8000 gh-111140: Fix edge case in PyLong_AsNativeBytes where large negative longs may require an extra byte by zooba · Pull Request #116053 · python/cpython · GitHub
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gh-111140: Fix edge case in PyLong_AsNativeBytes where large negative longs may require an extra byte #116053

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Merged
merged 19 commits into from
Apr 5, 2024
Merged

gh-111140: Fix edge case in PyLong_AsNativeBytes where large negative longs may require an extra byte #116053

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d5b0e98
gh-111140: Fix edge case in PyLong_AsNativeBytes where large negative…
zooba Feb 28, 2024
f162348
Improved edge case testing and fuzz tests
zooba Mar 1, 2024
0efa143
Test comments
zooba Mar 1, 2024
15b3752
Fix for x86
zooba Mar 4, 2024
fbe42e7
Merge remote-tracking branch 'upstream/main' into gh-111140
zooba Mar 19, 2024
8dedad7
More precise check for unsigned non-overflow
zooba Mar 19, 2024
d3deebf
Merge branch 'main' into gh-111140
zooba Mar 28, 2024
621708c
Change endianness arguments to flags to allow additional arguments
zooba Mar 28, 2024
c607f04
Space
zooba Mar 28, 2024
140c011
Fix tests, one result, and docs
zooba Mar 28, 2024
77097bf
Fix test
zooba Mar 28, 2024
a8e2540
Update NEWS
zooba Mar 28, 2024
5ba6451
Missed colon
zooba Mar 28, 2024
68b9d8e
Doc fixes and improved test failure output
zooba Apr 2, 2024
37ad671
Remove my deliberate failure
zooba Apr 2, 2024
1e615cf
Reword PyLong_AsNativeBytes docs (#33)
encukou Apr 4, 2024
d1b886c
Apply PR feedback
zooba Apr 4, 2024
38ecfe2
Merge branch 'gh-111140' of https://github.com/zooba/cpython into gh-…
zooba Apr 4, 2024
614a6ae
Check for new error
zooba Apr 4, 2024
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gh-111140: Fix edge case in PyLong_AsNativeBytes where large negative… 
… longs may require an extra byte
  • Loading branch information
@zooba
zooba committed Feb 28, 2024
commit d5b0e98e955e18394f74fc644ccc8bc136516cec
22 changes: 14 additions & 8 deletions Lib/test/test_capi/test_long.py
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Original file line number Diff line number Diff line change
Expand Up @@ -451,11 +451,15 @@ def test_long_asnativebytes(self):
(MAX_SSIZE, SZ),
(MAX_USIZE, SZ + 1),
(-MAX_SSIZE, SZ),
(-MAX_USIZE, SZ + 1),
(-MAX_USIZE, SZ),
(2**255-1, 32),
(-(2**255-1), 32),
(2**255, 33),
(-(2**255), 32), # edge case
(2**256-1, 33),
(-(2**256-1), 33),
(-(2**256-1), 32), # edge case
(2**256, 33),
(-(2**256), 33),
]:
with self.subTest(f"sizeof-{v:X}"):
buffer = bytearray(b"\x5a")
Expand Down Expand Up @@ -500,10 +504,10 @@ def test_long_asnativebytes(self):
(256, b'\x01\x00', 2),
# Extracts successfully (unsigned), but requests 9 bytes
(2**63, b'\x80' + b'\x00' * 7, 9),
# "Extracts", but requests 9 bytes
(-2**63, b'\x80' + b'\x00' * 7, 9),
(2**63, b'\x00\x80' + b'\x00' * 7, 9),
(-2**63, b'\xff\x80' + b'\x00' * 7, 9),
# Extracts successfully and only requests 8 bytes
(-2**63, b'\x80' + b'\x00' * 7, 8),
(-2**63, b'\xff\x80' + b'\x00' * 7, 8),

(2**255-1, b'\x7f' + b'\xff' * 31, 32),
(-(2**255-1), b'\x80' + b'\x00' * 30 + b'\x01', 32),
Expand All @@ -516,9 +520,11 @@ def test_long_asnativebytes(self):
# into a 32-byte buffer, though negative number may be unrecoverable
(2**256-1, b'\xff' * 32, 33),
(2**256-1, b'\x00' + b'\xff' * 32, 33),
(-(2**256-1), b'\x00' * 31 + b'\x01', 33),
(-(2**256-1), b'\xff' + b'\x00' * 31 + b'\x01', 33),
(-(2**256-1), b'\xff\xff' + b'\x00' * 31 + b'\x01', 33),
# Negative 256 bits of integer will only request 32 bytes, since the
# top-most bit is the sign bit as well as the magnitude.
(-(2**256-1), b'\x00' * 31 + b'\x01', 32),
(-(2**256-1), b'\xff' + b'\x00' * 31 + b'\x01', 32),
(-(2**256-1), b'\xff\xff' + b'\x00' * 31 + b'\x01', 32),

# The classic "Windows HRESULT as negative number" case
# HRESULT hr;
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C API/2024-02-28-15-50-01.gh-issue-111140.mpwcUg.rst
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@@ -0,0 +1,3 @@
Allow :c:func:`PyLong_AsNativeBytes` to extract negative numbers requiring
every single bit of the target buffer into the buffer without requesting a
larger one.
15 changes: 8 additions & 7 deletions Objects/longobject.c
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Expand Up @@ -1199,17 +1199,18 @@ PyLong_AsNativeBytes(PyObject* vv, void* buffer, Py_ssize_t n, int endianness)
_PyLong_AsByteArray(v, buffer, (size_t)n, little_endian, 1, 0);
}

// More efficient calculation for number of bytes required?
/* Calculates the number of bits required for the *absolute* value
* of v. This does not take sign into account, only magnitude. */
size_t nb = _PyLong_NumBits((PyObject *)v);
/* Normally this would be((nb - 1) / 8) + 1 to avoid rounding up
* multiples of 8 to the next byte, but we add an implied bit for
* the sign and it cancels out. */
size_t n_needed = (nb / 8) + 1;
res = (Py_ssize_t)n_needed;
if ((size_t)res != n_needed) {
PyErr_SetString(PyExc_OverflowError,
"value too large to convert");
res = -1;
res = (Py_ssize_t)(nb / 8) + 1;
/* The edge case of a negative value where the sign bit is at the
* MSB of one byte needs special handling to avoid requesting an
* extra byte, even though it could be properly represented. */
if (_PyLong_IsNegative(v) && !(nb % 8)) {
res -= 1;
}
}

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0