Add negative zero as an input to automatically generated tests

Although negative zero is officially unsupported, we've had bugs related to
it in the past. So do test functions with a negative zero input.

There will likely be cases where we don't want to accept negative zero as if
it was valid, because it's too hard to handle. We'll add exceptions on a
case by case basis.

For the functions that are currently tested by the generated tests, the new
test cases pass.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/mbedtls_dev/bignum_common.py

@@ -38,7 +38,13 @@ def invmod(a: int, n: int) -> int:
     raise ValueError("Not invertible")
 
 def hex_to_int(val: str) -> int:
-    return int(val, 16) if val else 0
+    """Implement the syntax accepted by mbedtls_test_read_mpi().
+
+    This is a superset of what is accepted by mbedtls_test_read_mpi_core().
+    """
+    if val == '' or val == '-':
+        return 0
+    return int(val, 16)
 
 def quote_str(val) -> str:
     return "\"{}\"".format(val)

tests/scripts/generate_bignum_tests.py

@@ -78,11 +78,16 @@ class BignumOperation(bignum_common.OperationCommon, BignumTarget, metaclass=ABC
     #pylint: disable=abstract-method
     """Common features for bignum operations in legacy tests."""
     input_values = [
-        "", "0", "7b", "-7b",
+        "", "0", "-", "-0",
+        "7b", "-7b",
         "0000000000000000123", "-0000000000000000123",
         "1230000000000000000", "-1230000000000000000"
     ]
 
+    def description_suffix(self) -> str:
+        """Text to add at the end of the test case description."""
+        return ""
+
     def description(self) -> str:
         """Generate a description for the test case.
 
@@ -96,6 +101,9 @@ class BignumOperation(bignum_common.OperationCommon, BignumTarget, metaclass=ABC
                 self.symbol,
                 self.value_description(self.arg_b)
             )
+            description_suffix = self.description_suffix()
+            if description_suffix:
+                self.case_description += " " + description_suffix
         return super().description()
 
     @staticmethod
@@ -107,6 +115,8 @@ class BignumOperation(bignum_common.OperationCommon, BignumTarget, metaclass=ABC
         """
         if val == "":
             return "0 (null)"
+        if val == "-":
+            return "negative 0 (null)"
         if val == "0":
             return "0 (1 limb)"
 
@@ -171,9 +181,21 @@ class BignumAdd(BignumOperation):
         ]
     )
 
-    def result(self) -> List[str]:
-        return [bignum_common.quote_str("{:x}").format(self.int_a + self.int_b)]
+    def __init__(self, val_a: str, val_b: str) -> None:
+        super().__init__(val_a, val_b)
+        self._result = self.int_a + self.int_b
 
+    def description_suffix(self) -> str:
+        if (self.int_a >= 0 and self.int_b >= 0):
+            return "" # obviously positive result or 0
+        if (self.int_a <= 0 and self.int_b <= 0):
+            return "" # obviously negative result or 0
+        # The sign of the result is not obvious, so indicate it
+        return ", result{}0".format('>' if self._result > 0 else
+                                    '<' if self._result < 0 else '=')
+
+    def result(self) -> List[str]:
+        return [bignum_common.quote_str("{:x}".format(self._result))]
 
 if __name__ == '__main__':
     # Use the section of the docstring relevant to the CLI as description