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[3.13] gh-71810: Fix _PyLong_AsByteArray() undefined behavior (GH-138873) #138884

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Merged
vstinner merged 1 commit into from
Sep 14, 2025
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[3.13] gh-71810: Fix _PyLong_AsByteArray() undefined behavior (GH-138873) #138884

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gh-71810: Fix _PyLong_AsByteArray() undefined behavior (GH-138873) 
Don't read p[-1] when p is an empty string: when n==0.
(cherry picked from commit 8b5ce31)

Co-authored-by: Victor Stinner <vstinner@python.org>
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@vstinner @miss-islington
vstinner authored and miss-islington committed Sep 14, 2025
commit fd1444e0b093ef3b871c189fe7b8668f5c354529
10 changes: 8 additions & 2 deletions Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1046,8 +1046,14 @@ _PyLong_AsByteArray(PyLongObject* v,
just above didn't get to ensure there's a sign bit, and the
loop below wouldn't add one either. Make sure a sign bit
exists. */
unsigned char msb = *(p - pincr);
int sign_bit_set = msb >= 0x80;
int sign_bit_set;
if (n > 0) {
unsigned char msb = *(p - pincr);
sign_bit_set = msb >= 0x80;
}
else {
sign_bit_set = 0;
}
assert(accumbits == 0);
if (sign_bit_set == do_twos_comp)
return 0;
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