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

[Jason A. Donenfeld]

Hi Ramses,

On Thu, Aug 27, 2026 at 01:52:56PM +0200, Ramses de Norre via B4 Relay wrote:
> 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>
 
Thanks for this. I've queued this up in the wireguard tree.

One thing I'm wondering is whether your Fixes: line is correct. Would
c4f796c4f16b be a better candidate than de799101519a? Or maybe even
4d25ca2d6801 is best?

Jason