netdev - Re: [PATCH v2] net: packetmmap: fix only tx timestamp on request

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Date:   Wed, 12 May 2021 10:24:06 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Richard Sanger <rsanger@...d.net.nz>
Cc:     Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v2] net: packetmmap: fix only tx timestamp on request

On Tue, May 11, 2021 at 9:32 PM Richard Sanger <rsanger@...d.net.nz> wrote:
>
> The packetmmap tx ring should only return timestamps if requested via
> setsockopt PACKET_TIMESTAMP, as documented. This allows compatibility
> with non-timestamp aware user-space code which checks
> tp_status == TP_STATUS_AVAILABLE; not expecting additional timestamp
> flags to be set in tp_status.
>
> Fixes: b9c32fb27170 ("packet: if hw/sw ts enabled in rx/tx ring, report which ts we got")
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Signed-off-by: Richard Sanger <rsanger@...d.net.nz>

Code LGTM.

It would be good to capture more of the context: when these spurious
timestamps can appear (network namespaces) and as of which commit (the
one that changes orphaning). As is, I would not be able to understand
the issue addressed from this commit message alone.

Instead of adding context to the commit, you could also add a Link tag
to the archived email thread, if you prefer.

And perhaps: [PATCH net v3] net/packet: return software transmit
timestamp only when requested



> ---
>  net/packet/af_packet.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index ba96db1..ae906eb 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -422,7 +422,8 @@ static __u32 tpacket_get_timestamp(struct sk_buff *skb, struct timespec64 *ts,
>             ktime_to_timespec64_cond(shhwtstamps->hwtstamp, ts))
>                 return TP_STATUS_TS_RAW_HARDWARE;
>
> -       if (ktime_to_timespec64_cond(skb->tstamp, ts))
> +       if ((flags & SOF_TIMESTAMPING_SOFTWARE) &&
> +           ktime_to_timespec64_cond(skb->tstamp, ts))
>                 return TP_STATUS_TS_SOFTWARE;
>
>         return 0;
> @@ -2340,7 +2341,12 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>
>         skb_copy_bits(skb, 0, h.raw + macoff, snaplen);
>
> -       if (!(ts_status = tpacket_get_timestamp(skb, &ts, po->tp_tstamp)))
> +       /* Always timestamp; prefer an existing software timestamp taken
> +        * closer to the time of capture.
> +        */
> +       ts_status = tpacket_get_timestamp(skb, &ts,
> +                                         po->tp_tstamp | SOF_TIMESTAMPING_SOFTWARE);
> +       if (!ts_status)
>                 ktime_get_real_ts64(&ts);
>
>         status |= ts_status;
> --
> 2.7.4
>