$ gdb  -c CoreDump -symbols=/usr/lib/debug/usr/bin/Xorg  /usr/bin/Xorg
GNU gdb (GDB) 7.5.91.20130417-cvs-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/bin/Xorg...Reading symbols from /usr/lib/debug/usr/bin/Xorg...done.
done.
[New LWP 1635]
[New LWP 1729]

warning: Can't read pathname for load map: Input/output error.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7fffc79fe000
Core was generated by `/usr/bin/X :0 vt8 -br -nolisten tcp -auth /var/run/xauth/A:0-lF9oac'.
Program terminated with signal 6, Aborted.
#0  0x00007fe18459d037 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56      ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt full
#0  0x00007fe18459d037 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
        resultvar = 0
        pid = 1635
        selftid = 1635
#1  0x00007fe1845a0698 in __GI_abort () at abort.c:90
        save_stage = 2
        act = {__sigaction_handler = {sa_handler = 0x7fffc7902d82, sa_sigaction = 0x7fffc7902d82}, sa_mask = {__val = {140606566205732, 140606602996640,
              1891, 4294967296, 140606564825233, 0, 0, 1, 4, 140736541495440, 0, 0, 0, 140606566205075, 140606599540736, 140606566221176}},
          sa_flags = -2036339712, sa_restorer = 0x7fe1869fed10 <__PRETTY_FUNCTION__.23574>}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2  0x00007fe184595e03 in __assert_fail_base (fmt=0x7fe1846ec578 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
    assertion=assertion@entry=0x7fe1869fec00 "pScreen->myNum < xf86NumScreens", file=file@entry=0x7fe1869feba0 "../../../../hw/xfree86/common/xf86Helper.c",
    line=line@entry=1891, function=function@entry=0x7fe1869fed10 <__PRETTY_FUNCTION__.23574> "xf86ScreenToScrn") at assert.c:92
        str = 0x7fe188d9c360 "\240\016ވ\341\177"
        total = 4096
#3  0x00007fe184595eb2 in __GI___assert_fail (assertion=assertion@entry=0x7fe1869fec00 "pScreen->myNum < xf86NumScreens",
    file=file@entry=0x7fe1869feba0 "../../../../hw/xfree86/common/xf86Helper.c", line=line@entry=1891,
    function=function@entry=0x7fe1869fed10 <__PRETTY_FUNCTION__.23574> "xf86ScreenToScrn") at assert.c:101
No locals.
#4  0x00007fe1868dc57d in xf86ScreenToScrn (pScreen=<optimized out>) at ../../../../hw/xfree86/common/xf86Helper.c:1891
No locals.
#5  0x00007fe182fe6145 in radeon_dri2_destroy_buffer2 (pScreen=<optimized out>, drawable=<optimized out>, buffers=<optimized out>)
    at ../../src/radeon_dri2.c:356
        scrn = <optimized out>
#6  radeon_dri2_destroy_buffer2 (pScreen=<optimized out>, drawable=<optimized out>, buffers=0x7fe188d8c470) at ../../src/radeon_dri2.c:347
No locals.
#7  0x00007fe182fe6cb0 in radeon_dri2_client_state_changed (ClientStateCallback=<optimized out>, data=<optimized out>, calldata=<optimized out>)
    at ../../src/radeon_dri2.c:599
        pClientEventsPriv = 0x7fe188d7b0d0
        ref = 0x7fe188d7ec90
        clientinfo = <optimized out>
        pClient = <optimized out>
#8  0x00007fe1868955d4 in _CallCallbacks (pcbl=pcbl@entry=0x7fe186c733a0 <ClientStateCallback>, call_data=call_data@entry=0x7fffc7901240)
    at ../../dix/dixutils.c:735
        cbl = 0x7fe1888c3ea0
        cbr = 0x7fe1888c30c0
        pcbr = <optimized out>
#9  0x00007fe18688ffb4 in CallCallbacks (call_data=0x7fffc7901240, pcbl=0x7fe186c733a0 <ClientStateCallback>) at ../../include/callback.h:83
No locals.
---Type <return> to continue, or q <return> to quit---
#10 CloseDownClient (client=client@entry=0x7fe188d7b010) at ../../dix/dispatch.c:3395
        clientinfo = {client = 0x7fe188d7b010, prefix = 0x0, setup = 0x0}
        really_close_down = <optimized out>
#11 0x00007fe186890b35 in Dispatch () at ../../dix/dispatch.c:440
        clientReady = 0x7fe188bff990
        result = -1
        client = 0x7fe188d7b010
        nready = 0
        icheck = 0x7fe186c733b0 <checkForInput>
        start_tick = 100
#12 0x00007fe18687f5aa in main (argc=8, argv=0x7fffc7901438, envp=<optimized out>) at ../../dix/main.c:298
        i = <optimized out>
        alwaysCheckForInput = {0, 1}
(gdb)