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# Kernel Panic Procedures

This article is a work in progress or otherwise under review
and does not represent current policy.

## Contents

1. Synopsis
1. Preliminary Notes
1. Obtaining a Kernel Dump
1. Finding which line caused the crash
1. Backtrace through trap() in GDB
1. Example Crash: Force Panic from WSCons via KVM: Dell DRAC4
1. What Now
1. Processing the core dump

## Synopsis

Although a few official documents exist on the topics of using
the advanced kernel debugging using KGDB (Kernelized GNU Debugger (GDB)),
there are few documents which formalize a <b>"Kernel Panic/Crash Reporting Procedure"</b>
using a combination of DDB (the minimalist in-kernel debugger) in
combination with GDB after the crash.


## Preliminary Notes

If the problem is easily re-created, try to obtain a kernel backtrace

The DDB is the minimalist kernel Debugger added by `options DDB` to the kernel

Obtain a backtrace at the `db{0}>` prompt using the `bt` command

Search the [Mailing List Archives]( and [Query the PR database]( for reports of similar issues.

Post the problem for the discussion on the appropriate mailing list.

## Obtaining a Kernel Dump

A kernel dump is possible to obtain from many kernel panics.
When at the DB prompt, simply execute:

	db{0}> sync

The dump of memory will be written to the swap partition.

At boot time the swap file coredump will be saved to `/var/crash`.
The default settings to control this behaviour is in `/etc/defaults/rc.conf`
and can be overriden in `/etc/rc.conf`

	savecore_flags="-N /netbsd -z"

A [[!template id=man name="gzip" section="1"]] compressed file will be available for analysis with [[!template id=man name="gdb" section="1"]] or [[!template id=man name="crash" section="8"]]. To load the core dump into `gdb`, after uncompressing it with [[!template id=man name="gunzip" section="1"]], use `target kvm /path/to/netbsd.core`.

Your swap partition must be at least the size of your physical RAM

Your `/var/crash` partition must have sufficient space to hold the same file.

## Finding which line caused the crash

With a back trace, it's possible to translate the an address to a line in source code.

	Stopped in pid 496.1 (gdb) at netbsd:breakpoint+0x5: leave

To find the address of breakpoint function in the running kernel, use [[!template id=man name="nm" section="1"]].

	nm /netbsd | grep breakpoint
	ffffffff8021df70 T breakpoint
	ffffffff8079d944 T db_breakpoint_cmd
	ffffffff81644b38 d db_breakpoint_list
	ffffffff81644b30 d db_breakpoints_inserted
	ffffffff8079d892 T db_clear_breakpoints
	ffffffff8079d7d0 t db_find_breakpoint
	ffffffff8079d824 T db_find_breakpoint_here
	ffffffff81644b40 d db_free_breakpoints
	ffffffff81644b48 d db_next_free_breakpoint
	ffffffff8079d835 T db_set_breakpoints

Then add `0x5` to the address (`0x5` is obtained from the panic message above, not a fixed value for all) and use [[!template id=man name="addr2line" section="1"]]

	addr2line -e /netbsd ffffffff8021df75

## Backtrace through trap() in GDB

In [[!template id=man name="gdb" section="1"]] import the stack script and run the `stack` command.

	(gdb) source /usr/src/sys/arch/i386/gdbscripts/stack

See [port-i386/10313]( for more info.

## Example Crash: Force Panic from WSCons via KVM: Dell DRAC4

You can invoke the kernel debugger from the console on amd64/i386 using the special key sequence: Control+Alt+Esc. See the "Entering the debugger" section of [[!template id=man name="ddb" section="9"]] for the key sequence on other platforms.

Once in the debugger, you can instruct the KDB to run a preliminary backtrace to get a general idea of what went wrong using the `bt` command.

You can then force a sync of the file system and and dump of the kernel memory into the swap partition using the `sync` command.

On the subsequent boot, the `/etc/rc.d/savecore` script will perform the necessary tasks to archive and [[!template id=man name="gzip" section="1"]] the dump.

