Emacs Abnormal Termination

Update: This bug was fixed in Emacs 24.4 (released October 2014).

A few months ago I filed a bug report for Emacs (upstream) when I stumbled across Emacs aborting under very specific circumstances. I was editing in markdown-mode and a regular expression replacement on lists would reliably, and frustratingly, cause Emacs to crash.

Through a sort-of binary search I only loaded only half of markdown-mode to see in which half it would trigger, then I cut that half in half again and repeated recursively until I had it down to a small expression that causes a --no-init-file (-q) Emacs to abort. It almost looks like I found it through fuzz testing. Change or remove anything even slightly and it no longer triggers the abort.

To trigger it, there’s an after-change-functions hook that performs a regular expression search immediately after a replace-regexp. A peek at the backtrace with gdb shows that this somehow causes the point to leave the bounds of the buffer. Emacs detects this as an assertion before dereferencing anything, and it aborts, thus preventing a buffer overflow vulnerability. This is important for my Emacs web server because if there’s a way to trigger this bug in the web server I’d much rather have it abort than run arbitrary shellcode injected in by a malicious HTTP request.

My bug report has seen no activity since I posted it. I can understand why. The circumstances to trigger it are unlikely and it’s a very old bug, so it’s low priority. It’s also a huge pain to debug. Hacking on Emacs from Lisp is pleasant but hacking on Emacs from C is not. The bug likely sits in the bowels of the complicated regular expression engine, making it even more unpleasant. I personally have no interest in trying to fix it myself.

So, since it looks like it’s here for the long haul it’s kind of fun to implement an abort function on top of it, allowing Elisp programs to terminate Emacs abnormally — you know, in case kill-emacs isn’t fun enough.

(defun abort ()
  "Ask Emacs to abnormally terminate itself (bug#12077)."
    (insert "#\n*\n")
    (goto-char (point-min))
    (add-hook 'after-change-functions
              (lambda (a b c) (re-search-forward "")))
    (replace-regexp "^\\*" " *")))

It’s interactive so you could even bind a key to it.

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Chris Wellons

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