8000 gh-96735: Fix undefined behaviour in struct unpacking functions by mdickinson · Pull Request #96739 · python/cpython · GitHub
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gh-96735: Fix undefined behaviour in struct unpacking functions #96739

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Merged
merged 10 commits into from
Sep 25, 2022
Merged

gh-96735: Fix undefined behaviour in struct unpacking functions #96739

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22fdce6
Fix undefined behaviour in bu_ulonglong
mdickinson Sep 10, 2022
e12943b
Add regression test
mdickinson Sep 10, 2022
8efefd8
Also apply the fix to lu_longlong
mdickinson Sep 10, 2022
cba4b8b
Merge branch 'main' into fix-struct-unpack-undefined-behaviour
mdickinson Sep 10, 2022
0d49e73
Fix similar undefined behaviour issues in bu_int and lu_int
mdickinson Sep 10, 2022
bfd714f
Merge remote-tracking branch 'mdickinson/fix-struct-unpack-undefined-…
mdickinson Sep 10, 2022
5aeef09
Remove redundant test
mdickinson Sep 10, 2022
204e924
Fix accumulator type
mdickinson Sep 10, 2022
994703e
Branch-free sign-extension
mdickinson Sep 13, 2022
1294440
Fix unconventional spacing
mdickinson Sep 13, 2022
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Also apply the fix to lu_longlong
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@mdickinson
mdickinson committed Sep 10, 2022
commit 8efefd8b5c91312c574ae1996052f45d7387e86a
6 changes: 5 additions & 1 deletion Lib/test/test_struct.py
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Expand Up @@ -729,7 +729,11 @@ def test_issue45034_signed(self):
def test_long_long_unpack_undefined_behaviour(self):
# Regression test for python/cpython#96735.
self.assertEqual(
struct.unpack('!q', b'\xff\xff\xff\xff\xff\xff\xff\xff'),
struct.unpack('<q', b'\xff\xff\xff\xff\xff\xff\xff\xff'),
(-1,)
)
self.assertEqual(
struct.unpack('>q', b'\xff\xff\xff\xff\xff\xff\xff\xff'),
(-1,)
)

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15 changes: 9 additions & 6 deletions Modules/_struct.c
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Expand Up @@ -1059,16 +1059,19 @@ lu_uint(_structmodulestate *state, const char *p, const formatdef *f)
static PyObject *
lu_longlong(_structmodulestate *state, const char *p, const formatdef *f)
{
long long x = 0;
Py_ssize_t i = f->size;
unsigned long long x = 0;

/* This function is only ever used in the case f->size == 8. */
assert(f->size == 8);
Py_ssize_t i = 8;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | bytes[--i];
} while (i > 0);
/* Extend the sign bit. */
if (SIZEOF_LONG_LONG > f->size)
x |= -(x & ((long long)1 << ((8 * f->size) - 1)));
return PyLong_FromLongLong(x);

/* Extend sign, avoiding implementation-defined or undefined behaviour. */
return PyLong_FromLongLong(x & 0x8000000000000000 ?
-1 - (long long)(0xFFFFFFFFFFFFFFFF - x) : (long long)x);
}

static PyObject *
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0