platform/netlink: accept pre-allocated receive buffer for nl_recv()

Add parameter to accept a pre-allocated buffer for nl_recv(). In
practice, rtnetlink messages are not larger than 32k, so we can always
pre-allocate it and avoid the need to heap allocate it.

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

@@ -9037,7 +9037,7 @@ event_handler_recvmsgs(NMPlatform *platform, gboolean handle_events)
 continue_reading:
     nm_clear_g_free(&buf);
 
-    n = nl_recv(sk, &nla, &buf, &creds, &creds_has);
+    n = nl_recv(sk, NULL, 0, &nla, &buf, &creds, &creds_has);
 
     if (n <= 0) {
         if (n == -NME_NL_MSG_TRUNC) {

src/libnm-platform/nm-netlink.c

@@ -1176,7 +1176,7 @@ nl_recvmsgs(struct nl_sock *sk, const struct nl_cb *cb)
     gboolean               creds_has;
 
 continue_reading:
-    n = nl_recv(sk, &nla, &buf, &creds, &creds_has);
+    n = nl_recv(sk, NULL, 0, &nla, &buf, &creds, &creds_has);
     if (n <= 0)
         return n;
 
@@ -1401,8 +1401,37 @@ nl_send_auto(struct nl_sock *sk, struct nl_msg *msg)
     return nl_send(sk, msg);
 }
 
+/**
+ * nl_recv():
+ * @sk: the netlink socket
+ * @buf0: NULL or a receive buffer of length @buf0_len
+ * @buf0_len: the length of the optional receive buffer.
+ * @nla: (out): the source address on success.
+ * @buf: (out): pointer to the result buffer on success. This is
+ *   either @buf0 or an allocated buffer that gets returned.
+ * @out_creds: (out) (allow-none): optional out buffer for the credentials
+ *   on success.
+ * @out_creds_has: (out) (allow-none): result indicating whether
+ *   @out_creds was filled.
+ *
+ * If @buf0_len is zero, the function will g_malloc() a new receive buffer of size
+ * nl_socket_get_msg_buf_size(). If @buf0_len is larger than zero, then @buf0
+ * is used as receive buffer. That is also the buffer returned by @buf.
+ *
+ * If NL_MSG_PEEK is not enabled and the receive buffer is too small, then
+ * the message was lost and -NME_NL_MSG_TRUNC gets returned.
+ * If NL_MSG_PEEK is enabled, then we first peek. If the buffer is too small,
+ * we g_malloc() a new buffer. In any case, we proceed to receive the buffer.
+ * NL_MSG_PEEK is great because it means no messages are lost. But it's bad,
+ * because we always need two syscalls on every receive.
+ *
+ * Returns: a negative error code or the length of the received message in
+ *   @buf.
+ */
 int
 nl_recv(struct nl_sock     *sk,
+        unsigned char      *buf0,
+        size_t              buf0_len,
         struct sockaddr_nl *nla,
         unsigned char     **buf,
         struct ucred       *out_creds,
@@ -1443,8 +1472,13 @@ nl_recv(struct nl_sock     *sk,
         || (!(sk->s_flags & NL_MSG_PEEK_EXPLICIT) && sk->s_bufsize == 0))
         flags |= MSG_PEEK | MSG_TRUNC;
 
+    if (buf0_len > 0) {
+        iov.iov_len  = buf0_len;
+        iov.iov_base = buf0;
+    } else {
         iov.iov_len  = sk->s_bufsize ?: (((size_t) nm_utils_getpagesize()) * 4u);
         iov.iov_base = g_malloc(iov.iov_len);
+    }
 
     if (out_creds && (sk->s_flags & NL_SOCK_PASSCRED)) {
         msg.msg_controllen = sizeof(msg_contol_buf);
@@ -1482,7 +1516,7 @@ retry:
         /* Provided buffer is not long enough, enlarge it
          * to size of n (which should be total length of the message)
          * and try again. */
-        iov.iov_base = g_realloc(iov.iov_base, n);
+        iov.iov_base = g_realloc(iov.iov_base != buf0 ? iov.iov_base : NULL, n);
         iov.iov_len  = n;
         flags        = 0;
         goto retry;
@@ -1517,6 +1551,7 @@ retry:
 
 abort:
     if (retval <= 0) {
+        if (iov.iov_base != buf0)
             g_free(iov.iov_base);
         return retval;
     }

src/libnm-platform/nm-netlink.h

@@ -518,6 +518,8 @@ int nl_socket_add_memberships(struct nl_sock *sk, int group, ...);
 int nl_connect(struct nl_sock *sk, int protocol);
 
 int nl_recv(struct nl_sock     *sk,
+            unsigned char      *buf0,
+            size_t              buf0_len,
             struct sockaddr_nl *nla,
             unsigned char     **buf,
             struct ucred       *out_creds,