verify we get exactly 3 fds from client

luad.c

@@ -50,29 +50,60 @@ int open_socket(char * pathname) {
     return fd;
 }
 
-int * fds_from_msg(struct msghdr *msg, size_t peer_fds_count) {
-    int *peer_fds = (int *) calloc(peer_fds_count, sizeof(int));
+int * fds_from_msg(struct msghdr *msg, int * peer_fds) {
+    // * there may be multiple SCM_RIGHTS messages
+    // * we expect three fds
+    // * if we get some number other than three, we should
+    //   close all of them and return NULL because someone is up to no good
+    int i = 0;
+    peer_fds[0] = -1;
+    peer_fds[1] = -1;
+    peer_fds[2] = -1;
+
     for (struct cmsghdr* cmsg = CMSG_FIRSTHDR(msg);
          cmsg != NULL;
          cmsg = CMSG_NXTHDR(msg, cmsg)) {
         if(cmsg->cmsg_level == SOL_SOCKET &&
            cmsg->cmsg_type == SCM_RIGHTS) {
-            memcpy(peer_fds, CMSG_DATA(cmsg), peer_fds_count * sizeof(int));
+            int passed_fd_count = (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof (int);
+            if(i <= 3) {
+                memcpy(&peer_fds[i], CMSG_DATA(cmsg), passed_fd_count * sizeof(int));
+
+            } else {
+                int * reject_fds = (int *) CMSG_DATA(cmsg);
+                for(int fd = 0; fd < passed_fd_count; fd++) {
+                    close(reject_fds[fd]);
+                }
+            }
+            i += passed_fd_count;
         }
     }
-    return peer_fds;
+    if(i != 3) {
+        /* if the peer sent too many fds, we closed the excess ones
+         * in the reject_fds clause above and now we'll close the
+         * first three.
+         * if they sent too few, we'll close whatever they sent
+         */
+
+        if(peer_fds[0] >=0) close(peer_fds[0]);
+        if(peer_fds[1] >=0) close(peer_fds[1]);
+        if(peer_fds[2] >=0) close(peer_fds[2]);
+        return NULL;
+    } else {
+        return peer_fds;
+    }
 }
 
 int start_lua_child(lua_State *L,
                      char * command_line, int command_line_length,
-                     int * fds, int fds_count) {
+                     int * fds) {
     char * word = command_line;
     char * command_line_end = command_line + command_line_length;
     char * pathname = NULL;
     int i = 0;
 
     fflush(stdout); fflush(stderr);
-    for(int i=0; i < fds_count; i++) {
+    for(int i=0; i < 3; i++) {
         if(fds[i] > 0) dup2(fds[i], i);
     }
 
@@ -98,11 +129,11 @@ int start_lua_child(lua_State *L,
 
 pid_t fork_lua(lua_State *L,
               char * command_line, int command_line_length,
-               int * fds, int fds_count) {
+               int * fds) {
     pid_t pid = fork();
     if(pid == 0) {
         exit(start_lua_child(L, command_line, command_line_length,
-                             fds, fds_count));
+                             fds));
     } else if(pid > 0) {
         free(command_line);
         return pid;
@@ -213,16 +244,22 @@ int main(int argc, char **argv)
         if(pollfds[0].revents) {
             if(find_peer(0)) {
                 int len = recvmsg(socket, &msg , 0);
-                int *peer_fds = fds_from_msg(&msg, 5);
-                char *argline = malloc(len);
-                if(! argline) {
-                    die("malloc failed");
-                }
-                memmove(argline, buf, len);
+                int peer_fds[3];
+                if(fds_from_msg(&msg, peer_fds)) {
+                    char *argline = malloc(len);
+                    if(! argline) {
+                        die("malloc failed");
+                    }
+                    memmove(argline, buf, len);
 
-                pid_t child_pid = fork_lua(L, argline, len, peer_fds, 5);
-                if(child_pid >= 0) {
-                    register_peer(child_pid, &peer_addr, msg.msg_namelen);
+                    pid_t child_pid = fork_lua(L, argline, len, peer_fds);
+                    if(child_pid >= 0) {
+                        register_peer(child_pid, &peer_addr, msg.msg_namelen);
+                    }
+                    /* close peer fds in parent */
+                    close(peer_fds[0]);
+                    close(peer_fds[1]);
+                    close(peer_fds[2]);
                 }
             }
         }