Re: [PATCH net] wireguard: queueing: preserve tstamp_type in wg_reset_packet()

[Eric Dumazet]

On Thu, Aug 27, 2026 at 3:28 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Ramses de Norre via B4 Relay
> <devnull+ramses.well-founded.dev@xxxxxxxxxx> writes:
>
> > From: Ramses de Norre <ramses@xxxxxxxxxxxxxxxx>
> >
> > Sending traffic through a wireguard tunnel on a host using the fq
> > qdisc fills the log with:
> >
> >   fq: likely mono tstamp with tstamp_type 0
> >
> > An skb carries a timestamp in skb->tstamp and, separately, a
> > skb->tstamp_type field recording which clock that timestamp came from.
> > The two have to agree.
> >
> > When wireguard encapsulates a packet it calls wg_reset_packet(), which
> > clears the fields that must not leak from the inner packet into the
> > tunnel packet. It does so in two steps:
> >
> >   skb_scrub_packet(skb, true);
> >   memset(&skb->headers, 0, sizeof(skb->headers));
> >
> > skb_scrub_packet() deliberately keeps skb->tstamp when it holds a
> > monotonic timestamp: that value is the time the packet is scheduled to
> > be sent, and the qdisc still needs it. The memset then zeroes
> > skb->tstamp_type, because that field sits inside the headers group
> > while skb->tstamp does not. The packet therefore leaves wireguard
> > carrying a monotonic timestamp labelled as a realtime one.
> >
> > Nothing noticed until commit c4f796c4f16b ("net_sched: sch_fq: convert
> > skb->tstamp if not monotonic"): fq used to assume every timestamp was
> > monotonic. It now consults tstamp_type, spots the mismatch, warns, and
> > falls back to treating the value as monotonic. Pacing still ends up
> > correct, so the log spam is the actual problem.
> >
> > Save tstamp_type before the memset and restore it when encapsulating,
> > next to the hash fields that are already carried over this way. When
> > decapsulating it stays zeroed, which is right: an incoming packet's
> > timestamp is a realtime receive timestamp.
> >
> > Fixes: de799101519a ("net: Add skb_clear_tstamp() to keep the mono delivery_time")
> > Signed-off-by: Ramses de Norre <ramses@xxxxxxxxxxxxxxxx>
>
> Seems reasonable:
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Thanks.