On Thu, Feb 27, 2020 at 10:51:39AM -0500, Charalampos Stratakis wrote:
> Hello folks,
>
> I recently observed a failure on the s390x fedora rawhide buildbot, on the
> clang builds, when clang got updated to version 10:
> https://bugs.python.org/issue39689
>
> The call:
> struct.unpack('>?', b'\xf0')
> means to unpack a "native bool", i.e. native size and alignment. Internally,
> this does:
>
> static PyObject *
> nu_bool(const char *p, const formatdef *f)
> {
> _Bool x;
> memcpy((char *)&x, p, sizeof x);
> return PyBool_FromLong(x != 0);
> }
>
> i.e., copies "sizeof x" (1 byte) of memory to a temporary buffer x, and then
> treats that as _Bool.
>
> While I don't have access to the C standard, I believe it says that
> assignment of a true value to _Bool can coerce to a unique "true" value. It
> seems that if a char doesn't have the exact bit pattern for true or false,
> casting to _Bool is undefined behavior. Is that correct?
>
> Clang 10 on s390x seems to take advantage of this: it probably only looks at
> the last bit(s) so a _Bool with a bit pattern of 0xf0 turns out false.
> But the tests assume that 0xf0 should unpack to True.
I don't think it's specific to Clang 9, or the s390x arch. Have a look to
https://godbolt.org/z/3n-LqN
clang indeed just checks for the lowest bit. Is it correct? I think so. _Bool
can only holds two value, 0 and 1, [0] which is different from an int whose
value is
true or false whether its different or equal to 0. GCC and Clang agree on that:
https://godbolt.org/z/koc4Pb
So yeah, according to that rule, the value set in `p` wasn't from a _Bool if it
has the 0xf0 value. So you're re-interepreting memory between two different
types type-punning, and that's UB.
Quick and obvious fix:
static PyObject *
nu_bool(const char *p, const formatdef *f)
{
char x;
memcpy((char *)&x, p, sizeof x);
return PyBool_FromLong(x != 0);
}
----
[0]
the standard says
6.3.1.2 Boolean type
When any scalar value is converted to_Bool,the result is 0
if the value compares equalto 0; otherwise, the result is 1.
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