https://github.com/python/cpython/commit/8b5ce31c2b44d9bf82e6119e90a52dd530bfd1db
commit: 8b5ce31c2b44d9bf82e6119e90a52dd530bfd1db
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2025-09-14T11:25:53+02:00
summary:
gh-71810: Fix _PyLong_AsByteArray() undefined behavior (#138873)
Don't read p[-1] when p is an empty string: when n==0.
files:
M Objects/longobject.c
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 63b48572ff20d3..dba190d840ca42 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -1214,8 +1214,14 @@ _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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