When x is the most negative value of a two's complement type,
`(unsigned_type)(-x)` has undefined behavior, whereas `-(unsigned_type)x`
has well-defined behavior and does what was intended.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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>
Although our build scripts support that, it's annoying, because it makes
"test_suite_XXX" ambiguous between "all the data for
test_suite_XXX.function" and "just test_suite_XXX.data".
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
