lib/51749: Xorg server crashes when run from xdm

[martin%NetBSD.org@localhost]

>Number:         51749
>Category:       lib
>Synopsis:       Xorg server crashes when run from xdm
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Dec 28 15:40:00 +0000 2016
>Originator:     Martin Husemann
>Release:        NetBSD 7.99.53
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD night-owl.duskware.de 7.99.53 NetBSD 7.99.53 (NIGHT-OWL) #450: Wed Dec 28 12:18:50 CET 2016 martin%night-owl.duskware.de@localhost:/usr/src/sys/arch/amd64/compile/NIGHT-OWL amd64
Architecture: x86_64
Machine: amd64
>Description:

With a very current -current Xorg crashes when started via xdm. A simple
startx works though. I see this on two different machines, one amd64, one
sparc64.

The issue is not in the kernel (i.e.: booting with an old kernel does not
fix it).

Xorg dumps core, but the core is pretty unusable (could it be that gdb's
core target has a silent limit on the number of program headers, so not
all memory mapped in the process and dumped gets loaded in gdb?):

[New process 1]
[New process 2]
Core was generated by `Xorg'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007f7feda2a7d9 in ?? ()
[Current thread is 1 (process 1)]
(gdb) bt
#0  0x00007f7feda2a7d9 in ?? ()
#1  0x00007f7ff7b3d800 in ?? ()
#2  0x00007f7fffffa488 in ?? ()
#3  0x00000000006ac2f8 in ?? ()
#4  0x00007f7fffffa488 in ?? ()
#5  0x00000000006ac2f8 in ?? ()
#6  0x0000000000000025 in ?? ()
#7  0x0000000000000097 in ?? ()
#8  0x00007f7fec60a441 in ?? ()
#9  0x00007f7fffffa488 in ?? ()
#10 0x0000000000000000 in ?? ()


A -current as of ~1 week ago worked.

The "lib" category of this PR is just a wild guess, as there seem to have been
no changes anywhere in xsrc that could cause this.

>How-To-Repeat:
Add xdm=YES to /etc/rc.conf, watch your X server die

>Fix:
n/a