keys: Implement update for the big_key type

Implement the ->update op for the big_key type.

Signed-off-by: David Howells <[e-mail]>
Acked-by: Jason A. Donenfeld <[e-mail]>

include/keys/big_key-type.h

···
 extern void big_key_destroy(struct key *key);
 extern void big_key_describe(const struct key *big_key, struct seq_file *m);
 extern long big_key_read(const struct key *key, char *buffer, size_t buflen);
+extern int big_key_update(struct key *key, struct key_preparsed_payload *prep);
 
 #endif /* _KEYS_BIG_KEY_TYPE_H */

security/keys/big_key.c

···
 	.destroy		= big_key_destroy,
 	.describe		= big_key_describe,
 	.read			= big_key_read,
-	/* no ->update(); don't add it without changing chacha20poly1305's nonce */
+	.update			= big_key_update,
 };
 
 /*
···
 	}
 	kzfree(key->payload.data[big_key_data]);
 	key->payload.data[big_key_data] = NULL;
+}
+
+/*
+ * Update a big key
+ */
+int big_key_update(struct key *key, struct key_preparsed_payload *prep)
+{
+	int ret;
+
+	ret = key_payload_reserve(key, prep->datalen);
+	if (ret < 0)
+		return ret;
+
+	if (key_is_positive(key))
+		big_key_destroy(key);
+
+	return generic_key_instantiate(key, prep);
 }
 
 /*