Undefined Behavior triggered in dictobject

[marler8997]

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:

import argparse
parser = argparse.ArgumentParser()

Here's the ubsan error message:

thread 15767 panic: load of misaligned address 0x386b491 for type 'PyDictUnicodeEntry *', which requires 8 byte alignment
Objects/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 that oldentries contains an unaligned value, but, numentries is also 0, 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

[methane]

See #127568

[StanFromIreland]

@methane If this is resolved then why is the issue needed?

[methane]

I am waiting for the issue author confirm the error is resolved in main branch.

[marler8997]

I haven't confirmed but it does look like #127568 will have solved, this. I'll reopen if I find otherwise.