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Fri, 22 May 2026 14:57:36 -0700 (PDT) X-Received: by 2002:a05:6830:3c88:b0:7dc:d967:63de with SMTP id 46e09a7af769-7e5feded159mr3649447a34.3.1779487056360; Fri, 22 May 2026 14:57:36 -0700 (PDT) Received: from localhost ([2a03:2880:12ff:6::]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-7e60667af4csm1842003a34.27.2026.05.22.14.57.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 22 May 2026 14:57:35 -0700 (PDT) From: Neil Spring To: netdev@vger.kernel.org Cc: edumazet@google.com, ncardwell@google.com, kuniyu@google.com, davem@davemloft.net, kuba@kernel.org, dsahern@kernel.org, pabeni@redhat.com, horms@kernel.org, shuah@kernel.org, linux-kselftest@vger.kernel.org, ntspring@meta.com, bpf@vger.kernel.org, martin.lau@linux.dev, daniel@iogearbox.net Subject: [PATCH net-next v8 1/2] tcp: rehash onto different local ECMP path on retransmit timeout Date: Fri, 22 May 2026 14:57:32 -0700 Message-ID: <20260522215733.929238-2-ntspring@meta.com> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20260522215733.929238-1-ntspring@meta.com> References: <20260522215733.929238-1-ntspring@meta.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Proofpoint-GUID: iAyX42RgcwkWACBXStOEmPVBdz1Gerno X-Authority-Analysis: v=2.4 cv=fL0JG5ae c=1 sm=1 tr=0 ts=6a10d152 cx=c_pps a=+3WqYijBVYhDct2f5Fivkw==:117 a=xqWC_Br6kY4A:10 a=NGcC8JguVDcA:10 a=f7IdgyKtn90A:10 a=VkNPw1HP01LnGYTKEx00:22 a=7x6HtfJdh03M6CCDgxCd:22 a=855S8uPTkML1Oy45N9_h:22 a=VabnemYjAAAA:8 a=TRHSsKDO3c-AA1P1OBkA:9 a=gKebqoRLp9LExxC7YDUY:22 X-Proofpoint-ORIG-GUID: iAyX42RgcwkWACBXStOEmPVBdz1Gerno X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTIyMDIxOCBTYWx0ZWRfX9rIxruaqEOat FqqpK3kcvL5Cna2YyUl09A3tmKgdfYiLDi2Q2js/uDu/MdFcnjMZdGV8jXqKASjYN0/4yAQ2pmq TqM56EPhOeRz5AADpEtpsvKzsy4o57oqU3usYwe9ptftcDbao0EVH4klHEFmHivf2NplXHCNUl4 v2+mWSEHFlCcXGUmQAH4G9TVsidwwThX/dYnFwQqlK6wwvH3bx6AfO5JnTsoHysQiweRkA66hPp amqOf0fQmqeaBaHPciRP2L7gr5RL3B2zcnZMChHyQDmq6D6oFvgCiyOLb8pNUtcoko9YV2ksjrI jYDF3P+66VbSXD/VCVEiSRdNGl5CMmrcTttGIhIDzEngg1eahhh2gURCjr9LkIBU47XXnePlMrQ qYUtALwB6fZerNEbR4nUOb/mMcPdB7ApKFnf2NGvXVcA08Rq8hmkOPbTTF6oxm/DnRKVmoEc2Sf aEavxGCVotAPzamLzEg== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-22_06,2026-05-18_01,2025-10-01_01 Currently sk_rethink_txhash() re-rolls the socket's txhash on RTO, PLB, and spurious-retransmission events, but the cached route is reused and the new hash is not propagated into the ECMP path selection logic. Two changes are needed to make rehash select a different local ECMP path: 1. Add __sk_dst_reset() alongside sk_rethink_txhash() in tcp_write_timeout(), tcp_rcv_spurious_retrans(), and tcp_plb_check_rehash() so the cached dst is invalidated and the next transmit triggers a fresh route lookup. 2. Set fl6->mp_hash from sk_txhash (or tcp_rsk(req)->txhash for SYN/ACK retransmits and syncookies) in tcp_v6_connect(), inet6_sk_rebuild_header(), inet6_csk_route_req(), inet6_csk_route_socket(), and cookie_v6_check() so fib6_select_path() picks a path based on the new hash. The mp_hash assignment in inet6_csk_route_socket() is guarded by sk_protocol == IPPROTO_TCP so that non-TCP callers (e.g., L2TP via inet6_csk_xmit) fall through to rt6_multipath_hash() and retain their existing