Subject: 24.3.50; race condition in x_frame_rehighlight

From: Andreas Politz <[email-masked]>
Date: Sat, 28 Sep 2013 01:47:33 +0200

There is a race condition in x_frame_rehighlight regarding input
redirection, triggering a null-pointer access.  This kinds of
errors are usually difficult to reproduce.  I used the following
code, while simultaneously changing focus rapidly via the
window-manager.

(defun fn (&optional parms)
  (let* ((frame (make-frame parms)))
    (sit-for 1e-100)
    (redirect-frame-focus (selected-frame) frame)
    frame))

(while t
  (let ((f1 (fn
	     '((width . 20)
	       (height . 30))))
	(f2 (fn
	     '((width . 20)
	       (height . 30)
	       (top . 400)))))
    (sleep-for (/ (float (random 1000)) 5000))
    (delete-other-frames)))

Take a look at this part of the attached back-trace.

#0  0x00000000004f9b0e in frame_highlight (f=0x132b510) at xterm.c:3204
#4  0x00000000004fa4ae in x_detect_focus_change (dpyinfo=0x15ba800, frame=0x11c7e68, 
    event=0x7fffffffb300, bufp=0x7fffffffae50) at xterm.c:3522
#14 0x00000000004ff413 in XTread_socket (...) at xterm.c:7066
#19 0x00000000005409e7 in unblock_input () at keyboard.c:7116
#20 0x0000000000503f82 in x_free_frame_resources (f=0x132b510) at xterm.c:9383
#21 0x0000000000503fbf in x_destroy_window (f=0x132b510) at xterm.c:9397
#22 0x00000000004274b7 in delete_frame (frame=20100373, force=12634498) at frame.c:1362
#23 0x000000000042784e in Fdelete_frame (frame=20100373, force=12634498) at frame.c:1495

Note that the freed frame in #20 is the same as the one about to
be highlighted in #0.  delete_frame would later execute

    f->terminal = 0;             /* Now the frame is dead.  */

but won't, since x_destroy_window has not returned yet.  But
x_free_frame_resources has executed

    f->output_data.x = NULL;

, so FRAME_LIVE_P(f) is still true, but FRAME_X_DISPLAY is no
good at this moment.  Then in x_frame_rehighlight the deleted
frame becomes the x_highlight_frame.

(gdb) p /x dpyinfo->x_focus_frame
$30 = 0x11c7e68
(gdb) p /x dpyinfo->x_highlight_frame 
$27 = 0x132b510
(gdb) pp dpyinfo->x_focus_frame.focus_frame
#<frame emacs@luca 0x132b510>
(gdb) p /x dpyinfo->x_highlight_frame.output_data.x
$36 = 0x0
(gdb) p /x dpyinfo->x_highlight_frame.terminal
$37 = 0x110e398

The second if condition is false (FRAME_LIVE_P) and
frame_highlight gets called with the halfway deleted frame, calls
FRAME_X_DISPLAY and that's the end.

-ap

In GNU Emacs 24.3.50.4 (x86_64-unknown-linux-gnu, GTK+ Version 2.20.1)
 of 2013-09-27 on luca
Bzr revision: 114421 [email-masked]
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
System Description:	Debian GNU/Linux 6.0.7 (squeeze)

Important settings:
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  value of $LC_MESSAGES: C
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

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Recent input:
i f SPC c o n d i M-/ SPC i s SPC f a l s e , SPC d 
u e SPC t o SPC M-h M-h DEL DEL SPC ( F R A M-/ M-/ 
M-/ M-/ ) SPC a n d SPC f r a m e M-/ M-/ M-h M-h h 
i M-/ SPC g e t s SPC c a l l e d SPC w i t h SPC M-q 
SPC t h e M-h a SPC f r a m e M-b DEL DEL C-p C-p C-p 
SPC < = = <backspace> <backspace> - - C-b C-b C-b C-k 
C-p C-p C-p C-n C-n C-n C-n C-n C-n M-f SPC d <backspace> 
M-SPC M-f C-p C-p C-p C-j C-y C-y C-p C-n C-n C-k M-f 
C-e C-n SPC M-b t h e SPC d e l e t e d SPC M-f , SPC 
c a l l s SPC C-x o C-u C-SPC C-SPC C-SPC C-c i f r 
M-p <return> C-s x _ C-s C-M-d C-M-SPC M-w C-x o C-y 
SPC a n d SPC M-q SPC t h a t ' s SPC t h e SPC e n 
d . C-x o C-x o M-q C-x o C-x b C-s C-s <return> M-< 
C-x C-w / t m p / g <backspace> <return> y y C-x o 
C-p M-f SPC h a l f w a y M-q C-n C-l C-j C-j - a p 
C-x C-w b u g <return> C-x h C-g M-x i s p e l l <return> 
a 0 a a a 1 a a a a a a a a a a C-x C-s C-x h M-w M-x 
r e p o r t - e m <tab> b u <tab> <return>

Recent messages:
Mark set [2 times]
Quit
Spell-checking bug using aspell with en dictionary...done
Saving file /tmp/bug...
Wrote /tmp/bug
Mark set [2 times]
Saved text until "RAME_X_DISPLAY and that's the end.

-ap
"

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