fishdescriptor: Copy fd passing C sender code from libxl

Copy int libxl__sendmsg_fds from libxl so we can clone and hack it.

Source was:
 xen.git#38ab259f559be5457f6866ba24185e013f27defb
 tools/libxl/libxl_utils.c

libxl is LGPL2.1-only.  We will upgrade this licence to be compatible
with the rest of chiark utils (GPL3+) in a moment.  This is permitted
by LGPL-2.1 section 3.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

diff --git a/fishdescriptor/donate.c b/fishdescriptor/donate.c
new file mode 100644 (file)
index 0000000..b7ef5f8
--- /dev/null
+++ b/fishdescriptor/donate.c
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2009 Citrix Ltd.
+ * Copyright (C) 2017 Citrix Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation; version 2.1 only. with the special
+ * exception on linking described in file LICENSE.
+ */
+
+int libxl__sendmsg_fds(libxl__gc *gc, int carrier,
+                       const void *data, size_t datalen,
+                       int nfds, const int fds[], const char *what) {
+    struct msghdr msg = { 0 };
+    struct cmsghdr *cmsg;
+    size_t spaceneeded = nfds * sizeof(fds[0]);
+    char control[CMSG_SPACE(spaceneeded)];
+    struct iovec iov;
+    int r;
+
+    iov.iov_base = (void*)data;
+    iov.iov_len  = datalen;
+
+    /* compose the message */
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+    msg.msg_control = control;
+    msg.msg_controllen = sizeof(control);
+
+    /* attach open fd */
+    cmsg = CMSG_FIRSTHDR(&msg);
+    cmsg->cmsg_level = SOL_SOCKET;
+    cmsg->cmsg_type = SCM_RIGHTS;
+    cmsg->cmsg_len = CMSG_LEN(spaceneeded);
+    memcpy(CMSG_DATA(cmsg), fds, spaceneeded);
+
+    msg.msg_controllen = cmsg->cmsg_len;
+
+    r = sendmsg(carrier, &msg, 0);
+    if (r < 0) {
+        LOGE(ERROR, "failed to send fd-carrying message (%s)", what);
+        return ERROR_FAIL;
+    }
+
+    return 0;
+}