[PATCH net-next] wireguard: queuing: preserve napi_id on decapsulation

[Miroslav Lichvar <mlichvar@redhat.com>]

The socket timestamping option SOF_TIMESTAMPING_OPT_PKTINFO needs the
skb napi_id in order to provide the index of the device that captured
the receive hardware timestamp. However, wireguard resets most of the
skb headers, including the napi_id, which prevents the timestamping
option from working as expected and applications that rely on it (e.g.
chrony) from using the captured timestamps.

Preserve the napi_id in wg_reset_packet() on decapsulation in order to
make the timestamping option useful with wireguard tunnels and enable
highly-accurate synchronization.

Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
---

Notes:
    This is a minimal change that fixes the described problem for me, but
    I don't really understand the code well enough to see if there are any
    major side effects. If there is a better way to fix the option for wg or
    tunnels in general, please let me know.

 drivers/net/wireguard/queueing.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
index 79b6d70de236..bcf03bc01992 100644
--- a/drivers/net/wireguard/queueing.h
+++ b/drivers/net/wireguard/queueing.h
@@ -75,6 +75,7 @@ static inline bool wg_check_packet_protocol(struct sk_buff *skb)
 
 static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
 {
+	unsigned int napi_id = skb_napi_id(skb);
 	u8 l4_hash = skb->l4_hash;
 	u8 sw_hash = skb->sw_hash;
 	u32 hash = skb->hash;
@@ -84,6 +85,10 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
 		skb->l4_hash = l4_hash;
 		skb->sw_hash = sw_hash;
 		skb->hash = hash;
+	} else {
+#if defined(CONFIG_NET_RX_BUSY_POLL) || defined(CONFIG_XPS)
+		skb->napi_id = napi_id;
+#endif
 	}
 	skb->queue_mapping = 0;
 	skb->nohdr = 0;
-- 
2.51.0