Bug report

Bug description:

When building with ubsan enabled, an unaligned access error gets triggered in dictobject.c. I triggered this on linux x86_64 when interpreting the following script:

importargparseparser=argparse.ArgumentParser()

Here's the ubsan error message:

thread 15767 panic: load of misaligned address 0x386b491 for type 'PyDictUnicodeEntry *', which requires 8 byte alignmentObjects/dictobject.c:1504:21: 0x2916e0e in dictresize (cpython/Objects/dictobject.c)                    memcpy(newentries, oldentries, numentries * sizeof(PyDictUnicodeEntry));                    ^

Adding logs around the call to memcpy reveals thatoldentries contains an unaligned value, but,numentries is also0, which, means most implementations would probably do what the caller expects, namely, nothing.

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-136660: fix UB in dictobject memcpy when there are 0 entries #136662