warn about use of uninstalled kernel headers

User applications frequently hit problems when they try to use
the kernel headers directly, rather than the exported headers.

This adds an explicit warning for this case, and points to
a URL holding an explanation of why this is wrong and what
to do about it.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

include/linux/kernel.h

@@ -659,6 +659,12 @@ extern int do_sysinfo(struct sysinfo *info);
 
 #endif /* __KERNEL__ */
 
+#ifndef __EXPORTED_HEADERS__
+#ifndef __KERNEL__
+#warning Attempt to use kernel headers from user space, see http://kernelnewbies.org/KernelHeaders
+#endif /* __KERNEL__ */
+#endif /* __EXPORTED_HEADERS__ */
+
 #define SI_LOAD_SHIFT	16
 struct sysinfo {
 	long uptime;			/* Seconds since boot */

scripts/headers_install.pl

@@ -20,7 +20,7 @@ use strict;
 
 my ($readdir, $installdir, $arch, @files) = @ARGV;
 
-my $unifdef = "scripts/unifdef -U__KERNEL__";
+my $unifdef = "scripts/unifdef -U__KERNEL__ -D__EXPORTED_HEADERS__";
 
 foreach my $file (@files) {
 	local *INFILE;