headers_check.pl: disallow extern's

Since prototypes with "extern" refer to kernel functions, they make no
sense in userspace, so reject them automatically.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
[sam: made it into a warning]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/headers_check.pl

@@ -14,7 +14,9 @@
 #    Only include files located in asm* and linux* are checked.
 #    The rest are assumed to be system include files.
 #
-# 2) TODO: check for leaked CONFIG_ symbols
+# 2) It is checked that prototypes does not use "extern"
+#
+# 3) TODO: check for leaked CONFIG_ symbols
 
 use strict;
 
@@ -33,6 +35,7 @@ foreach my $file (@files) {
 	while ($line = <FH>) {
 		$lineno++;
 		check_include();
+		check_prototypes();
 	}
 	close FH;
 }
@@ -54,3 +57,10 @@ sub check_include
 		}
 	}
 }
+
+sub check_prototypes
+{
+	if ($line =~ m/^\s*extern\b/) {
+		printf STDERR "$filename:$lineno: extern's make no sense in userspace\n";
+	}
+}