Introduce helper to detect kernel declarations.

This is to avoid playing roulette with mainline kernel versions vs
stable versions vs RHEL versions.

Makefile.in

@@ -20,10 +20,12 @@ ccflags-y = @KOPTS@
 
 all: ipt_NETFLOW.ko libipt_NETFLOW.so libip6t_NETFLOW.so @SNMPTARGET@
 
-ipt_NETFLOW.ko: version.h ipt_NETFLOW.c ipt_NETFLOW.h compat.h Makefile
+ipt_NETFLOW.ko: version.h ipt_NETFLOW.c ipt_NETFLOW.h compat_def.h compat.h Makefile
 	@echo Compiling for kernel $(KVERSION)
 	make -C $(KDIR) M=$(CURDIR) modules CONFIG_DEBUG_INFO=y
 	@touch $@
+compat_def.h: gen_compat_def
+	./gen_compat_def > $@
 sparse: | version.h ipt_NETFLOW.c ipt_NETFLOW.h compat.h Makefile
 	@rm -f ipt_NETFLOW.ko ipt_NETFLOW.o
 	@echo Compiling for kernel $(KVERSION)
@@ -41,7 +43,7 @@ mclean:
 lclean:
 	-rm -f *.so *_sh.o
 clean: mclean lclean
-	-rm -f *.so *.o modules.order version.h
+	-rm -f *.so *.o modules.order version.h compat_def.h
 
 snmp_NETFLOW.so: snmp_NETFLOW.c
 	$(CC) -fPIC -shared -o $@ $< -lnetsnmp

compat.h

@@ -6,7 +6,7 @@
 
 #ifndef COMPAT_NETFLOW_H
 #define COMPAT_NETFLOW_H
-
+#include "compat_def.h"
 
 #ifndef NIPQUAD
 # define NIPQUAD(addr) \
@@ -612,7 +612,7 @@ int in6_pton(const char *src, int srclen,
 #endif
 
 /* Offset changes made in 613dbd95723aee7abd16860745691b6c7bda20dc */
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28) && LINUX_VERSION_CODE < KERNEL_VERSION(4,10,0)
+#ifndef HAVE_XT_FAMILY
 # if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,35)
 #  define xt_action_param xt_target_param
 # endif

gen_compat_def

@@ -0,0 +1,71 @@
+#!/bin/bash -efu
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Generate defines based on kernel having
+# some symbols declared
+#
+# Copyright (C) 2019 <[email protected]>
+#
+
+fatal() {
+  echo "Error: $*" >&2
+  exit 1
+}
+
+eval $(grep ^KDIR Makefile | tr -d ' ')
+[ "$KDIR" ] || fatal "KDIR is not found"
+
+WD=cc-test-build
+mkdir -p $WD
+cd $WD || fatal "cannot cd to $WD"
+
+kbuild_test_compile() {
+  local cmd
+
+  cat > test.c
+  echo obj-m = test.o > Makefile
+  cmd="make -s -C $KDIR M=$PWD modules"
+  echo "$cmd" > log
+  if $cmd >> log 2>&1; then
+    [ "$2" ] && echo "// $2 is declared ${3:+with <$3>}"
+    echo "#define HAVE_$1"
+    echo
+  else
+    echo "// ${2:-symbol} is undeclared${3:+ with <$3>}. Compile:"
+    sed  "s/^/\/\/   /" test.c
+    echo "// Output:"
+    sed  "s/^/\/\/   /" log
+    echo
+    if ! grep -q undeclared log; then
+      echo "Error: unexpected error from compiler" >&2
+      cat log >&2
+      echo >&2
+      exit 3
+    fi
+  fi
+}
+
+kbuild_test_symbol() {
+kbuild_test_compile ${1^^} $1 ${2-} <<EOF
+#include <linux/module.h>
+${2:+#include <$2>}
+MODULE_LICENSE("GPL");
+void *test = $1;
+EOF
+}
+
+echo "// Autogenerated for $KDIR"
+echo
+
+# helpers introduced in 613dbd95723aee7abd16860745691b6c7bda20dc
+kbuild_test_symbol xt_family linux/netfilter_ipv4/ip_tables.h
+
+echo "// End of compat_def.h"
+
+cd $OLDPWD
+rm -rf $WD
+
+# debug output for Travis
+if [ -z "${PWD/*travis*}" ]; then
+  cat compat_def.h >&2
+fi