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

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

On Mon, Aug 31, 2026 at 12:24:15PM +0200, Ramses wrote:
> Aug 31, 2026, 05:25 by Jason@zx2c4.com:
> 
> > Hi Ramses,
> >
> > 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>
> >>
> >
> > 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
> >
> Hi Jason
> 
> I think you are right, but I think d98d58a00261 is actually the commit that introduced the issue then (but no warnings were logged yet at that point).
> The patch doesn't apply cleanly to that version though, I think it only applies starting from 6.11 because of a rename, so we can't backport all the way back. It's probably fine though, since the warnings weren't being logged yet at that point.
> Do you want me to send an updated patch, or will you just fix this up in your tree?

I'll fix it in my tree to be:
   Fixes: d98d58a00261 ("net: Set skb->mono_delivery_time and clear it after sch_handle_ingress()")