net: socket: recvmsg() doesn't update msg_controllen accordingly

[axelnxp]

Describe the bug

The bug is exactly the same than it was with #68352.
This was fixed at the time, but I think the changes from #73713 introduced the regression.
msg_controllen is still set to the initial length, while csmg_len is set to 0.

If I bypass this added code, I don't reproduce the issue.

Maybe the problem comes from add_timestamping ? Does it update the csmg_len accordingly (if applicable) ?

To Reproduce
One can reuse the matter function where the issue is observed:
https://github.com/project-chip/connectedhomeip/blob/master/src/inet/UDPEndPointImplSockets.cpp#L611
And make sure no control data is received.

Expected behavior
msg_controllen field shall be updated by recvmsg according to the control data received.

Impact
Impact is high, in the current state, any Matter application over Zephyr would be impacted, likely unable to even perform a commissioning.

[axelnxp]

cc @jukkar

[jukkar]

Cc: @awojasinski please take a look, seems to be related to the ptp pr

[axelnxp]

I've been able to get more info on the root cause.
In my case, add_timestamping returns -ENOTSUP because net_context_get_option sets timestamping to 0.
In this case, csmg_len is not updated, so msg_controllen should be cleared, as no pkt info has been added.

So the problem comes from the clear_controllen logic.

I came up with the following patch, the idea is to not clear msg_controllen only when adding pkt info was a success.
I can submit the PR if this patch looks fine to you.

diff --git a/subsys/net/lib/sockets/sockets.c b/subsys/net/lib/sockets/sockets.c
index 25328939457..118cf0f9f93 100644
--- a/subsys/net/lib/sockets/sockets.c
+++ b/subsys/net/lib/sockets/sockets.c
@@ -1573,17 +1573,19 @@ static inline ssize_t zsock_recv_dgram(struct net_context *ctx,
                                bool clear_controllen = true;
 
                                if (IS_ENABLED(CONFIG_NET_CONTEXT_TIMESTAMPING)) {
-                                       clear_controllen = false;
                                        if (add_timestamping(ctx, pkt, msg) < 0) {
                                                msg->msg_flags |= ZSOCK_MSG_CTRUNC;
+                                       } else {
+                                               clear_controllen = false;
                                        }
                                }
 
                                if (IS_ENABLED(CONFIG_NET_CONTEXT_RECV_PKTINFO) &&
                                    net_context_is_recv_pktinfo_set(ctx)) {
-                                       clear_controllen = false;
                                        if (add_pktinfo(ctx, pkt, msg) < 0) {
                                                msg->msg_flags |= ZSOCK_MSG_CTRUNC;
+                                       } else {
+                                               clear_controllen = false;
                                        }
                                }

[jukkar]

@axelnxp looks good, please send a PR for it.

[axelnxp]

@jukkar done, refer to #77345