bug#53136: 28.0.90; segfault in lock_file

[Eli Zaretskii]

> Resent-From: Po Lu <luangruo@yahoo.com>
> Original-Sender: "Debbugs-submit" <debbugs-submit-bounces@debbugs.gnu.org>
> Resent-CC: bug-gnu-emacs@gnu.org
> Resent-Sender: help-debbugs@gnu.org
> Date: Sun, 09 Jan 2022 14:04:23 +0800
> From:  Po Lu via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> This write in `lock_file':
> 
>       memcpy (dot, replacement, replacementlen);
> 
> Ends up writing one byte outside dot.
> 
> I could not find the problem, but someone else may want to take a look.

Details, please: the values of all the relevant variables at that
point, including the contents of all the string variables.  And what
is your value of MAX_LFINFO?  I don't see how this can be efficiently
investigated without this data.  In particular, 'dot' has no storage
of its own, it's just a pointer into the lock_info.user[] array.