Bug 89296

Note that xfce settings daemon uses the left handed property to determine if a device is from libinput, so this is most likely related:

#ifdef HAVE_LIBINPUT
static gboolean
xfce_pointers_is_libinput (Display *xdisplay,
                           XDevice *device)
{
    Atom     prop, type;
    gulong   n_items, bytes_after;
    gint     rc, format;
    guchar  *data;

    prop = XInternAtom (xdisplay, LIBINPUT_PROP_LEFT_HANDED, False);
    gdk_error_trap_push ();
    rc = XGetDeviceProperty (xdisplay, device, prop, 0, 1, False,
                             XA_INTEGER, &type, &format, &n_items,
                             &bytes_after, &data);
    gdk_error_trap_pop ();
    if (rc == Success)
    {
        XFree (data);
        return (n_items > 0);
    }

    return FALSE;
}
#endif /* HAVE_LIBINPUT */

Created attachment 113792 [details]
Reproducer

Simple reproducer, build and run this test program would crash the X server here.

Corresponding backtrace in Xorg:

Program received signal SIGSEGV, Segmentation fault.
libinput_device_config_left_handed_is_available (device=0x0) at libinput.c:1685
1685		if (!device->config.left_handed)
(gdb) p device
$1 = (struct libinput_device *) 0x0
(gdb) bt
#0  libinput_device_config_left_handed_is_available (device=0x0)
    at libinput.c:1685
#1  0x00007f4b2c658e05 in LibinputSetPropertyLeftHanded (
    checkonly=<optimized out>, val=<optimized out>, atom=<optimized out>, 
    dev=<optimized out>) at libinput.c:1473
#2  LibinputSetProperty (dev=<optimized out>, atom=<optimized out>, 
    val=<optimized out>, checkonly=<optimized out>) at libinput.c:1574
#3  0x0000000000530d4f in XIChangeDeviceProperty (dev=0x2211bd0, 
    property=property@entry=275, type=<optimized out>, format=<optimized out>, 
    mode=<optimized out>, len=<optimized out>, value=0x21b46f4, sendevent=1)
    at xiproperty.c:772
#4  0x0000000000531428 in change_property (client=0x2420060, data=0x21b46f4, 
    len=<optimized out>, mode=<optimized out>, format=<optimized out>, 
    type=<optimized out>, property=275, dev=<optimized out>)
    at xiproperty.c:353
#5  ProcXChangeDeviceProperty (client=0x2420060) at xiproperty.c:911
#6  0x0000000000439137 in Dispatch () at dispatch.c:432
#7  0x000000000043d2d6 in dix_main (argc=16, argv=0x7fff251f1bd8, 
    envp=<optimized out>) at main.c:296
#8  0x00007f4b33adcfe0 in __libc_start_main (main=0x4276c0 <main>, argc=16, 
    argv=0x7fff251f1bd8, init=<optimized out>, fini=<optimized out>, 
    rtld_fini=<optimized out>, stack_end=0x7fff251f1bc8) at libc-start.c:289
#9  0x00000000004276ee in _start ()

Crash occurs while accessing a specific device "HID 04d9:1400" which is listed as both a keyboard and a pointer device:

15:16 $ xinput
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
⎜   ↳ Logitech USB-PS/2 Optical Mouse         	id=11	[slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad              	id=14	[slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                   	id=15	[slave  pointer  (2)]
⎜   ↳ HID 04d9:1400                           	id=10	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
    ↳ Power Button                            	id=6	[slave  keyboard (3)]
    ↳ Video Bus                               	id=7	[slave  keyboard (3)]
    ↳ Sleep Button                            	id=8	[slave  keyboard (3)]
    ↳ HID 04d9:1400                           	id=9	[slave  keyboard (3)]
    ↳ Integrated Camera                       	id=12	[slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard            	id=13	[slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                  	id=16	[slave  keyboard (3)]

lsusb gives:

Bus 001 Device 025: ID 04d9:1400 Holtek Semiconductor, Inc. PS/2 keyboard + mouse controller

Possible fix http://patchwork.freedesktop.org/patch/43211/ (works for me, but there might be a better fix)

Do you have a full Xorg.log? if the libinput device ends up null, you should see an error message.

Updated patch: http://patchwork.freedesktop.org/patch/43317/

commit 98ae01b9ae8616d3c2047f5510205aa4e3bc52b5
Author: Olivier Fourdan <ofourdan@redhat.com>
Date:   Wed Feb 25 11:49:19 2015 +0100

    Ignore property changes if the device is disabled