netdev - Re: [PATCH net-next v2 0/2] timestamp: control SOF_TIMESTAMPING_RX

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Message-ID: <66d082422d85_3895fa29427@willemb.c.googlers.com.notmuch>
Date: Thu, 29 Aug 2024 10:14:26 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 willemb@...gle.com
Cc: netdev@...r.kernel.org, 
 Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 0/2] timestamp: control
 SOF_TIMESTAMPING_RX_SOFTWARE feature per socket

Jason Xing wrote:
> From: Jason Xing <kernelxing@...cent.com>
> 
> Prior to this series, when one socket is set SOF_TIMESTAMPING_RX_SOFTWARE
> which measn the whole system turns on this button, other sockets that only
> have SOF_TIMESTAMPING_SOFTWARE will be affected and then print the rx
> timestamp information even without SOF_TIMESTAMPING_RX_SOFTWARE flag.
> In such a case, the rxtimestamp.c selftest surely fails, please see
> testcase 6.
> 
> In a normal case, if we only set SOF_TIMESTAMPING_SOFTWARE flag, we
> can't get the rx timestamp because there is no path leading to turn on
> netstamp_needed_key button in net_enable_timestamp(). That is to say, if
> the user only sets SOF_TIMESTAMPING_SOFTWARE, we don't expect we are
> able to fetch the timestamp from the skb.

I already happened to stumble upon a counterexample.

The below code requests software timestamps, but does not set the
generate flag. I suspect because they assume a PTP daemon (sfptpd)
running that has already enabled that.

https://github.com/Xilinx-CNS/onload/blob/master/src/tests/onload/hwtimestamping/rx_timestamping.c

I suspect that there will be more of such examples in practice. In
which case we should scuttle this. Please do a search online for
SOF_TIMESTAMPING_SOFTWARE to scan for this pattern.
 
> More than this, we can find there are some other ways to turn on
> netstamp_needed_key, which will happenly allow users to get tstamp in
> the receive path. Please see net_enable_timestamp().
> 
> How to solve it?
> 
> setsockopt interface is used to control each socket separately but in
> this case it is affected by other sockets. For timestamp itself, it's
> not feasible to convert netstamp_needed_key into a per-socket button
> because when the receive stack just handling the skb from driver doesn't
> know which socket the skb belongs to.
> 
> According to the original design, we should not use both generation flag
> (SOF_TIMESTAMPING_RX_SOFTWARE) and report flag (SOF_TIMESTAMPING_SOFTWARE)
> together to test if the application is allowed to receive the timestamp
> report in the receive path. But it doesn't hold for receive timestamping
> case. We have to make an exception.
> 
> So we have to test the generation flag when the applications do recvmsg:
> if we set both of flags, it means we want the timestamp; if not, it means
> we don't expect to see the timestamp even the skb carries.
> 
> As we can see, this patch makes the SOF_TIMESTAMPING_RX_SOFTWARE under
> setsockopt control. And it's a per-socket fine-grained now.
> 
> v2
> Link: https://lore.kernel.org/all/20240825152440.93054-1-kerneljasonxing@gmail.com/
> Discussed with Willem
> 1. update the documentation accordingly
> 2. add more comments in each patch
> 3. remove the previous test statements in __sock_recv_timestamp()
> 
> Jason Xing (2):
>   tcp: make SOF_TIMESTAMPING_RX_SOFTWARE feature per socket
>   net: make SOF_TIMESTAMPING_RX_SOFTWARE feature per socket
> 
>  Documentation/networking/timestamping.rst |  7 +++++++
>  include/net/sock.h                        |  7 ++++---
>  net/bluetooth/hci_sock.c                  |  4 ++--
>  net/core/sock.c                           |  2 +-
>  net/ipv4/ip_sockglue.c                    |  2 +-
>  net/ipv4/ping.c                           |  2 +-
>  net/ipv4/tcp.c                            | 11 +++++++++--
>  net/ipv6/datagram.c                       |  4 ++--
>  net/l2tp/l2tp_ip.c                        |  2 +-
>  net/l2tp/l2tp_ip6.c                       |  2 +-
>  net/nfc/llcp_sock.c                       |  2 +-
>  net/rxrpc/recvmsg.c                       |  2 +-
>  net/socket.c                              | 11 ++++++++---
>  net/unix/af_unix.c                        |  2 +-
>  14 files changed, 40 insertions(+), 20 deletions(-)
> 
> -- 
> 2.37.3
>