https://github.com/python/cpython/commit/a96096c8897c5ab4511804280c737962ca9c9213
commit: a96096c8897c5ab4511804280c737962ca9c9213
branch: 3.14
author: Jelle Zijlstra <[email protected]>
committer: gpshead <[email protected]>
date: 2026-11-09T03:25:42Z
summary:
[3.14] gh-159739: Handle overlapping buffers in struct.pack_into (GH-159985)
(#160484)
`struct.pack_into()` can use the same bytearray as both the destination and an
`s` or `p` value. The copy then passes overlapping regions to `memcpy`, which
is undefined in C and can produce platform-dependent results or a sanitizer
abort. `memmove` makes the overlapping copy defined.
(cherry picked from commit 31a7d994a559a421814ca9e8edfcab21ae4a5b36)
files:
A Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
M Modules/_struct.c
diff --git
a/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
new file mode 100644
index 00000000000000..11c88411bb9125
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
@@ -0,0 +1,2 @@
+Avoid undefined behavior when :func:`struct.pack_into` copies a bytearray into
+overlapping output.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 788dd39164f68c..c4afd44cff0e3b 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -2202,7 +2202,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const
*args, int offset,
if (n > code->size)
n = code->size;
if (n > 0)
- memcpy(res, p, n);
+ memmove(res, p, n);
} else if (e->format == 'p') {
Py_ssize_t n;
int isstring;
@@ -2228,7 +2228,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const
*args, int offset,
n = code->size - 1;
}
if (n > 0)
- memcpy(res + 1, p, n);
+ memmove(res + 1, p, n);
if (n > 255)
n = 255;
if (n > 0) {
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