The bug has been opened on https://launchpad.net/bugs/94737

"Binary package hint: gdm

crash when I used xdmcp server
...
libx11-6 2:1.1.1-1ubuntu1
...
http://librarian.launchpad.net/6898243/ThreadStacktrace.txt
ThreadStacktrace.txt
..."

Debug backtrace for the crash:

206                     map->syms[offset]= 0;
(gdb) thread apply all bt full

Thread 1 (process 5372):
#0  0xb76df41b in _XkbReadGetMapReply (dpy=0x8086108, rep=0xbf8e97a4, xkb=0x8557fc0, nread_rtrn=0x0)
    at ../../../src/xkb/XKBGetMap.c:206
        sz = <value optimized out>
        prev_syms = (KeySym *) 0x855c7b8
        extraData = <value optimized out>
        mask = <value optimized out>
#1  0xb76df864 in _XkbHandleGetMapReply (dpy=0x8086108, xkb=0x8557fc0) at ../../../src/xkb/XKBGetMap.c:526
        rep = {type = 1 '\001', deviceID = 4 '\004', sequenceNumber = 843, length = 1010, pad1 = 0, minKeyCode = 8 '\b', 
  maxKeyCode = 255 '�', present = 71, firstType = 0 '\0', nTypes = 18 '\022', totalTypes = 18 '\022', 
  firstKeySym = 8 '\b', totalSyms = 289, nKeySyms = 248 '�', firstKeyAct = 0 '\0', totalActs = 0, nKeyActs = 0 '\0', 
  firstKeyBehavior = 0 '\0', nKeyBehaviors = 0 '\0', totalKeyBehaviors = 0 '\0', firstKeyExplicit = 0 '\0', 
  nKeyExplicit = 0 '\0', totalKeyExplicit = 0 '\0', firstModMapKey = 8 '\b', nModMapKeys = 248 '
  totalModMapKeys = 14 '\016', firstVModMapKey = 0 '\0', nVModMapKeys = 0 '\0', totalVModMapKeys = 0 '\0', pad2 = 0 '\0', 
  virtualMods = 65535}
#2  0xb76e0204 in XkbGetUpdatedMap (dpy=0x8086108, which=71, xkb=0x8557fc0) at ../../../src/xkb/XKBGetMap.c:543
        req = <value optimized out>
        status = 0
#3  0xb76e02c4 in XkbGetMap (dpy=0x8086108, which=71, deviceSpec=256) at ../../../src/xkb/XKBGetMap.c:561
        xkb = (XkbDescPtr) 0x8557fc0
#4  0xb7bd4950 in get_xkb (keymap_x11=0x855a038) at gdkkeys-x11.c:256
        display_x11 = (GdkDisplayX11 *) 0x808e098
        xdisplay = (Display *) 0x8086108
#5  0xb7bd4f3f in update_direction (keymap_x11=0x85c3ae8, group=65825) at gdkkeys-x11.c:591
        xkb = <value optimized out>
        group_atom = 0
#6  0xb7bd5178 in IA__gdk_keymap_get_direction (keymap=0x855a038) at gdkkeys-x11.c:713
        state_rec = {group = 0 '\0', locked_group = 0 '\0', base_group = 0, latched_group = 0, mods = 0 '\0', 
  base_mods = 0 '\0', latched_mods = 0 '\0', locked_mods = 0 '\0', compat_state = 0 '\0', grab_mods = 0 '\0', 
  compat_grab_mods = 0 '\0', lookup_mods = 0 '\0', compat_lookup_mods = 0 '\0', ptr_buttons = 0}
        __PRETTY_FUNCTION__ = "IA__gdk_keymap_get_direction"
#7  0xb7ccfb25 in gtk_entry_ensure_layout (entry=0x82080a8, include_preedit=1) at gtkentry.c:3251
No locals.
#8  0xb7cd3033 in gtk_entry_adjust_scroll (entry=0x82080a8) at gtkentry.c:3705
        min_offset = <value optimized out>
        max_offset = <value optimized out>
        text_area_width = 152
        text_width = <value optimized out>
        inner_border = {left = 2, right = 2, top = 2, bottom = 2}
        strong_x = <value optimized out>
        weak_x = <value optimized out>
        strong_xoffset = <value optimized out>
        weak_xoffset = <value optimized out>
        xalign = 1.9375
        layout = <value optimized out>
---Type <return> to continue, or q <return> to quit---
        logical_rect = {x = 136347816, y = 1079488016, width = -1218125536, height = -1208607828}
#9  0xb7cd31c7 in recompute_idle_func (data=0x82080a8) at gtkentry.c:3111
        entry = (GtkEntry *) 0x85c3ae8
#10 0xb78db091 in g_idle_dispatch (source=0x8556bf0, callback=0x10121, user_data=0x82080a8) at gmain.c:3928
No locals.
#11 0xb78dcdf2 in IA__g_main_context_dispatch (context=0x80953c0) at gmain.c:2045
No locals.
#12 0xb78dfdcf in g_main_context_iterate (context=0x80953c0, block=1, dispatch=1, self=0x80a3d18) at gmain.c:2677
        got_ownership = <value optimized out>
        max_priority = 115
        timeout = 0
        some_ready = 1
        nfds = <value optimized out>
        allocated_nfds = <value optimized out>
        fds = (GPollFD *) 0x8096df0
        __PRETTY_FUNCTION__ = "g_main_context_iterate"
#13 0xb78e0179 in IA__g_main_loop_run (loop=0x8517778) at gmain.c:2881
        got_ownership = 0
        self = (GThread *) 0x80a3d18
        __PRETTY_FUNCTION__ = "IA__g_main_loop_run"
#14 0xb7d4c044 in IA__gtk_main () at gtkmain.c:1177
        tmp_list = (GList *) 0x0
        functions = (GList *) 0x0
        init = (GtkInitFunction *) 0x0
        loop = (GMainLoop *) 0x8517778
#15 0x08051925 in ?? ()
No symbol table info available.
#16 0x0854f320 in ?? ()
No symbol table info available.
#17 0x080992b0 in ?? ()
No symbol table info available.
#18 0x080526b0 in ?? ()
No symbol table info available.
#19 0x00000000 in ?? ()
No symbol table info available.

(gdb) p *xkb->map->syms
$1 = 0