kbuild: prevent exported headers from including <stdlib.h>, <stdbool.h>

Some UAPI headers included <stdlib.h>, like this:

  #ifndef __KERNEL__
  #include <stdlib.h>
  #endif

As it turned out, they just included it for no good reason.

After some fixes, now I can compile-test UAPI headers
(CONFIG_UAPI_HEADER_TEST=y) without including <stdlib.h> from the
system header search paths.

To avoid somebody getting it back again, this commit adds the dummy
header, usr/dummy-include/stdlib.h

I added $(srctree)/usr/dummy-include to the header search paths.
Because it is searched before the system directories, if someone
tries to include <stdlib.h>, they will see the error message.

While I am here, I also replaced $(objtree)/usr/include with $(obj),
but it has no functional change.

If we can make kernel headers self-contained (that is, none of exported
kernel headers includes system headers), we will be able to add the
-nostdinc flag, but that is much far from where we stand now.

As a realistic solution, we can ban header inclusion individually by
putting a dummy header into usr/dummy-include/.

Currently, no header include <stdbool.h>. I put it as well before somebody
attempts to use it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

usr/dummy-include/stdbool.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _STDBOOL_H
+#define _STDBOOL_H
+
+#error "Please do not include <stdbool.h> from exported headers"
+
+#endif /* _STDBOOL_H */

usr/dummy-include/stdlib.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _STDLIB_H
+#define _STDLIB_H
+
+#error "Please do not include <stdlib.h> from exported headers"
+
+#endif /* _STDLIB_H */

usr/include/Makefile

@@ -15,7 +15,7 @@ UAPI_CFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
 # USERCFLAGS might contain sysroot location for CC.
 UAPI_CFLAGS += $(USERCFLAGS)
 
-override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
+override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I $(obj) -I $(srctree)/usr/dummy-include
 
 # The following are excluded for now because they fail to build.
 #