platform: use pre-allocated receive buffer for rtnetlink socket

Instead of allocating a receive buffer for each nl_recv() call, re-use a
pre-allocated buffer.

The buffer is part of NMPlatform and kept around. As we would not have more
than one NMPlatform instance per netns, we waste a limited amount of
memory.

The buffer gets initialized with 32k, which should be large enough for
any rtnetlink message that we might receive. As before, if the buffer
would be too small, then the first large message would be lost (as we don't
peek). But then we would allocate a larger buffer, resync the platform cache
and recover.

src/libnm-platform/nm-linux-platform.c

@@ -451,6 +451,17 @@ typedef struct {
         int is_handling;
     } delayed_action;
 
+    /* This is the receive buffer for netlink messages. This buffer should be large
+     * enough for any rtnetlink message. When too small, nl_recv() would notice the
+     * truncation and lose the message. In that case, we reallocate a larger buffer.
+     *
+     * We keep the receive buffer around for the entire lifetime of the platform instance.
+     * Usually we only have one platform instance per netns, so we don't waste too much. */
+    struct {
+        unsigned char *buf;
+        gsize          len;
+    } netlink_recv_buf;
+
 } NMLinuxPlatformPrivate;
 
 struct _NMLinuxPlatform {
@@ -9032,29 +9043,33 @@ event_handler_recvmsgs(NMPlatform *platform, gboolean handle_events)
     struct sockaddr_nl      nla = {0};
     struct ucred            creds;
     gboolean                creds_has;
-    gs_free unsigned char  *buf = NULL;
+    unsigned char          *buf;
 
 continue_reading:
-    nm_clear_g_free(&buf);
+    buf = NULL;
 
-    n = nl_recv(sk, NULL, 0, &nla, &buf, &creds, &creds_has);
+    n = nl_recv(sk,
+                priv->netlink_recv_buf.buf,
+                priv->netlink_recv_buf.len,
+                &nla,
+                &buf,
+                &creds,
+                &creds_has);
+
+    nm_assert((n <= 0 && !buf)
+              || (n > 0 && n <= priv->netlink_recv_buf.len && buf == priv->netlink_recv_buf.buf));
 
     if (n <= 0) {
         if (n == -NME_NL_MSG_TRUNC) {
-            int buf_size;
-
             /* the message receive buffer was too small. We lost one message, which
              * is unfortunate. Try to double the buffer size for the next time. */
-            buf_size = nl_socket_get_msg_buf_size(sk);
-            if (buf_size < 512 * 1024) {
-                buf_size *= 2;
-                _LOGT("netlink: recvmsg: increase message buffer size for recvmsg() to %d bytes",
-                      buf_size);
-                if (nl_socket_set_msg_buf_size(sk, buf_size) < 0)
-                    nm_assert_not_reached();
-                if (!handle_events)
-                    goto continue_reading;
-            }
+            priv->netlink_recv_buf.len *= 2;
+            priv->netlink_recv_buf.buf =
+                g_realloc(priv->netlink_recv_buf.buf, priv->netlink_recv_buf.len);
+            _LOGT("netlink: recvmsg: increase message buffer size for recvmsg() to %zu bytes",
+                  priv->netlink_recv_buf.len);
+            if (!handle_events)
+                goto continue_reading;
         }
 
         return n;
@@ -9487,6 +9502,9 @@ nm_linux_platform_init(NMLinuxPlatform *self)
 {
     NMLinuxPlatformPrivate *priv = NM_LINUX_PLATFORM_GET_PRIVATE(self);
 
+    priv->netlink_recv_buf.len = 32 * 1024;
+    priv->netlink_recv_buf.buf = g_malloc(priv->netlink_recv_buf.len);
+
     c_list_init(&priv->sysctl_clear_cache_lst);
     c_list_init(&priv->sysctl_list);
 
@@ -9555,10 +9573,10 @@ constructed(GObject *_object)
         _LOGD("could not enable extended acks on netlink socket");
 
     /* explicitly set the msg buffer size and disable MSG_PEEK.
-     * If we later encounter NME_NL_MSG_TRUNC, we will adjust the buffer size. */
+     * We use our own receive buffer priv->netlink_recv_buf.
+     * If we encounter NME_NL_MSG_TRUNC, we will increase the buffer
+     * and resync (as we would have lost the message without NL_MSG_PEEK). */
     nl_socket_disable_msg_peek(priv->nlh);
-    nle = nl_socket_set_msg_buf_size(priv->nlh, 32 * 1024);
-    g_assert(!nle);
 
     nle = nl_socket_add_memberships(priv->nlh,
                                     RTNLGRP_IPV4_IFADDR,
@@ -9726,6 +9744,8 @@ finalize(GObject *object)
     priv->udev_client = nm_udev_client_destroy(priv->udev_client);
 
     G_OBJECT_CLASS(nm_linux_platform_parent_class)->finalize(object);
+
+    g_free(priv->netlink_recv_buf.buf);
 }
 
 static void