flow-key-based ECMP behavior. The expression uses (txhash >> 1) ?: 1 so that the rare txhash == 1 still produces a valid non-zero mp_hash. This is conditioned on fib_multipath_hash_policy == 0 (L3) because policies 1-3 compute a deterministic hash from the flow keys (e.g., symmetric 5-tuple for policy 1) which must not be overridden by a random txhash. It is necessary to update mp_hash explicitly because the default ECMP hash derives from fl6->flowlabel via np->flow_label, which is not updated from sk_txhash (REPFLOW is off by default). ip6_make_flowlabel() cannot help either, as it runs after the route lookup. sk_set_txhash() is moved before ip6_dst_lookup_flow() in tcp_v6_connect() so the initial ECMP path is selected by the same txhash that subsequent route rebuilds will use. This avoids unintended path changes when the cached dst is naturally invalidated (e.g., by PMTU discovery or route changes). The dst reset is guarded by sk->sk_family == AF_INET6 since IPv4 ECMP does not currently use sk_txhash for path selection. For IPv4-mapped IPv6 sockets this produces a redundant dst reset on a cold path (RTO/PLB); the subsequent IPv4 route lookup returns the same result. tcp_rsk(req)->txhash initialization is moved before route_req() in tcp_conn_request() so that inet6_csk_route_req() reads a valid hash on the initial SYN/ACK. For syncookies, txhash is set to the cookie (ISN) before route_req() so the SYN-ACK uses the same ECMP path that cookie_v6_check() will select when the ACK arrives and the full socket is created. cookie_tcp_reqsk_init() likewise derives txhash from the cookie rather than calling net_tx_rndhash(), since the original request socket (and its txhash) was freed after the SYN-ACK. The ecn_ok clear for syncookies without timestamps stays after tcp_ecn_create_request() so it takes precedence. bpf_sk_assign_tcp_reqsk() is updated to initialize txhash via net_tx_rndhash(), matching cookie_tcp_reqsk_alloc(). Without this, inet6_csk_route_req() would read uninitialized slab memory from request sockets created by BPF syncookies. Signed-off-by: Neil Spring --- net/core/filter.c | 1 + net/ipv4/syncookies.c | 8 +++++++- net/ipv4/tcp_input.c | 18 +++++++++++------- net/ipv4/tcp_plb.c | 5 ++++- net/ipv4/tcp_timer.c | 2 ++ net/ipv6/af_inet6.c | 3 +++ net/ipv6/inet6_connection_sock.c | 8 ++++++++ net/ipv6/syncookies.c | 4 ++++ net/ipv6/tcp_ipv6.c | 13 +++++++++++-- 9 files changed, 51 insertions(+), 11 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 80a3b702a2d4..7fea9ad881e7 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -12301,6 +12301,7 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk, treq->req_usec_ts = !!attrs->usec_ts_ok; treq->ts_off = tsoff; + treq->txhash = net_tx_rndhash(); skb_orphan(skb); skb->sk = req_to_sk(req); diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c index df479277fb80..8591f2606ca6 100644 --- a/net/ipv4/syncookies.c +++ b/net/ipv4/syncookies.c @@ -280,9 +280,15 @@ static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb, treq->snt_synack = 0; treq->snt_tsval_first = 0; treq->tfo_listener = false; - treq->txhash = net_tx_rndhash(); treq->rcv_isn = ntohl(th->seq) - 1; treq->snt_isn = ntohl(th->ack_seq) - 1; + /* Use the cookie as txhash so the ECMP path matches