Ensure proper alignment of cmsg buffers

The cmsg macros expect a control message buffer to be aligned like
a struct cmsghdr.  The current layout around those stack-allocated
buffers probably provides the required alignment (usually 4 bytes).
Use a union to enforce proper alignment, in case future changes modify
the stack layout.

Spotted when chasing an unrelated bug with Otto Moerbeek (@omoerbeek).

diff --git a/util/netevent.c b/util/netevent.c
index f7bb9b8979351132de629e114c7ff7391a4fdac1..159d344d6896fbf6b05fe35658b1d98d8c7a760b 100644 (file)
--- a/util/netevent.c
+++ b/util/netevent.c
@@ -447,7 +447,10 @@ comm_point_send_udp_msg_if(struct comm_point *c, sldns_buffer* packet,
        ssize_t sent;
        struct msghdr msg;
        struct iovec iov[1];
-       char control[256];
+       union {
+               struct cmsghdr hdr;
+               char buf[256];
+       } control;
 #ifndef S_SPLINT_S
        struct cmsghdr *cmsg;
 #endif /* S_SPLINT_S */
@@ -465,9 +468,9 @@ comm_point_send_udp_msg_if(struct comm_point *c, sldns_buffer* packet,
        iov[0].iov_len = sldns_buffer_remaining(packet);
        msg.msg_iov = iov;
        msg.msg_iovlen = 1;
-       msg.msg_control = control;
+       msg.msg_control = control.buf;
 #ifndef S_SPLINT_S
-       msg.msg_controllen = sizeof(control);
+       msg.msg_controllen = sizeof(control.buf);
 #endif /* S_SPLINT_S */
        msg.msg_flags = 0;
 
@@ -584,7 +587,10 @@ comm_point_udp_ancil_callback(int fd, short event, void* arg)
        struct msghdr msg;
        struct iovec iov[1];
        ssize_t rcv;
-       char ancil[256];
+       union {
+               struct cmsghdr hdr;
+               char buf[256];
+       } ancil;
        int i;
 #ifndef S_SPLINT_S
        struct cmsghdr* cmsg;
@@ -608,9 +614,9 @@ comm_point_udp_ancil_callback(int fd, short event, void* arg)
                iov[0].iov_len = sldns_buffer_remaining(rep.c->buffer);
                msg.msg_iov = iov;
                msg.msg_iovlen = 1;
-               msg.msg_control = ancil;
+               msg.msg_control = ancil.buf;
 #ifndef S_SPLINT_S
-               msg.msg_controllen = sizeof(ancil);
+               msg.msg_controllen = sizeof(ancil.buf);
 #endif /* S_SPLINT_S */
                msg.msg_flags = 0;
                rcv = recvmsg(fd, &msg, 0);