9 @@ void tcp_enter_recovery(struct sock *sk, bool ece_ack) struct tcp_sock *tp = tcp_sk(sk); int mib_idx;
+ /* Start the clock with our fast retransmit, for undo and ETIMEDOUT. */ + tcp_retrans_stamp_cleanup(sk); + if (tcp_is_reno(tp)) mib_idx = LINUX_MIB_TCPRENORECOVERY; else