From: Asbjørn Sloth Tønnesen <ast@2e8.dk>
Date: Thu, 21 May 2026 12:43:35 +0000 (+0200)
Subject: Netlink: Skip request payload in acknowledgements
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=127ab72e5a2dd689d4de8b2f6deea4c7a37a31e9;p=thirdparty%2Fbird.git

Netlink: Skip request payload in acknowledgements

Set NETLINK_CAP_ACK, as we don't need a full copy of the request
payload, so it's "rather wasteful"[1] to not set NETLINK_CAP_ACK.

We don't check the return value of setsockopt(), as any capped
message will have NLM_F_CAPPED set in nlmsg_flags.

NETLINK_CAP_ACK was introduced in Linux v4.3 commit 0a6a3a23ea6e
("netlink: add NETLINK_CAP_ACK socket option"), and so AFAICT it
doesn't need to be added to netlink-sys.h.

[1] https://docs.kernel.org/userspace-api/netlink/intro.html
---

diff --git a/sysdep/linux/netlink.c b/sysdep/linux/netlink.c
index 9c519adb6..68f075fc0 100644
--- a/sysdep/linux/netlink.c
+++ b/sysdep/linux/netlink.c
@@ -70,6 +70,14 @@ static linpool *nl_linpool;
 static struct nl_sock nl_scan = {.fd = -1};	/* Netlink socket for synchronous scan */
 static struct nl_sock nl_req  = {.fd = -1};	/* Netlink socket for requests */
 
+static void
+nl_set_cap_ack(struct nl_sock *nl UNUSED, int val UNUSED)
+{
+#ifdef SOL_NETLINK
+  setsockopt(nl->fd, SOL_NETLINK, NETLINK_CAP_ACK, &val, sizeof(val));
+#endif
+}
+
 static void
 nl_open_sock(struct nl_sock *nl)
 {
@@ -82,6 +90,8 @@ nl_open_sock(struct nl_sock *nl)
       nl->rx_buffer = xmalloc(NL_RX_SIZE);
       nl->last_hdr = NULL;
       nl->last_size = 0;
+
+      nl_set_cap_ack(nl, 1);
     }
 }