fix(legacy): Support negative amounts in layoutConfirmTx().

legacy/firmware/layout2.c

@@ -423,13 +423,30 @@ void layoutConfirmOpReturn(const uint8_t *data, uint32_t size) {
                     NULL);
 }
 
-void layoutConfirmTx(const CoinInfo *coin, uint64_t amount_out,
-                     uint64_t amount_fee) {
-  char str_out[32] = {0}, str_fee[32] = {0};
-  bn_format_uint64(amount_out, NULL, coin->coin_shortcut, coin->decimals, 0,
-                   false, str_out, sizeof(str_out));
-  bn_format_uint64(amount_fee, NULL, coin->coin_shortcut, coin->decimals, 0,
-                   false, str_fee, sizeof(str_fee));
+static bool formatAmountDifference(const CoinInfo *coin, uint64_t amount1,
+                                   uint64_t amount2, char *output,
+                                   size_t output_length) {
+  uint64_t abs_diff = 0;
+  const char *sign = NULL;
+  if (amount1 >= amount2) {
+    abs_diff = amount1 - amount2;
+  } else {
+    abs_diff = amount2 - amount1;
+    sign = "-";
+  }
+
+  return bn_format_uint64(abs_diff, sign, coin->coin_shortcut, coin->decimals,
+                          0, false, output, output_length) != 0;
+}
+
+void layoutConfirmTx(const CoinInfo *coin, uint64_t total_in,
+                     uint64_t total_out, uint64_t change_out) {
+  char str_out[32] = {0};
+  formatAmountDifference(coin, total_in, change_out, str_out, sizeof(str_out));
+
+  char str_fee[32] = {0};
+  formatAmountDifference(coin, total_in, total_out, str_fee, sizeof(str_fee));
+
   layoutDialogSwipe(&bmp_icon_question, _("Cancel"), _("Confirm"), NULL,
                     _("Really send"), str_out, _("from your wallet?"),
                     _("Fee included:"), str_fee, NULL);

legacy/firmware/layout2.h

@@ -49,8 +49,8 @@ void layoutHome(void);
 void layoutConfirmOutput(const CoinInfo *coin, const TxOutputType *out);
 void layoutConfirmOmni(const uint8_t *data, uint32_t size);
 void layoutConfirmOpReturn(const uint8_t *data, uint32_t size);
-void layoutConfirmTx(const CoinInfo *coin, uint64_t amount_out,
-                     uint64_t amount_fee);
+void layoutConfirmTx(const CoinInfo *coin, uint64_t total_in,
+                     uint64_t total_out, uint64_t change_out);
 void layoutConfirmReplacement(const char *description, uint8_t txid[32]);
 void layoutConfirmModifyFee(const CoinInfo *coin, uint64_t fee_old,
                             uint64_t fee_new);

legacy/firmware/signing.c

@@ -1586,7 +1586,7 @@ static bool signing_confirm_tx(void) {
     }
 
     // last confirmation
-    layoutConfirmTx(coin, total_in - change_out, fee);
+    layoutConfirmTx(coin, total_in, total_out, change_out);
     if (!protectButton(ButtonRequestType_ButtonRequest_SignTx, false)) {
       fsm_sendFailure(FailureType_Failure_ActionCancelled, NULL);
       signing_abort();