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

[Toke Høiland-Jørgensen]

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>