modpost: do not try to match the SHT_NUL section.

Trying to match the SHT_NUL section isn't useful and causes build failures
on parisc and mn10300 since the addition of section strict white-listing
and __ex_table sanitizing.
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 050e57fd ("modpost: add strict white-listing when referencing....")
Fixes: 52dc0595 ("modpost: handle relocations mismatch in __ex_table.")
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

scripts/mod/modpost.c

@@ -1064,6 +1064,15 @@ static const struct sectioncheck *section_mismatch(
 	int elems = sizeof(sectioncheck) / sizeof(struct sectioncheck);
 	const struct sectioncheck *check = &sectioncheck[0];
 
+	/*
+	 * The target section could be the SHT_NUL section when we're
+	 * handling relocations to un-resolved symbols, trying to match it
+	 * doesn't make much sense and causes build failures on parisc and
+	 * mn10300 architectures.
+	 */
+	if (*tosec == '\0')
+		return NULL;
+
 	for (i = 0; i < elems; i++) {
 		if (match(fromsec, check->fromsec)) {
 			if (check->bad_tosec[0] && match(tosec, check->bad_tosec))