make our version of nla_for_each_nested() public

caputils/ws80211_utils.c contains a re-definition of the linux kernel's
nla_for_each_nested() macro that applies the correct casts to allow
compilation with a C++ compiler.

Make this definition public by moving it into a new wsutil/netlink.h
file. Include the kernel's original definition before we overwrite it. This
way, it's not necessary for a .c file to include wsutil/netlink.h after
the system includes.

Use our nla_for_each_nested() version in extcap/dpauxmon.c to squelch the
following compiler warning:

[1664/2251] Building C object
extcap/CMakeFiles/dpauxmon.dir/dpauxmon.c.o
../extcap/dpauxmon.c: In function ‘family_handler’:
../extcap/dpauxmon.c:168:13: warning: request for implicit conversion
        from ‘void *’ to ‘struct nlattr *’ not permitted in C++ [-Wc++-compat]
  nla_for_each_nested(mcgrp, tb[CTRL_ATTR_MCAST_GROUPS], rem_mcgrp) {

Change-Id: I6ba40ef6343c5d168c1b0c4554f13202911ded76
Reviewed-on: https://code.wireshark.org/review/27688
Reviewed-by: Anders Broman <a.broman58@gmail.com>

caputils/ws80211_utils.c

@@ -41,6 +41,8 @@ DIAG_ON_PEDANTIC
 
 #include <linux/nl80211.h>
 
+#include <wsutil/netlink.h>
+
 #ifdef HAVE_NL80211_SPLIT_WIPHY_DUMP
 static int ws80211_get_protocol_features(int* features);
 #endif /* HAVE_NL80211_SPLIT_WIPHY_DUMP */
@@ -195,27 +197,6 @@ static struct ws80211_interface *
 	return NULL;
 }
 
-/*
- * And now for a steaming heap of suck.
- *
- * The nla_for_each_nested() macro defined by at least some versions of the
- * Linux kernel's headers doesn't do the casting required when compiling
- * with a C++ compiler or with -Wc++-compat, so we get warnings, and those
- * warnings are fatal when we compile this file.
- *
- * So we replace it with our own version, which does the requisite cast.
- */
-
-/**
- * nla_for_each_nested - iterate over nested attributes
- * @pos: loop counter, set to current attribute
- * @nla: attribute containing the nested attributes
- * @rem: initialized to len, holds bytes currently remaining in stream
- */
-#undef nla_for_each_nested
-#define nla_for_each_nested(pos, nla, rem) \
-	nla_for_each_attr(pos, (struct nlattr *)nla_data(nla), nla_len(nla), rem)
-
 #ifdef HAVE_NL80211_SPLIT_WIPHY_DUMP
 static int get_features_handler(struct nl_msg *msg, void *arg)
 {

extcap/dpauxmon.c

@@ -16,6 +16,7 @@
 
 #include <wsutil/strtoi.h>
 #include <wsutil/filesystem.h>
+#include <wsutil/netlink.h>
 #include <writecap/pcapio.h>
 
 #include <netlink/netlink.h>

wsutil/CMakeLists.txt

@@ -45,6 +45,7 @@ set(WSUTIL_PUBLIC_HEADERS
 	interface.h
 	jsmn.h
 	mpeg-audio.h
+	netlink.h
 	nstime.h
 	os_version_info.h
 	pint.h

wsutil/netlink.h

@@ -0,0 +1,66 @@
+/* netlink.h
+ * netlink-related definitions shared between libwireshark and other parts
+ *
+ * Copyright 2018, Martin Kaiser
+ *
+ * Wireshark - Network traffic analyzer
+ * By Gerald Combs <gerald@wireshark.org>
+ * Copyright 1998 Gerald Combs
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#ifndef _WS_NETLINK_H
+#define _WS_NETLINK_H
+
+#include "config.h"
+
+#if defined(HAVE_LIBNL)
+
+/*
+ * Pull in the include files where the kernel's nla_for_each_nested is defined.
+ * This is to make sure that the kernel's definition will not overwrite our
+ * version if msg.h or attr.h are included again explicitly after this file.
+ */
+DIAG_OFF_PEDANTIC
+#include <netlink/msg.h>
+DIAG_ON_PEDANTIC
+#include <netlink/attr.h>
+
+/*
+ * And now for a steaming heap of suck.
+ *
+ * The nla_for_each_nested() macro defined by at least some versions of the
+ * Linux kernel's headers doesn't do the casting required when compiling
+ * with a C++ compiler or with -Wc++-compat, so we get warnings, and those
+ * warnings are fatal when we compile this file.
+ *
+ * So we replace it with our own version, which does the requisite cast.
+ */
+
+/**
+ * nla_for_each_nested - iterate over nested attributes
+ * @pos: loop counter, set to current attribute
+ * @nla: attribute containing the nested attributes
+ * @rem: initialized to len, holds bytes currently remaining in stream
+ */
+#undef nla_for_each_nested
+#define nla_for_each_nested(pos, nla, rem) \
+    nla_for_each_attr(pos, (struct nlattr *)nla_data(nla), nla_len(nla), rem)
+
+#endif /* HAVE_LIBNL */
+
+#endif /* _WS_NETLINK_H */
+
+/*
+ * Editor modelines  -  http://www.wireshark.org/tools/modelines.html
+ *
+ * Local variables:
+ * c-basic-offset: 4
+ * tab-width: 8
+ * indent-tabs-mode: nil
+ * End:
+ *
+ * vi: set shiftwidth=4 tabstop=8 expandtab:
+ * :indentSize=4:tabSize=8:noTabs=true:
+ */