https://github.com/python/cpython/commit/31a7d994a559a421814ca9e8edfcab21ae4a5b36
commit: 31a7d994a559a421814ca9e8edfcab21ae4a5b36
branch: main
author: Jelle Zijlstra <[email protected]>
committer: JelleZijlstra <[email protected]>
date: 2026-10-21T19:13:16Z
summary:
gh-159739: Handle overlapping buffers in struct.pack_into (#159985)
`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.
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 1e5d52b758f7e2..fad026ac8a4652 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -2409,7 +2409,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const
*args,
if (n > code->size)
n = code->size;
if (n > 0)
- memcpy(res, p, n);
+ memmove(res, p, n);
} else if (strcmp(e->format, "p") == 0) {
Py_ssize_t n;
int isstring;
@@ -2435,7 +2435,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const
*args,
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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