tidy up open_socket and explain need for abstract bind

luac.c

@@ -44,23 +44,28 @@ size_t strlcpy(char *dst, const char *src, size_t dsize)
     return(src - osrc - 1);	/* count does not include NUL */
 }
 
-int open_socket(char * pathname){
-
-    struct sockaddr_un addr;
-    addr.sun_family = AF_UNIX;
-    addr.sun_path[0] = '\0';    /* abstract path */
-    strncpy(&addr.sun_path[1], pathname, sizeof(addr.sun_path) - 2);
-
+int open_socket(struct sockaddr_un *addr, struct sockaddr_un * peer_addr) {
     int fd = socket(AF_LOCAL, SOCK_DGRAM, PF_LOCAL);
     if(fd < 0) {
         perror("socket");
         exit(EXIT_FAILURE);
     }
 
-    if((bind(fd, (struct sockaddr *) &addr, sizeof addr)) < 0) {
+    // local address is in the abstract namespace and ok to include
+    // trailing nul bytes
+    if((bind(fd, (struct sockaddr *) addr, sizeof (struct sockaddr_un))) < 0) {
         perror("bind");
         exit(EXIT_FAILURE);
     }
+
+    // peer address is in the actual filesystem, so we need to pass
+    // the real length
+    int namelen = sizeof peer_addr->sun_family + strlen(peer_addr->sun_path);
+    if((connect(fd, (struct sockaddr *) peer_addr, namelen)) < 0) {
+      perror("connect");
+      exit(EXIT_FAILURE);
+    }
+
     return fd;
 }
 
@@ -101,15 +106,19 @@ int main(int argc, char **argv)
     if(!iobuf) return 1;
     int iobuf_len = encode_args(iobuf, 1024, argc, argv);
 
-    int maxnamelen=sizeof (((struct sockaddr_un *) 0)->sun_path);
-    char *sockname = calloc(maxnamelen, 1);
+
+    /* for sendmsg with peer address to work on the luad side, we need to
+     * bind our connected docket on this side to something, so we create
+     * a hopefully unique abstract name based on the pid and argv */
+
+    struct sockaddr_un my_addr = { AF_UNIX, "" };
+    int maxnamelen = sizeof my_addr.sun_path;
     pid_t mypid = getpid();
-    memcpy(sockname, (void *) &mypid, sizeof mypid);
-    memcpy(sockname + sizeof mypid, iobuf, maxnamelen - sizeof mypid);
+    memcpy(my_addr.sun_path + 1, (void *) &mypid, sizeof mypid);
+    memcpy(my_addr.sun_path + 1 + sizeof mypid, iobuf, maxnamelen - sizeof mypid - 1);
 
     struct sockaddr_un peer_addr = { AF_UNIX, "/tmp/mysock" };
-    int socket = open_socket(sockname);
-    connect(socket, (struct sockaddr *) &peer_addr, sizeof peer_addr);
+    int socket = open_socket(&my_addr, &peer_addr);
 
     struct msghdr msg = { 0 };
     struct cmsghdr *cmsg;