[PATCH v4 16/26] scripts: Fix the inclusion order in modpost

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

In the process of creating the source file of a module modpost injects a
set of includes that are not required if the compilation unit is
statically built into the kernel.

The order of inclusion of the headers can cause redefinition problems
(e.g.):

   In file included from include/linux/elf.h:5:0,
                    from include/linux/module.h:18,
                    from crypto/arc4.mod.c:2:
>> arch/parisc/include/asm/elf.h:324:0: warning: "ELF_OSABI" redefined
    #define ELF_OSABI  ELFOSABI_LINUX

   In file included from include/linux/elfnote.h:62:0,
                    from include/linux/build-salt.h:4,
                    from crypto/arc4.mod.c:1:
   include/uapi/linux/elf.h:363:0: note: this is the location of
   the previous definition
    #define ELF_OSABI ELFOSABI_NONE

The issue was exposed during the development of the series [1].

[1] https://lore.kernel.org/lkml/20200306133242.26279-1-vincenzo.frascino@arm.com/

Reported-by: kbuild test robot <lkp@intel.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
 scripts/mod/modpost.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 7edfdb2f4497..0f354b1ee2aa 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -2251,8 +2251,12 @@ static int check_modname_len(struct module *mod)
  **/
 static void add_header(struct buffer *b, struct module *mod)
 {
-	buf_printf(b, "#include <linux/build-salt.h>\n");
 	buf_printf(b, "#include <linux/module.h>\n");
+	/*
+	 * Include build-salt.h after module.h in order to
+	 * inherit the definitions.
+	 */
+	buf_printf(b, "#include <linux/build-salt.h>\n");
 	buf_printf(b, "#include <linux/vermagic.h>\n");
 	buf_printf(b, "#include <linux/compiler.h>\n");
 	buf_printf(b, "\n");
-- 
2.25.1