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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Thu, Aug 27, 2026 at 01:52:56PM +0200, Ramses de Norre via B4 Relay wrote:
> From: Ramses de Norre <ramses@well-founded.dev>
> 
> 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@well-founded.dev>
> ---
>  drivers/net/wireguard/queueing.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
> index 79b6d70de236b..663b19b783953 100644
> --- a/drivers/net/wireguard/queueing.h
> +++ b/drivers/net/wireguard/queueing.h
> @@ -78,12 +78,14 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
>  	u8 l4_hash = skb->l4_hash;
>  	u8 sw_hash = skb->sw_hash;
>  	u32 hash = skb->hash;
> +	u8 tstamp_type = skb->tstamp_type;
>  	skb_scrub_packet(skb, true);
>  	memset(&skb->headers, 0, sizeof(skb->headers));
>  	if (encapsulating) {
>  		skb->l4_hash = l4_hash;
>  		skb->sw_hash = sw_hash;
>  		skb->hash = hash;
> +		skb->tstamp_type = tstamp_type;
>  	}
>  	skb->queue_mapping = 0;
>  	skb->nohdr = 0;

Seems reasonable to me. I'll put this through the paces testing and
queue it up when I'm back at my laptop on Monday.

Jason