diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -1745,8 +1745,11 @@
     static const size_t cmsg_len_end = (offsetof(struct cmsghdr, cmsg_len) +
                                         sizeof(cmsgh->cmsg_len));
 
-    /* Note that POSIX allows msg_controllen to be of signed type. */
-    if (cmsgh == NULL || msg->msg_control == NULL || msg->msg_controllen < 0)
+    /* Note that POSIX in recent versions allows msg_controllen to be a signed
+       type fitting in 2**31-1, but older versions it says it should be
+       unsigned and fit within 2**32-1. */
+    if (cmsgh == NULL || msg->msg_control == NULL ||
+           ((long long)msg->msg_controllen) < 0)
         return 0;
     if (space < cmsg_len_end)
         space = cmsg_len_end;