the + * SYN-ACK, where txhash was also set to the cookie. A + * random txhash would be inconsistent because the original + * request socket (and its txhash) was freed after sending + * the SYN-ACK. + */ + treq->txhash = treq->snt_isn; treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield; #if IS_ENABLED(CONFIG_MPTCP) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 7995a89bafc9..810c95a11c8c 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5020,8 +5020,10 @@ static void tcp_rcv_spurious_retrans(struct sock *sk, skb->protocol == htons(ETH_P_IPV6) && (tcp_sk(sk)->inet_conn.icsk_ack.lrcv_flowlabel != ntohl(ip6_flowlabel(ipv6_hdr(skb)))) && - sk_rethink_txhash(sk)) + sk_rethink_txhash(sk)) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH); + __sk_dst_reset(sk); + } /* Save last flowlabel after a spurious retrans. */ tcp_save_lrcv_flowlabel(sk, skb); @@ -7636,6 +7638,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, tcp_rsk(req)->af_specific = af_ops; tcp_rsk(req)->ts_off = 0; tcp_rsk(req)->req_usec_ts = false; + tcp_rsk(req)->txhash = net_tx_rndhash(); #if IS_ENABLED(CONFIG_MPTCP) tcp_rsk(req)->is_mptcp = 0; #endif @@ -7659,6 +7662,11 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, /* Note: tcp_v6_init_req() might override ir_iif for link locals */ inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb); + if (want_cookie) { + isn = cookie_init_sequence(af_ops, sk, skb, &req->mss); + tcp_rsk(req)->txhash = isn; + } + dst = af_ops->route_req(sk, skb, &fl, req, isn); if (!dst) goto drop_and_free; @@ -7698,11 +7706,8 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, tcp_ecn_create_request(req, skb, sk, dst); - if (want_cookie) { - isn = cookie_init_sequence(af_ops, sk, skb, &req->mss); - if (!tmp_opt.tstamp_ok) - inet_rsk(req)->ecn_ok = 0; - } + if (want_cookie && !tmp_opt.tstamp_ok) + inet_rsk(req)->ecn_ok = 0; #ifdef CONFIG_TCP_AO if (tcp_parse_auth_options(tcp_hdr(skb), NULL, &aoh)) @@ -7717,7 +7722,6 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, } #endif tcp_rsk(req)->snt_isn = isn; - tcp_rsk(req)->txhash = net_tx_rndhash(); tcp_rsk(req)->syn_tos = TCP_SKB_CB(skb)->ip_dsfield; tcp_openreq_init_rwin(req, sk, dst); sk_rx_queue_set(req_to_sk(req), skb); diff --git a/net/ipv4/tcp_plb.c b/net/ipv4/tcp_plb.c index c11a0cd3f8fe..849ac4aad480 100644 --- a/net/ipv4/tcp_plb.c +++ b/net/ipv4/tcp_plb.c @@ -78,7 +78,10 @@ void tcp_plb_check_rehash(struct sock *sk, struct tcp_plb_state *plb) if (plb->pause_until) return; - sk_rethink_txhash(sk); + if (sk_rethink_txhash(sk)) { + if (sk->sk_family == AF_INET6) + __sk_dst_reset(sk); + } plb->consec_cong_rounds = 0; WRITE_ONCE(tcp_sk(sk)->plb_rehash, tcp_sk(sk)->plb_rehash + 1); NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPLBREHASH); diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c index 322db13333c7..7c05f1072a06 100644 --- a/net/ipv4/tcp_timer.c +++ b/net/ipv4/tcp_timer.c @@ -300,6 +300,8 @@ static int tcp_write_timeout(struct sock *sk) if (sk_rethink_txhash(sk)) { WRITE_ONCE(tp->timeout_rehash, tp->timeout_rehash + 1); __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTREHASH); + if (sk->sk_family == AF_INET6) + __sk_dst_reset(sk); } return 0; diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c index 