PoC: scoped allow for hmac

Author: ppannuto

libtock-sync/crypto/hmac.c

@@ -1,5 +1,10 @@
 #include "hmac.h"
 
+// would be in a libtock-sync/support.h or internal.h or similar
+#define SCOPED_ALLOW(method, buffer, length) \
+  inline void unallow_##buffer(returncode_t* sentinel##buffer) { if (sentinel##buffer == RETURNCODE_SUCCESS) method(NULL, 0); } \
+  returncode_t ret_##buffer __attribute__((cleanup(unallow_##buffer))) = method(buffer, length);  \
+
 returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                      uint8_t* key_buffer, uint32_t key_length,
                                      uint8_t* input_buffer, uint32_t input_length,
@@ -9,29 +14,25 @@ returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
   ret = libtock_hmac_command_set_algorithm((uint32_t) hmac_type);
   if (ret != RETURNCODE_SUCCESS) return ret;
 
-  ret = libtock_hmac_set_readonly_allow_key_buffer(key_buffer, key_length);
-  if (ret != RETURNCODE_SUCCESS) return ret;
+  // Macro expands to this:
+  //
+  // inline void unallow_key_buffer(returncode_t* sentinel_key_buffer) { if (sentinel_key_buffer == RETURNCODE_SUCCESS) libtock_hmac_set_readonly_allow_key_buffer(NULL, 0); }
+  // returncode_t ret_key_buffer __attribute__((cleanup(unallow_key_buffer))) = libtock_hmac_set_readonly_allow_key_buffer(key_buffer, key_length);
 
-  ret = libtock_hmac_set_readonly_allow_data_buffer(input_buffer, input_length);
-  if (ret != RETURNCODE_SUCCESS) goto exit1;
+  SCOPED_ALLOW(libtock_hmac_set_readonly_allow_key_buffer, key_buffer, key_length);
+  if (ret_key_buffer != RETURNCODE_SUCCESS) return ret_key_buffer;
 
-  ret = libtock_hmac_set_readwrite_allow_destination_buffer(hmac_buffer, hmac_length);
-  if (ret != RETURNCODE_SUCCESS) goto exit2;
+  SCOPED_ALLOW(libtock_hmac_set_readonly_allow_data_buffer, input_buffer, input_length);
+  if (ret_input_buffer != RETURNCODE_SUCCESS) return ret_input_buffer;
+
+  SCOPED_ALLOW(libtock_hmac_set_readwrite_allow_destination_buffer, hmac_buffer, hmac_length);
+  if (ret_hmac_buffer != RETURNCODE_SUCCESS) return ret_hmac_buffer;
 
   ret = libtock_hmac_command_run();
-  if (ret != RETURNCODE_SUCCESS) goto exit3;
+  if (ret != RETURNCODE_SUCCESS) return ret;
 
   // Wait for the operation.
   ret = libtocksync_hmac_yield_wait_for();
 
-exit3:
-  libtock_hmac_set_readwrite_allow_destination_buffer(NULL, 0);
-
-exit2:
-  libtock_hmac_set_readonly_allow_data_buffer(NULL, 0);
-
-exit1:
-  libtock_hmac_set_readonly_allow_key_buffer(NULL, 0);
-
   return ret;
 }