workaround another sendmsg kernel bug on 64-bit machines

the kernel wrongly expects the cmsg length field to be size_t instead
of socklen_t. in order to work around the issue, we have to impose a
length limit and copy to a local buffer. the length limit should be
more than sufficient for any real-world use; these headers are only
used for passing file descriptors and permissions between processes
over unix sockets.

src/network/sendmsg.c

@@ -1,5 +1,7 @@
 #include <sys/socket.h>
 #include <limits.h>
+#include <string.h>
+#include <errno.h>
 #include "syscall.h"
 #include "libc.h"
 
@@ -7,10 +9,21 @@ ssize_t sendmsg(int fd, const struct msghdr *msg, int flags)
 {
 #if LONG_MAX > INT_MAX
 	struct msghdr h;
+	struct cmsghdr chbuf[1024/sizeof(struct cmsghdr)+1], *c;
 	if (msg) {
 		h = *msg;
 		h.__pad1 = h.__pad2 = 0;
 		msg = &h;
+		if (h.msg_controllen) {
+			if (h.msg_controllen > 1024) {
+				errno = ENOMEM;
+				return -1;
+			}
+			memcpy(chbuf, h.msg_control, h.msg_controllen);
+			h.msg_control = chbuf;
+			for (c=CMSG_FIRSTHDR(&h); c; c=CMSG_NXTHDR(&h,c))
+				c->__pad1 = 0;
+		}
 	}
 #endif
 	return socketcall_cp(sendmsg, fd, msg, flags, 0, 0, 0);