From 7d8eef4d7263616b9c3c1819aed257533eaf6c51 Mon Sep 17 00:00:00 2001 From: Jelle Zijlstra Date: Wed, 21 Oct 2026 12:13:16 -0700 Subject: [PATCH] gh-159739: Handle overlapping buffers in struct.pack_into (GH-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. (cherry picked from commit 31a7d994a559a421814ca9e8edfcab21ae4a5b36) Co-authored-by: Jelle Zijlstra --- .../Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst | 2 ++ Modules/_struct.c | 4 ++-- 2 files changed, 4 insertions(+), 2 deletions(-) create mode 100644 Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst 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 7d88e7e662af4d..20955e9cc5563d 100644 --- a/Modules/_struct.c +++ b/Modules/_struct.c @@ -2366,7 +2366,7 @@ 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; @@ -2392,7 +2392,7 @@ 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) {