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. _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/S24XTMGGHJCN2FRZJWVHM5ZRMN3QORPK/ Code of Conduct: http://python.org/psf/codeofconduct/