0a88b376141d..7a2b1de7487c 100644 --- a/net/ipv6/af_inet6.c +++ b/net/ipv6/af_inet6.c @@ -823,6 +823,9 @@ int inet6_sk_rebuild_header(struct sock *sk) fl6->flowi6_uid = sk_uid(sk); security_sk_classify_flow(sk, flowi6_to_flowi_common(fl6)); + if (ip6_multipath_hash_policy(sock_net(sk)) == 0 && sk->sk_txhash) + fl6->mp_hash = (sk->sk_txhash >> 1) ?: 1; + rcu_read_lock(); final_p = fl6_update_dst(fl6, rcu_dereference(np->opt), &np->final); rcu_read_unlock(); diff --git a/net/ipv6/inet6_connection_sock.c b/net/ipv6/inet6_connection_sock.c index 37534e116899..7ca24eef614c 100644 --- a/net/ipv6/inet6_connection_sock.c +++ b/net/ipv6/inet6_connection_sock.c @@ -48,6 +48,10 @@ struct dst_entry *inet6_csk_route_req(const struct sock *sk, fl6->flowi6_uid = sk_uid(sk); security_req_classify_flow(req, flowi6_to_flowi_common(fl6)); + if (ip6_multipath_hash_policy(sock_net(sk)) == 0 && + tcp_rsk(req)->txhash) + fl6->mp_hash = (tcp_rsk(req)->txhash >> 1) ?: 1; + if (!dst) { dst = ip6_dst_lookup_flow(sock_net(sk), sk, fl6, final_p); if (IS_ERR(dst)) @@ -70,6 +74,10 @@ struct dst_entry *inet6_csk_route_socket(struct sock *sk, fl6->saddr = np->saddr; fl6->flowlabel = np->flow_label; IP6_ECN_flow_xmit(sk, fl6->flowlabel); + + if (sk->sk_protocol == IPPROTO_TCP && + ip6_multipath_hash_policy(sock_net(sk)) == 0 && sk->sk_txhash) + fl6->mp_hash = (sk->sk_txhash >> 1) ?: 1; fl6->flowi6_oif = sk->sk_bound_dev_if; fl6->flowi6_mark = sk->sk_mark; fl6->fl6_sport = inet->inet_sport; diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c index 4f6f0d751d6c..70759cd64b34 100644 --- a/net/ipv6/syncookies.c +++ b/net/ipv6/syncookies.c @@ -245,6 +245,10 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb) fl6.flowi6_uid = sk_uid(sk); security_req_classify_flow(req, flowi6_to_flowi_common(&fl6)); + if (ip6_multipath_hash_policy(net) == 0 && + tcp_rsk(req)->txhash) + fl6.mp_hash = (tcp_rsk(req)->txhash >> 1) ?: 1; + dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p); if (IS_ERR(dst)) { SKB_DR_SET(reason, IP_OUTNOROUTES); diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c index 2c3f7a739709..ecdc8f84d203 100644 --- a/net/ipv6/tcp_ipv6.c +++ b/net/ipv6/tcp_ipv6.c @@ -258,6 +258,8 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr, if (!ipv6_addr_any(&sk->sk_v6_rcv_saddr)) saddr = &sk->sk_v6_rcv_saddr; + sk_set_txhash(sk); + fl6->flowi6_proto = IPPROTO_TCP; fl6->daddr = sk->sk_v6_daddr; fl6->saddr = saddr ? *saddr : np->saddr; @@ -275,6 +277,15 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr, security_sk_classify_flow(sk, flowi6_to_flowi_common(fl6)); + /* Non-zero mp_hash bypasses rt6_multipath_hash() in + * fib6_select_path(), letting txhash control ECMP path + * selection so that sk_rethink_txhash() rehashes onto a + * different path. Policies 1-3 derive a deterministic + * hash from the flow keys and must not be overridden. + */ + if (ip6_multipath_hash_policy(net) == 0 && sk->sk_txhash) + fl6->mp_hash = (sk->sk_txhash >> 1) ?: 1; + dst = ip6_dst_lookup_flow(net, sk, fl6, final_p); if (IS_ERR(dst)) { err = PTR_ERR(dst); @@ -313,8 +324,6 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr, if (err) goto late_failure; - sk_set_txhash(sk); - if (likely(!tp->repair)) { union tcp_seq_and_ts_off st; -- 2.53.0-Meta