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gh-146196: Fix potential Undefined Behavior in _PyUnicodeWriter_WriteASCIIString #146201

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vstinner merged 2 commits into from
Mar 20, 2026
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gh-146196: Fix potential Undefined Behavior in _PyUnicodeWriter_WriteASCIIString #146201

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gh-146196: Fix potential Undefined Behavior in _PyUnicodeWriter_Write…
ashm-dev Mar 20, 2026
63bbe18
Update Misc/NEWS.d/next/Core_and_Builtins/2026-03-20-13-55-14.gh-issu…
vstinner Mar 20, 2026
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gh-146196: Fix potential Undefined Behavior in _PyUnicodeWriter_Write… 
…ASCIIString
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@ashm-dev
ashm-dev committed Mar 20, 2026
commit 694c82a5fd156130679c18b9e61fa64f2a8deae5
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-20-13-55-14.gh-issue-146196.Zg70Kb.rst
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@@ -0,0 +1,3 @@
Fix potential Undefined Behavior in :c:func:`PyUnicodeWriter_WriteASCII` by
adding a zero-length check and using memmove() instead of memcpy(). Patch by
Shamil Abdulaev.
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4 changes: 4 additions & 0 deletions Objects/unicode_writer.c
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Expand Up @@ -465,6 +465,10 @@ _PyUnicodeWriter_WriteASCIIString(_PyUnicodeWriter *writer,
if (len == -1)
len = strlen(ascii);

if (len == 0) {
return 0;
}

assert(ucs1lib_find_max_char((const Py_UCS1*)ascii, (const Py_UCS1*)ascii + len) < 128);

if (writer->buffer == NULL && !writer->overallocate) {
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