Support negative zero as MPI test input
The bignum module does not officially support "negative zero" (an
mbedtls_mpi object with s=-1 and all limbs zero). However, we have a
history of bugs where a function that should produce an official
zero (with s=1), produces a negative zero in some circumstances. So it's
good to check that the bignum functions are robust when passed a negative
zero as input. And for that, we need a way to construct a negative zero
from test case arguments.
There are checks that functions don't produce negative zeros as output in
the test suite. Skip those checks if there's a negative zero input: we
don't want functions to _create_ negative zeros, but we don't mind if
they _propagate_ negative zeros.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index cc23fd7..7c83714 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -89,6 +89,10 @@
mbedtls_test_info.step = step;
}
+#if defined(MBEDTLS_BIGNUM_C)
+unsigned mbedtls_test_case_uses_negative_0 = 0;
+#endif
+
void mbedtls_test_info_reset( void )
{
mbedtls_test_info.result = MBEDTLS_TEST_RESULT_SUCCESS;
@@ -98,6 +102,9 @@
mbedtls_test_info.filename = 0;
memset( mbedtls_test_info.line1, 0, sizeof( mbedtls_test_info.line1 ) );
memset( mbedtls_test_info.line2, 0, sizeof( mbedtls_test_info.line2 ) );
+#if defined(MBEDTLS_BIGNUM_C)
+ mbedtls_test_case_uses_negative_0 = 0;
+#endif
}
int mbedtls_test_equal( const char *test, int line_no, const char* filename,
@@ -396,6 +403,15 @@
int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
{
+ int negative = 0;
+ /* Always set the sign bit to -1 if the input has a minus sign, even for 0.
+ * This creates an invalid representation, which mbedtls_mpi_read_string()
+ * avoids but we want to be able to create that in test data. */
+ if( s[0] == '-' )
+ {
+ ++s;
+ negative = 1;
+ }
/* mbedtls_mpi_read_string() currently retains leading zeros.
* It always allocates at least one limb for the value 0. */
if( s[0] == 0 )
@@ -403,7 +419,15 @@
mbedtls_mpi_free( X );
return( 0 );
}
- else
- return( mbedtls_mpi_read_string( X, 16, s ) );
+ int ret = mbedtls_mpi_read_string( X, 16, s );
+ if( ret != 0 )
+ return( ret );
+ if( negative )
+ {
+ if( mbedtls_mpi_cmp_int( X, 0 ) == 0 )
+ ++mbedtls_test_case_uses_negative_0;
+ X->s = -1;
+ }
+ return( 0 );
}
#endif