module: Factor out elf_validity_cache_index_str

Pull out index validation for the symbol string section.

Note that this does not validate the *contents* of the string table,
only shape and presence of the section.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Author: maurer

kernel/module/main.c

@@ -2014,6 +2014,31 @@ static int elf_validity_cache_index_sym(struct load_info *info)
 	return 0;
 }
 
+/**
+ * elf_validity_cache_index_str() - Validate and cache strtab index
+ * @info: Load info to cache strtab index in.
+ *        Must have &load_info->sechdrs and &load_info->secstrings populated.
+ *        Must have &load_info->index.sym populated.
+ *
+ * Looks at the symbol table's associated string table, makes sure it is
+ * in-bounds, and caches it.
+ *
+ * Return: %0 if valid, %-ENOEXEC on failure.
+ */
+static int elf_validity_cache_index_str(struct load_info *info)
+{
+	unsigned int str_idx = info->sechdrs[info->index.sym].sh_link;
+
+	if (str_idx == SHN_UNDEF || str_idx >= info->hdr->e_shnum) {
+		pr_err("Invalid ELF sh_link!=SHN_UNDEF(%d) or (sh_link(%d) >= hdr->e_shnum(%d)\n",
+		       str_idx, str_idx, info->hdr->e_shnum);
+		return -ENOEXEC;
+	}
+
+	info->index.str = str_idx;
+	return 0;
+}
+
 /*
  * Check userspace passed ELF module against our expectations, and cache
  * useful variables for further processing as we go.
@@ -2037,7 +2062,6 @@ static int elf_validity_cache_index_sym(struct load_info *info)
 static int elf_validity_cache_copy(struct load_info *info, int flags)
 {
 	int err;
-	int str_idx;
 
 	err = elf_validity_cache_sechdrs(info);
 	if (err < 0)
@@ -2054,16 +2078,11 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	err = elf_validity_cache_index_sym(info);
 	if (err < 0)
 		return err;
-
-	str_idx = info->sechdrs[info->index.sym].sh_link;
-	if (str_idx == SHN_UNDEF || str_idx >= info->hdr->e_shnum) {
-		pr_err("Invalid ELF sh_link!=SHN_UNDEF(%d) or (sh_link(%d) >= hdr->e_shnum(%d)\n",
-				str_idx, str_idx, info->hdr->e_shnum);
-		return -ENOEXEC;
-	}
+	err = elf_validity_cache_index_str(info);
+	if (err < 0)
+		return err;
 
 	/* Sets internal strings. */
-	info->index.str = str_idx;
 	info->strtab = (char *)info->hdr + info->sechdrs[info->index.str].sh_offset;
 
 	/* This is temporary: point mod into copy of data. */