You can then load the core dump into [[!template id=man name="gdb" section="1"]] or [[!template id=man name="crash" section="8"]]

## What Now

You can submit the feedback as [a PR to the NetBSD GNATS system](

## Processing the core dump

Hubert Feyrer has a [great guide to analyzing kernel panic core dumps](

Additionally, the following command below can be used to create a relatively useful backtrace:

localhost# cd /var/crash
localhost# gunzip -d *gz
localhost# gdb  --symbols=/netbsd.gdb --quiet --eval-command="file /netbsd.gdb" \ 
                --eval-command="target kvm netbsd.1.core" --eval-command "bt" \ 
                --eval-command "list" --eval-command "info all-registers" 2>&1
Load new symbol table from "/netbsd.gdb"? (y or n) y
Reading symbols from /netbsd.gdb...done.
#0  0xc047c9f8 in cpu_reboot (howto=256, bootstr=0x0) at /usr/src/sys/arch/i386/i386/machdep.c:927
927                     dumpsys();
#0  0xc047c9f8 in cpu_reboot (howto=256, bootstr=0x0) at /usr/src/sys/arch/i386/i386/machdep.c:927
#1  0xc01c3f2a in db_sync_cmd (addr=-1065223264, have_addr=false, count=-1071881791, modif=0xcc883c04 "[BINARY]") at /usr/src/sys/ddb/db_command.c:1304
#2  0xc01c45fa in db_command (last_cmdp=0xc07dfe3c) at /usr/src/sys/ddb/db_command.c:926
#3  0xc01c4856 in db_command_loop () at /usr/src/sys/ddb/db_command.c:583
#4  0xc01c7320 in db_trap (type=1, code=0) at /usr/src/sys/ddb/db_trap.c:101
#5  0xc0478855 in kdb_trap (type=1, code=0, regs=0xcc883e3c) at /usr/src/sys/arch/i386/i386/db_interface.c:229
#6  0xc047efe2 in trap (frame=0xcc883e3c) at /usr/src/sys/arch/i386/i386/trap.c:350
#7  0xc010cb80 in calltrap ()
#8  0xc047717c in breakpoint ()
#9  0xc02e3676 in wskbd_translate (id=0xc0833ae0, type=2, value=&lt;value optimized out&gt;) at /usr/src/sys/dev/wscons/wskbd.c:1586
#10 0xc02e386e in wskbd_input (dev=0xcc888800, type=2, value=1) at /usr/src/sys/dev/wscons/wskbd.c:682
#11 0xc054c27a in pckbd_input (vsc=0xcc0cc6a8, data=1) at /usr/src/sys/dev/pckbport/pckbd.c:584
#12 0xc02ba80d in pckbcintr (vsc=0xcc0d6ebc) at /usr/src/sys/dev/ic/pckbc.c:607
#13 0xc0465798 in intr_biglock_wrapper (vp=0xc2e853c0) at /usr/src/sys/arch/x86/x86/intr.c:617
#14 0xc01036d9 in Xintr_ioapic_edge3 ()
#15 0xc0477234 in x86_mwait ()
Previous frame inner to this frame (corrupt stack?)
922             /* Disable interrupts. */
923             splhigh();
925             /* Do a dump if requested. */
926             if ((howto &amp; (RB_DUMP | RB_HALT)) == RB_DUMP)
927                     dumpsys();
929     haltsys:
930             doshutdownhooks();
eax            0x0      0
ecx            0x0      0
edx            0x0      0
ebx            0x100    256
esp            0xcc883bb8       0xcc883bb8
ebp            0xcc883bc0       0xcc883bc0
esi            0xc07dfe3c       -1065484740
edi            0x0      0
eip            0xc047c9f8       0xc047c9f8 &lt;cpu_reboot+368&gt;
eflags         0x0      [ ]
cs             0x0      0
ss             0x0      0
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0
st0            0        (raw 0x00000000000000000000)
st1            0        (raw 0x00000000000000000000)
st2            0        (raw 0x00000000000000000000)
st3            0        (raw 0x00000000000000000000)
st4            0        (raw 0x00000000000000000000)
st5            0        (raw 0x00000000000000000000)
st6            0        (raw 0x00000000000000000000)
st7            0        (raw 0x00000000000000000000)
fctrl          0x0      0
fstat          0x0      0
ftag           0x0      0
fiseg          0x0      0
fioff          0x0      0
foseg          0x0      0
fooff          0x0      0
fop            0x0      0

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