kbuild: soften modpost checks when doing cross builds

The module alias support in the kernel have a consistency
check where it is checked that the size of a structure
in the kernel and on the build host are the same.
For cross builds this check does not make sense so detect
when we do cross builds and silently skip the check in these
situations.
This fixes a build bug for a wireless driver when cross building
for arm.

Acked-by: Michael Buesch <[e-mail]>
Tested-by: Gordon Farquharson <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Cc: [e-mail]

Makefile

···
 # Alternatively CROSS_COMPILE can be set in the environment.
 # Default value for CROSS_COMPILE is not to prefix executables
 # Note: Some architectures assign CROSS_COMPILE in their arch/*/Makefile
-
+export KBUILD_BUILDHOST := $(SUBARCH)
 ARCH		?= $(SUBARCH)
 CROSS_COMPILE	?=
 

scripts/Makefile.modpost

···
 # Stop after building .o files if NOFINAL is set. Makes compile tests quicker
 _modpost: $(if $(KBUILD_MODPOST_NOFINAL), $(modules:.ko:.o),$(modules))
 
+ifneq ($(KBUILD_BUILDHOST),$(ARCH))
+        cross_build := 1
+endif
 
 # Step 2), invoke modpost
 #  Includes step 3,4
···
  $(if $(CONFIG_DEBUG_SECTION_MISMATCH),,-S)      \
  $(if $(CONFIG_MARKERS),-K $(kernelmarkersfile)) \
  $(if $(CONFIG_MARKERS),-M $(markersfile))	 \
- $(if $(KBUILD_EXTMOD)$(KBUILD_MODPOST_WARN),-w)
+ $(if $(KBUILD_EXTMOD)$(KBUILD_MODPOST_WARN),-w) \
+ $(if $(cross_build),-c)
 
 quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules
       cmd_modpost = $(modpost) -s

scripts/mod/file2alias.c

···
                 sprintf(str + strlen(str), "*");                \
 } while(0)
 
+unsigned int cross_build = 0;
 /**
  * Check that sizeof(device_id type) are consistent with size of section
  * in .o file. If in-consistent then userspace and kernel does not agree
  * on actual size which is a bug.
  * Also verify that the final entry in the table is all zeros.
+ * Ignore both checks if build host differ from target host and size differs.
  **/
 static void device_id_check(const char *modname, const char *device_id,
 			    unsigned long size, unsigned long id_size,
···
 	int i;
 
 	if (size % id_size || size < id_size) {
+		if (cross_build != 0)
+			return;
 		fatal("%s: sizeof(struct %s_device_id)=%lu is not a modulo "
 		      "of the size of section __mod_%s_device_table=%lu.\n"
 		      "Fix definition of struct %s_device_id "

scripts/mod/modpost.c

···
 	int opt;
 	int err;
 
-	while ((opt = getopt(argc, argv, "i:I:msSo:awM:K:")) != -1) {
+	while ((opt = getopt(argc, argv, "i:I:cmsSo:awM:K:")) != -1) {
 		switch (opt) {
 		case 'i':
 			kernel_read = optarg;
···
 		case 'I':
 			module_read = optarg;
 			external_module = 1;
+			break;
+		case 'c':
+			cross_build = 1;
 			break;
 		case 'm':
 			modversions = 1;

scripts/mod/modpost.h

···
 };
 
 /* file2alias.c */
+extern unsigned int cross_build;
 void handle_moddevtable(struct module *mod, struct elf_info *info,
 			Elf_Sym *sym, const char *symname);
 void add_moddevtable(struct buffer *buf, struct module *mod);