The Use Case

There are numerous reasons for needing a debugging kernel. For example, during a kernel panic. This was the case with us. Therefore, we installed a kernel with DDB enabled. In the event of a kernel panic, the system would drop to the debugger and the software engineers were then able to begin diagnosing the problem.

Preparing To Build The Kernel

Prerequisites

• FreeBSD source tree
• A kernel config

FreeBSD Sources

Operators can use cvs or Subversion to install the sources. Information on FreeBSD source trees can be found here.

The Kernel Config

Create a kernel config by opening /usr/src/sys/${arch}/conf/DEBUG. Where ${arch} is the machine architecture the kernel is to be built for. In this case, we build an amd64 kernel on an amd64 machine. The kernel config should appear as follows:

include GENERIC
options KDB_UNATTENDED
options DDB

Compiling and Installing The Kernel

Compiling and installing the debugging kernel is as simple as executing:

# cd /usr/src
# make buildkernel KERNCONF=DEBUG INSTKERNNAME=DEBUG
# make installkernel KERCONF=DEBUG INSTKERNNAME=DEBUG

Executing/Using The Kernel

From the loader menu

At the loader menu, press 6 to escape to the loader(8) prompt and type:

boot DEBUG

From the running system

The DEBUG kernel can be loaded by default by adding/modifying the kernel directive in loader.conf as follows. Each subsequent boot will load the DEBUG kernel.

kernel=DEBUG

Dropping into the debugger

During a kernel panic, it will simply reboot unless debug.debugger_on_panic is enabled. To enable this execute:

sysctl debug.debugger_on_panic=1

To induce a panic:

sysctl debug.kdb.panic=1