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

On Thu, Aug 27, 2026 at 3:28 PM Toke Høiland-Jørgensen [off-list ref] wrote:

Ramses de Norre via B4 Relay
[off-list ref] writes:

quoted

From: Ramses de Norre <redacted>

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 <redacted>

Seems reasonable:

Reviewed-by: Toke Høiland-Jørgensen <toke@kernel.org>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Thanks.