bug#47462: 27.1; Fatal error 11: Segmentation fault

bug#47462: 27.1; Fatal error 11: Segmentation fault

[Al Haji-Ali]

Executing the following code (for example with `eval-last-sexp`) seg faults
my Emacs 27.1 with `emacs -Q`

(progn (defmacro watch-variable (symbol)
	 `(add-variable-watcher
	   ,symbol
	   (lambda nil
	     (message "variable changed %S" ,symbol)
	     (backtrace))))
       (watch-variable load-file-rep-suffixes))


In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo version 1.15.10)
 of 2020-09-14 built on thinkpad
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04.2 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --prefix=/home/abdo/.local/ --with-json --with-modules
 --with-harfbuzz --with-compress-install --with-threads
 --with-included-regex --with-zlib --with-sound=alsa --without-gconf
 --with-mailutils --with-xft --with-xml2 --with-cairo --with-gconf
 --with-xwidgets --without-toolkit-scroll-bars 'CFLAGS=-O3 -mtune=native
 -march=native -fomit-frame-pointer
 -I/usr/lib/gcc/x86_64-linux-gnu/9/include/
 -L/usr/lib/gcc/x86_64-linux-gnu/9''

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GCONF GSETTINGS GLIB
NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT
LIBOTF ZLIB GTK3 X11 XDBE XIM MODULES THREADS XWIDGETS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

Important settings:
  value of $LC_CTYPE: en_GB.UTF-8
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame minibuffer cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
xwidget-internal cairo move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 44010 9472)
 (symbols 48 6010 1)
 (strings 32 15457 1779)
 (string-bytes 1 507214)
 (vectors 16 9269)
 (vector-slots 8 124636 10078)
 (floats 8 19 43)
 (intervals 56 219 0)
 (buffers 1000 11))

-- Al

bug#47462: 27.1; Fatal error 11: Segmentation fault

[Eli Zaretskii]

> From: Al Haji-Ali <abdo.haji.ali@gmail.com>
> Date: Mon, 29 Mar 2021 09:03:52 +0100
> 
> Executing the following code (for example with `eval-last-sexp`) seg faults
> my Emacs 27.1 with `emacs -Q`
> 
> (progn (defmacro watch-variable (symbol)
> 	 `(add-variable-watcher
> 	   ,symbol
> 	   (lambda nil
> 	     (message "variable changed %S" ,symbol)
> 	     (backtrace))))
>        (watch-variable load-file-rep-suffixes))

Thanks, I added protection on master from this kind of invalid usage.
With Emacs 27, just don't do that.