modpost: add array range check to sec_name()

The section index is always positive, so the argument, secindex, should
be unsigned.

Also, inserted the array range check.

If sym->st_shndx is a special section index (between SHN_LORESERVE and
SHN_HIRESERVE), there is no corresponding section header.

For example, if a symbol specifies an absolute value, sym->st_shndx is
SHN_ABS (=0xfff1).

The current users do not cause the out-of-range access of
info->sechddrs[], but it is better to avoid such a pitfall.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -336,8 +336,16 @@ static const char *sech_name(const struct elf_info *info, Elf_Shdr *sechdr)
 				      sechdr->sh_name);
 }
 
-static const char *sec_name(const struct elf_info *info, int secindex)
+static const char *sec_name(const struct elf_info *info, unsigned int secindex)
 {
+	/*
+	 * If sym->st_shndx is a special section index, there is no
+	 * corresponding section header.
+	 * Return "" if the index is out of range of info->sechdrs[] array.
+	 */
+	if (secindex >= info->num_sections)
+		return "";
+
 	return sech_name(info, &info->sechdrs[secindex]);
 }