Subject: Re: [PATCH net-next v3 2/4] net_tstamp: add SCM_TS_OPT_ID for TCP sockets

From: Vadim Fedorenko <[email-masked]>
Date: Sun, 8 Sep 2024 20:55:55 +0100

On 08/09/2024 20:19, Willem de Bruijn wrote:
> Vadim Fedorenko wrote:
>> On 07/09/2024 00:48, Willem de Bruijn wrote:
>>> Vadim Fedorenko wrote:
>>>> On 06/09/2024 16:25, Willem de Bruijn wrote:
>>>>> 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 <[email-masked]>
>>>>>>>>> ---
>>>>>>>>>       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.
>>>>
>>>> I have done proof of concept code which uses hwtsamp as a storage for
>>>> custom tskey in case of TCP:
>>>>
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index a52638363ea5..40ed49e61bf7 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -5414,8 +5414,6 @@ static void __skb_complete_tx_timestamp(struct
>>>> sk_buff *skb,
>>>>            serr->header.h4.iif = skb->dev ? skb->dev->ifindex : 0;
>>>>            if (READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_OPT_ID) {
>>>>                    serr->ee.ee_data = skb_shinfo(skb)->tskey;
>>>> -               if (sk_is_tcp(sk))
>>>> -                       serr->ee.ee_data -= atomic_read(&sk->sk_tskey);
>>>>            }
>>>>
>>>>            err = sock_queue_err_skb(sk, skb);
>>>> @@ -5450,6 +5448,8 @@ void skb_complete_tx_timestamp(struct sk_buff *skb,
>>>>             * but only if the socket refcount is not zero.
>>>>             */
>>>>            if (likely(refcount_inc_not_zero(&sk->sk_refcnt))) {
>>>> +               if (sk_is_tcp(sk) && (READ_ONCE(sk->sk_tsflags) &
>>>> SOF_TIMESTAMPING_OPT_ID) && skb_hwtstamps(skb)->hwtstamp)
>>>> +                       skb_shinfo(skb)->tskey =
>>>> (u32)skb_hwtstamps(skb)->hwtstamp;
>>>>                    *skb_hwtstamps(skb) = *hwtstamps;
>>>>                    __skb_complete_tx_timestamp(skb, sk, SCM_TSTAMP_SND,
>>>> false);
>>>>                    sock_put(sk);
>>>> @@ -5509,6 +5509,12 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
>>>>                    skb_shinfo(skb)->tskey = skb_shinfo(orig_skb)->tskey;
>>>>            }
>>>>
>>>> +       if (sk_is_tcp(sk) && (tsflags & SOF_TIMESTAMPING_OPT_ID)) {
>>>> +               if (skb_hwtstamps(orig_skb)->hwtstamp)
>>>> +                       skb_shinfo(skb)->tskey =
>>>> (u32)skb_hwtstamps(orig_skb)->hwtstamp;
>>>> +               else
>>>> +                       skb_shinfo(skb)->tskey -=
>>>> atomic_read(&sk->sk_tskey);
>>>> +       }
>>>>            if (hwtstamps)
>>>>                    *skb_hwtstamps(skb) = *hwtstamps;
>>>>            else
>>>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>>>> index 0d3decc13a99..1a161a2155b5 100644
>>>> --- a/net/ipv4/tcp.c
>>>> +++ b/net/ipv4/tcp.c
>>>> @@ -488,9 +488,8 @@ static void tcp_tx_timestamp(struct sock *sk, struct
>>>> sockcm_cookie *sockc)
>>>>                            tcb->txstamp_ack = 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;
>>>> +                               skb_hwtstamps(skb)->hwtstamp =
>>>> sockc->ts_opt_id;
>>>> +                       shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
>>>>                    }
>>>>            }
>>>>     }
>>>>
>>>>
>>>> Looks like we can add u32 tskey field in skb_shared_hwtstamps and reuse
>>>> it. netdev_data field is only used on RX timestamp side, so should be
>>>> fine to reuse. WDYT?
>>>
>>> We cannot really extend the struct. skb_shared_info is scarce.
>>> hwtstamps is a union. But on tx the hw tstamp is queued using
>>> skb_tstamp_tx, not through this shinfo data at all.
>>>
>>> It just seems weird to have a shinfo->tskey, but then ignore it and
>>> find yet another 32b field. Easier would be to find 1b to toggle
>>> whether tskey is to be interpreted as counter based or OPT_ID_CMSG.
>>>
>>> I don't immediate see a perfect solution either.
>>
>> Well, with this said, and given that having 2 different tskey values is
>> weird solution, I'm thinking of skipping TCP part until we can find some
>> proper way. Will it be OK to have UDP and RAW sockets only opted into
>> the feature?
> 
> Yes, I think that's fine.
> 
> Let's make sure that tcp sendmsg fails if the new cmsg is passed. So
> that we can add support for it later.

Thanks Willem, I'll prepare patchset tomorrow.

> We also have to give some thought what it means to coalesce skbs with
> non-sequential IDs, in the functions you mentioned before. Might be
> fine to just say: that's the application's issue. But even then we
> should document that behavior.

We can discuss it in a separate RFC series, there are more open
questions, I believe.