modpost: detect endianness on run-time

Endianness is currently detected on compile-time, but we can defer this
until run-time. This change avoids re-executing scripts/mod/mk_elfconfig
even if modpost in the linux-headers package needs to be rebuilt for a
foreign architecture.

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>

scripts/mod/mk_elfconfig.c

···
 main(int argc, char **argv)
 {
 	unsigned char ei[EI_NIDENT];
-	union { short s; char c[2]; } endian_test;
 
 	if (fread(ei, 1, EI_NIDENT, stdin) != EI_NIDENT) {
 		fprintf(stderr, "Error: input truncated\n");
···
 	default:
 		exit(1);
 	}
-	switch (ei[EI_DATA]) {
-	case ELFDATA2LSB:
-		printf("#define KERNEL_ELFDATA ELFDATA2LSB\n");
-		break;
-	case ELFDATA2MSB:
-		printf("#define KERNEL_ELFDATA ELFDATA2MSB\n");
-		break;
-	default:
-		exit(1);
-	}
-
-	endian_test.s = 0x0102;
-	if (memcmp(endian_test.c, "\x01\x02", 2) == 0)
-		printf("#define HOST_ELFDATA ELFDATA2MSB\n");
-	else if (memcmp(endian_test.c, "\x02\x01", 2) == 0)
-		printf("#define HOST_ELFDATA ELFDATA2LSB\n");
-	else
-		exit(1);
 
 	return 0;
 }

scripts/mod/modpost.c

···
 
 static bool extra_warn;
 
+bool target_is_big_endian;
+bool host_is_big_endian;
+
 /*
  * Cut off the warnings when there are too many. This typically occurs when
  * vmlinux is missing. ('make modules' without building vmlinux.)
···
 		/* Not an ELF file - silently ignore it */
 		return 0;
 	}
+
+	switch (hdr->e_ident[EI_DATA]) {
+	case ELFDATA2LSB:
+		target_is_big_endian = false;
+		break;
+	case ELFDATA2MSB:
+		target_is_big_endian = true;
+		break;
+	default:
+		fatal("target endian is unknown\n");
+	}
+
 	/* Fix endianness in ELF header */
 	hdr->e_type      = TO_NATIVE(hdr->e_type);
 	hdr->e_machine   = TO_NATIVE(hdr->e_machine);
···
 	const char *file;
 };
 
+static void check_host_endian(void)
+{
+	static const union {
+		short s;
+		char c[2];
+	} endian_test = { .c = {0x01, 0x02} };
+
+	switch (endian_test.s) {
+	case 0x0102:
+		host_is_big_endian = true;
+		break;
+	case 0x0201:
+		host_is_big_endian = false;
+		break;
+	default:
+		fatal("Unknown host endian\n");
+	}
+}
+
 int main(int argc, char **argv)
 {
 	struct module *mod;
···
 			exit(1);
 		}
 	}
+
+	check_host_endian();
 
 	list_for_each_entry_safe(dl, dl2, &dump_lists, list) {
 		read_dump(dl->file);

scripts/mod/modpost.h

···
 		    x); \
 })
 
-#if KERNEL_ELFDATA != HOST_ELFDATA
-
-#define TO_NATIVE(x) (bswap(x))
-
-#else /* endianness matches */
-
-#define TO_NATIVE(x) (x)
-
-#endif
+#define TO_NATIVE(x)	\
+	(target_is_big_endian == host_is_big_endian ? x : bswap(x))
 
 #define NOFAIL(ptr)   do_nofail((ptr), #ptr)
 
···
 void get_src_version(const char *modname, char sum[], unsigned sumlen);
 
 /* from modpost.c */
+extern bool target_is_big_endian;
+extern bool host_is_big_endian;
 char *read_text_file(const char *filename);
 char *get_line(char **stringp);
 void *sym_get_data(const struct elf_info *info, const Elf_Sym *sym);