lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Fri, 27 Dec 2019 16:49:33 -0800 From: Eric Dumazet <eric.dumazet@...il.com> To: Cambda Zhu <cambda@...ux.alibaba.com>, Eric Dumazet <edumazet@...gle.com> Cc: David Miller <davem@...emloft.net>, Yuchung Cheng <ycheng@...gle.com>, netdev <netdev@...r.kernel.org>, Dust Li <dust.li@...ux.alibaba.com> Subject: Re: [PATCH] tcp: Fix highest_sack and highest_sack_seq On 12/27/19 12:52 AM, Cambda Zhu wrote: > From commit 50895b9de1d3 ("tcp: highest_sack fix"), the logic about > setting tp->highest_sack to the head of the send queue was removed. > Of course the logic is error prone, but it is logical. Before we > remove the pointer to the highest sack skb and use the seq instead, > we need to set tp->highest_sack to NULL when there is no skb after > the last sack, and then replace NULL with the real skb when new skb > inserted into the rtx queue, because the NULL means the highest sack > seq is tp->snd_nxt. If tp->highest_sack is NULL and new data sent, > the next ACK with sack option will increase tp->reordering unexpectedly. > > This patch sets tp->highest_sack to the tail of the rtx queue if > it's NULL and new data is sent. The patch keeps the rule that the > highest_sack can only be maintained by sack processing, except for > this only case. > > Fixes: 50895b9de1d3 ("tcp: highest_sack fix") > Signed-off-by: Cambda Zhu <cambda@...ux.alibaba.com> > --- > net/ipv4/tcp_output.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > index 1f7735ca8f22..58c92a7d671c 100644 > --- a/net/ipv4/tcp_output.c > +++ b/net/ipv4/tcp_output.c > @@ -72,6 +72,9 @@ static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb) > __skb_unlink(skb, &sk->sk_write_queue); > tcp_rbtree_insert(&sk->tcp_rtx_queue, skb); > > + if (tp->highest_sack == NULL) > + tp->highest_sack = skb; > + > tp->packets_out += tcp_skb_pcount(skb); > if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) > tcp_rearm_rto(sk); > This patch seems to keep something in the fast path, even for flows never experiencing sacks. Why would we always painfully maintain tp->highest_sack to the left most skb in the rtx queue ? Given that tcp_highest_sack_seq() has an explicit check about tp->highest_sack being NULL, there is something I do not quite understand yet. Why keeping this piece of code ? if (tp->highest_sack == NULL) return tp->snd_nxt; Defensive programming should be replaced by better knowledge. Can you provide more explanations, or maybe a packetdrill test ? Maybe some other path (in slow path this time) misses a !tp->highest_sack test. Thanks.
Powered by blists - more mailing lists