[PATCH wireguard-go 3/3] conn: propagate ancillary data to coalesced split frames

[Artjom Vejsel <akawolf0@gmail.com>]

splitCoalescedMessages divides a single coalesced read into individual
frames by copying Buffers[0][start:end], setting N, and propagating
Addr to each child slot, but it does not propagate the control buffer
(OOB) or its length (NN). The parent buffer holds the cmsg returned by
recvmmsg for the entire coalesced datagram, including the PKTINFO that
identifies the destination address of the original packet. The child
slots retain whatever control content was left over from a previous
recv, with NN == 0 for unused slots and stale data for reused ones.

receiveIP then calls getSrcFromControl(msg.OOB[:msg.NN], ep) on each
emitted message. Because NN is zero for every split frame except the
first (which still aliases the parent's storage), StdNetEndpoint.src
is never populated for the rest, and setSrcControl on the
corresponding send writes empty control bytes. The kernel falls back
to route-based source selection, defeating the sticky source address
mechanism whenever UDP_GRO coalesces packets — which is the common
case on Linux 5.12+.

Copy the parent's control buffer and length into each split child. The
n == i case is skipped because the destination slot is the parent
itself and the slices already alias.

Signed-off-by: Artjom Vejsel <akawolf0@gmail.com>
---
 conn/bind_std.go | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/conn/bind_std.go b/conn/bind_std.go
index 72f3047..88c0ce3 100644
--- a/conn/bind_std.go
+++ b/conn/bind_std.go
@@ -528,6 +528,11 @@ func splitCoalescedMessages(msgs []ipv6.Message, firstMsgAt int, getGSO getGSOFu
 			copied := copy(msgs[n].Buffers[0], msg.Buffers[0][start:end])
 			msgs[n].N = copied
 			msgs[n].Addr = msg.Addr
+			if n != i {
+				oobCopied := copy(msgs[n].OOB[:cap(msgs[n].OOB)], msg.OOB[:msg.NN])
+				msgs[n].OOB = msgs[n].OOB[:oobCopied]
+				msgs[n].NN = oobCopied
+			}
 			start = end
 			end += gsoSize
 			if end > msg.N {
-- 
2.54.0