Re: [PATCH bpf-next v12 10/12] bpf: add BPF_SOCK_OPS_TS_SND_CB callback

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

Jason Xing wrote:
> On Thu, Feb 20, 2025 at 11:15 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
> >
> > On Thu, Feb 20, 2025 at 10:55 AM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> > >
> > > Jason Xing wrote:
> > > > This patch introduces a new callback in tcp_tx_timestamp() to correlate
> > > > tcp_sendmsg timestamp with timestamps from other tx timestamping
> > > > callbacks (e.g., SND/SW/ACK).
> > > >
> > > > Without this patch, BPF program wouldn't know which timestamps belong
> > > > to which flow because of no socket lock protection. This new callback
> > > > is inserted in tcp_tx_timestamp() to address this issue because
> > > > tcp_tx_timestamp() still owns the same socket lock with
> > > > tcp_sendmsg_locked() in the meanwhile tcp_tx_timestamp() initializes
> > > > the timestamping related fields for the skb, especially tskey. The
> > > > tskey is the bridge to do the correlation.
> > > >
> > > > For TCP, BPF program hooks the beginning of tcp_sendmsg_locked() and
> > > > then stores the sendmsg timestamp at the bpf_sk_storage, correlating
> > > > this timestamp with its tskey that are later used in other sending
> > > > timestamping callbacks.
> > > >
> > > > Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> > > > ---
> > > >  include/uapi/linux/bpf.h       | 5 +++++
> > > >  net/ipv4/tcp.c                 | 4 ++++
> > > >  tools/include/uapi/linux/bpf.h | 5 +++++
> > > >  3 files changed, 14 insertions(+)
> > > >
> > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > index 9355d617767f..86fca729fbd8 100644
> > > > --- a/include/uapi/linux/bpf.h
> > > > +++ b/include/uapi/linux/bpf.h
> > > > @@ -7052,6 +7052,11 @@ enum {
> > > >                                        * when SK_BPF_CB_TX_TIMESTAMPING
> > > >                                        * feature is on.
> > > >                                        */
> > > > +     BPF_SOCK_OPS_TS_SND_CB,         /* Called when every sendmsg syscall
> > > > +                                      * is triggered. It's used to correlate
> > > > +                                      * sendmsg timestamp with corresponding
> > > > +                                      * tskey.
> > > > +                                      */
> > > >  };
> > > >
> > > >  /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> > > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > > > index 12b9c4f9c151..4b9739cd3bc5 100644
> > > > --- a/net/ipv4/tcp.c
> > > > +++ b/net/ipv4/tcp.c
> > > > @@ -492,6 +492,10 @@ static void tcp_tx_timestamp(struct sock *sk, struct sockcm_cookie *sockc)
> > > >               if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK)
> > > >                       shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
> > > >       }
> > > > +
> > > > +     if (cgroup_bpf_enabled(CGROUP_SOCK_OPS) &&
> > > > +         SK_BPF_CB_FLAG_TEST(sk, SK_BPF_CB_TX_TIMESTAMPING) && skb)
> > > > +             bpf_skops_tx_timestamping(sk, skb, BPF_SOCK_OPS_TS_SND_CB);
> > > >  }
> > > >
> > > >  static bool tcp_stream_is_readable(struct sock *sk, int target)
> > > > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > > > index d3e2988b3b4c..2739ee0154a0 100644
> > > > --- a/tools/include/uapi/linux/bpf.h
> > > > +++ b/tools/include/uapi/linux/bpf.h
> > > > @@ -7042,6 +7042,11 @@ enum {
> > > >                                        * when SK_BPF_CB_TX_TIMESTAMPING
> > > >                                        * feature is on.
> > > >                                        */
> > > > +     BPF_SOCK_OPS_TS_SND_CB,         /* Called when every sendmsg syscall
> > > > +                                      * is triggered. It's used to correlate
> > > > +                                      * sendmsg timestamp with corresponding
> > > > +                                      * tskey.
> > > > +                                      */
> > >
> > > Feel free to decline this late in the review process, but a bit more
> > > bikeshedding..
> > >
> > > Can we spell out TSTAMP instead of TS in these definitions? Within
> > > the context of this series it is self-explanatory, but when reading
> > > kernel code the meaning of a two letter acronym is not that clear.
> >
> > Even though I feel reluctant to change across the whole series because
> > if so, I will adjust in many places. Of course, you're right about the
> > new name being clearer :)
> >
> > >
> > > And instead of SND can we use SENDMSG or something like that?
> > > SND here confused me as the software timestamp is SCM_TSTAMP_SND.
> >
> > I'm not sure about this. For TCP, it's not implemented in the
> > tcp_sendmsg_locked but tcp_tx_timestamp. Well, I have no strong
> > preference.
> >
> > You can make the final call :)
> 
> After taking a break, I feel full of energy and I will update them all
> as you requested :)

Awesome, thank you.