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>
Align the name of the bignum test suite with the source module (which was
renamed from mpi.c to bignum.c in the PolarSSL 1.x days). This also brings
it into line with the test suites for the low-level bignum interfaces.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
