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[34.150.132.193]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-45801db349asm16829301cf.76.2024.09.06.08.25.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Sep 2024 08:25:52 -0700 (PDT) Date: Fri, 06 Sep 2024 11:25:52 -0400 From: Willem de Bruijn To: Vadim Fedorenko , Willem de Bruijn , Willem de Bruijn Cc: netdev@vger.kernel.org, David Ahern , Jason Xing , Jakub Kicinski , Simon Horman , Paolo Abeni Message-ID: <66db1f004a0c_29a3852944d@willemb.c.googlers.com.notmuch> In-Reply-To: References: <20240904113153.2196238-1-vadfed@meta.com> <20240904113153.2196238-3-vadfed@meta.com> <3e4add99-6b57-4fe1-9ee1-519c80cf7cf5@linux.dev> <66d9debb2d2ea_1eae1a2943d@willemb.c.googlers.com.notmuch> Subject: Re: [PATCH net-next v3 2/4] net_tstamp: add SCM_TS_OPT_ID for TCP sockets Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Vadim Fedorenko wrote: > On 05/09/2024 17:39, Willem de Bruijn wrote: > > Vadim Fedorenko wrote: > >> On 04/09/2024 12:31, Vadim Fedorenko wrote: > >>> TCP sockets have different flow for providing timestamp OPT_ID value. > >>> Adjust the code to support SCM_TS_OPT_ID option for TCP sockets. > >>> > >>> Signed-off-by: Vadim Fedorenko > >>> --- > >>> net/ipv4/tcp.c | 13 +++++++++---- > >>> 1 file changed, 9 insertions(+), 4 deletions(-) > >>> > >>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c > >>> index 8a5680b4e786..5553a8aeee80 100644 > >>> --- a/net/ipv4/tcp.c > >>> +++ b/net/ipv4/tcp.c > >>> @@ -474,9 +474,10 @@ void tcp_init_sock(struct sock *sk) > >>> } > >>> EXPORT_SYMBOL(tcp_init_sock); > >>> > >>> -static void tcp_tx_timestamp(struct sock *sk, u16 tsflags) > >>> +static void tcp_tx_timestamp(struct sock *sk, struct sockcm_cookie *sockc) > >>> { > >>> struct sk_buff *skb = tcp_write_queue_tail(sk); > >>> + u32 tsflags = sockc->tsflags; > >>> > >>> if (tsflags && skb) { > >>> struct skb_shared_info *shinfo = skb_shinfo(skb); > >>> @@ -485,8 +486,12 @@ static void tcp_tx_timestamp(struct sock *sk, u16 tsflags) > >>> sock_tx_timestamp(sk, tsflags, &shinfo->tx_flags); > >>> if (tsflags & SOF_TIMESTAMPING_TX_ACK) > >>> tcb->txstamp_ack = 1; > >>> - if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK) > >>> - shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1; > >>> + if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK) { > >>> + if (tsflags & SOCKCM_FLAG_TS_OPT_ID) > >>> + shinfo->tskey = sockc->ts_opt_id; > >>> + else > >>> + shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1; > >>> + } > >>> } > >>> } > >>> > >>> @@ -1318,7 +1323,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size) > >>> > >>> out: > >>> if (copied) { > >>> - tcp_tx_timestamp(sk, sockc.tsflags); > >>> + tcp_tx_timestamp(sk, &sockc); > >>> tcp_push(sk, flags, mss_now, tp->nonagle, size_goal); > >>> } > >>> out_nopush: > >> > >> Hi Willem, > >> > >> Unfortunately, these changes are not enough to enable custom OPT_ID for > >> TCP sockets. There are some functions which rewrite shinfo->tskey in TCP > >> flow: > >> > >> tcp_skb_collapse_tstamp() > >> tcp_fragment_tstamp() > >> tcp_gso_tstamp() > >> > >> I believe the last one breaks tests, but the problem is that there is no > >> easy way to provide the flag of constant tskey to it. Only > >> shinfo::tx_flags are available at the caller side and we have already > >> discussed that we shouldn't use the last bit of this field. > >> > >> So, how should we deal with the problem? Or is it better to postpone > >> support for TCP sockets in this case? > > > > Are you sure that this is a problem. These functions pass on the > > skb_shinfo(skb)->ts_key from one skb to another. > > Yes, you are right, the problem is in a different place. > > __skb_complete_tx_timestamp receives skb with shinfo->tskey equal to > provided by cmsg. But for TCP sockets it unconditionally adjusts ee_data > value: > > if (sk_is_tcp(sk)) > serr->ee.ee_data -= atomic_read(&sk->sk_tskey); > > It happens because of assumption that for TCP sockets shinfo::tskey will > have sequence number and the logic has to recalculate it back to the > bytes sent. The same logic exists in all TCP TX timestamping functions > (mentioned in the previous mail) and may trigger some unexpected > behavior. To fix the issue we have to provide some kind of signal that > tskey value is provided from user-space and shouldn't be changed. And we > have to have it somewhere in skb info. Again, tx_flags looks like the > best candidate, but it's impossible to use. I'm thinking of using > special flag in tcp_skb_cb - gonna test more, but open for other > suggestions. Ai, that is tricky. tx_flags is full/scarce indeed. CB does not persist as the skb transitions between layers. The most obvious solution would be to set the flag in sk_tsflags itself. But then the cmsg would no long work on a per request basis: either the socket uses OPT_ID with counter or OPT_ID_CMSG. Good that we catch this now before the ABI is finalized. If necessary TCP semantics can diverge from datagrams. So we could punt on this. But it's not ideal.