kern/49603: Single-stepping into syscall reboots -current/amd64 under qemu

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         49603
>Category:       kern
>Synopsis:       Single-stepping into syscall reboots -current/amd64 under qemu
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jan 25 13:55:01 +0000 2015
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2014.12.14.18.14.15
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

When debugging a userland process using gdb under NetBSD-current/amd64
running in qemu virtual machine, single stepping into a system call
causes an instant reboot.  Root privileges are not required.

I ran an automated binary search to find when the problem first
appeard, and it pointed at src/sys/sys/ksyms.h 1.30, committed by
christos on CVS date 2014.12.14.18.14.15.

The reboot is consistently reproducible under qemu, but I have not
been able to reproduce it on physical hardware (tested on an AMD
Athlon64).  It also does not happen with the i386 port, only amd64.

>How-To-Repeat:

pkg_add py-anita
anita interact http://nyftp.netbsd.org/pub/NetBSD-daily/HEAD/201501250540Z/amd64/
(log in as root)
gdb /bin/sync
break sync
run
stepi
stepi
stepi

>Fix: