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Message-ID: <1f17d828-5d0f-4050-be4b-8840feb8de76@linux.dev>
Date: Fri, 6 Sep 2024 18:27:00 +0100
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
Willem de Bruijn <willemb@...gle.com>
Cc: netdev@...r.kernel.org, David Ahern <dsahern@...nel.org>,
Jason Xing <kerneljasonxing@...il.com>, Jakub Kicinski <kuba@...nel.org>,
Simon Horman <horms@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v3 2/4] net_tstamp: add SCM_TS_OPT_ID for TCP
sockets
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 <vadfed@...a.com>
>>>>> ---
>>>>> 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?
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