Re: [PATCH net-next v3 07/14] net-timestamp: add a new triggered point to set sk_tsflags_bpf in UDP layer

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> On Tue, Oct 29, 2024 at 9:33 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > Jason Xing wrote:
> > > On Tue, Oct 29, 2024 at 9:07 AM Willem de Bruijn
> > > <willemdebruijn.kernel@gmail.com> wrote:
> > > >
> > > > Jason Xing wrote:
> > > > > From: Jason Xing <kernelxing@tencent.com>
> > > > >
> > > > > This patch behaves like how cmsg feature works, that is to say,
> > > > > check and set on each call of udp_sendmsg before passing sk_tsflags_bpf
> > > > > to cork tsflags.
> > > > >
> > > > > Signed-off-by: Jason Xing <kernelxing@tencent.com>
> > > > > ---
> > > > >  include/net/sock.h             | 1 +
> > > > >  include/uapi/linux/bpf.h       | 3 +++
> > > > >  net/core/skbuff.c              | 2 +-
> > > > >  net/ipv4/udp.c                 | 1 +
> > > > >  tools/include/uapi/linux/bpf.h | 3 +++
> > > > >  5 files changed, 9 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/net/sock.h b/include/net/sock.h
> > > > > index 062f405c744e..cf7fea456455 100644
> > > > > --- a/include/net/sock.h
> > > > > +++ b/include/net/sock.h
> > > > > @@ -2828,6 +2828,7 @@ static inline bool sk_listener_or_tw(const struct sock *sk)
> > > > >  }
> > > > >
> > > > >  void sock_enable_timestamp(struct sock *sk, enum sock_flags flag);
> > > > > +void timestamp_call_bpf(struct sock *sk, int op, u32 nargs, u32 *args);
> > > > >  int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len, int level,
> > > > >                      int type);
> > > > >
> > > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > > index 6fc3bd12b650..055ffa7c965c 100644
> > > > > --- a/include/uapi/linux/bpf.h
> > > > > +++ b/include/uapi/linux/bpf.h
> > > > > @@ -7028,6 +7028,9 @@ enum {
> > > > >                                        * feature is on. It indicates the
> > > > >                                        * recorded timestamp.
> > > > >                                        */
> > > > > +     BPF_SOCK_OPS_TS_UDP_SND_CB,     /* Called when every udp_sendmsg
> > > > > +                                      * syscall is triggered
> > > > > +                                      */
> > > > >  };
> > > > >
> > > > >  /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > > > index 8b2a79c0fe1c..0b571306f7ea 100644
> > > > > --- a/net/core/skbuff.c
> > > > > +++ b/net/core/skbuff.c
> > > > > @@ -5622,7 +5622,7 @@ static void skb_tstamp_tx_output(struct sk_buff *orig_skb,
> > > > >       __skb_complete_tx_timestamp(skb, sk, tstype, opt_stats);
> > > > >  }
> > > > >
> > > > > -static void timestamp_call_bpf(struct sock *sk, int op, u32 nargs, u32 *args)
> > > > > +void timestamp_call_bpf(struct sock *sk, int op, u32 nargs, u32 *args)
> > > > >  {
> > > > >       struct bpf_sock_ops_kern sock_ops;
> > > > >
> > > > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > > > > index 9a20af41e272..e768421abc37 100644
> > > > > --- a/net/ipv4/udp.c
> > > > > +++ b/net/ipv4/udp.c
> > > > > @@ -1264,6 +1264,7 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> > > > >       if (!corkreq) {
> > > > >               struct inet_cork cork;
> > > > >
> > > > > +             timestamp_call_bpf(sk, BPF_SOCK_OPS_TS_UDP_SND_CB, 0, NULL);
> > > > >               skb = ip_make_skb(sk, fl4, getfrag, msg, ulen,
> > > > >                                 sizeof(struct udphdr), &ipc, &rt,
> > > > >                                 &cork, msg->msg_flags);
> > > > > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > > > > index 6fc3bd12b650..055ffa7c965c 100644
> > > > > --- a/tools/include/uapi/linux/bpf.h
> > > > > +++ b/tools/include/uapi/linux/bpf.h
> > > > > @@ -7028,6 +7028,9 @@ enum {
> > > > >                                        * feature is on. It indicates the
> > > > >                                        * recorded timestamp.
> > > > >                                        */
> > > > > +     BPF_SOCK_OPS_TS_UDP_SND_CB,     /* Called when every udp_sendmsg
> > > > > +                                      * syscall is triggered
> > > > > +                                      */
> > > >
> > > > If adding a timestamp as close to syscall entry as possible, give it a
> > > > generic name, not specific to UDP.
> > >
> > > Good suggestion, then it will also solve the remaining issue for TCP type:
> > > __when__ we should record the user timestamp which exists in the
> > > application SO_TIMESTAMPING feature.
> > >
> > > >
> > > > And please explain in the commit message the reason for a new
> > > > timestamp recording point: with existing timestamping the application
> > > > can call clock_gettime before (and optionally after) the send call.
> > > > An admin using BPF does not have this option, so needs this as part of
> > > > the BPF timestamping API.
> > >
> > > Will revise this part. Thanks for your description!
> >
> > Actually, I may have misunderstood the intention of this new hook.
> >
> > I thought it was to record an additional timestamp.
> 
> I planned to do it after this series. For now, without the new hook,
> it will not work for UDP type.

Why not? This is something specific to the SK BPF hooks, I suppose?

As soon as bpf_setsockopt is called, the timestamp callbacks should
start getting called?

> >
> > But it is (also?) to program skb_shared_info.tx_flags based on
> > instructions parsed from cmsg in __sock_cmsg_send.
> 
> I'm not sure if I grasp the key point you said.
> 
> For UDP, skb_shared_info.tx_flags will finally be initialized in
> __ip_append_data() based on cork->tx_flags.
> 
> cork->tx_flags is computed by sock_tx_timestamp() based on
> ipc->sockc.tsflags if cmsg feature is turned on.
> 
> __sock_tx_timestamp() uses "flags |= xxx" to initialize the
> cork->tx_flags, so that the cork->tx_flags will not be completely
> overridden by either the cmsg method or bpf program, that is to say,
> the cork->tx_flags can combine both of them.
> 
> Then another key point is that we do the check to see which one
> actually works in sk_tstamp_tx_flags() by testing sk->sk_tsflags or
> sk->sk_tsflags_bpf in patch [2/14]. It guarantees that.

Ack, thanks. So I was mistaken the second time around.