Chapter 10. Kernel Debugging

Contributed by Paul Richards, Jörg Wunsch and Robert Watson.
Table of Contents
10.1. Obtaining a Kernel Crash Dump
10.2. Debugging a Kernel Crash Dump with kgdb
10.3. On-Line Kernel Debugging Using DDB
10.4. On-Line Kernel Debugging Using Remote GDB
10.5. Debugging a Console Driver
10.6. Debugging Deadlocks
10.7. Kernel debugging with Dcons
10.8. Glossary of Kernel Options for Debugging

10.1. Obtaining a Kernel Crash Dump

When running a development kernel (e.g., FreeBSD-CURRENT), such as a kernel under extreme conditions (e.g., very high load averages, tens of thousands of connections, exceedingly high number of concurrent users, hundreds of jail(8)s, etc.), or using a new feature or device driver on FreeBSD-STABLE (e.g., PAE), sometimes a kernel will panic. In the event that it does, this chapter will demonstrate how to extract useful information out of a crash.

A system reboot is inevitable once a kernel panics. Once a system is rebooted, the contents of a system's physical memory (RAM) is lost, as well as any bits that are on the swap device before the panic. To preserve the bits in physical memory, the kernel makes use of the swap device as a temporary place to store the bits that are in RAM across a reboot after a crash. In doing this, when FreeBSD boots after a crash, a kernel image can now be extracted and debugging can take place.


A swap device that has been configured as a dump device still acts as a swap device. Dumps to non-swap devices (such as tapes or CDRWs, for example) are not supported at this time. A swap device is synonymous with a swap partition.

Several types of kernel crash dumps are available:

Full memory dumps

Hold the complete contents of physical memory.


Hold only memory pages in use by the kernel (FreeBSD 6.2 and higher).


Hold captured, scripted, or interactive debugger output (FreeBSD 7.1 and higher).

Minidumps are the default dump type as of FreeBSD 7.0, and in most cases will capture all necessary information present in a full memory dump, as most problems can be isolated only using kernel state.

10.1.1. Configuring the Dump Device

Before the kernel will dump the contents of its physical memory to a dump device, a dump device must be configured. A dump device is specified by using the dumpon(8) command to tell the kernel where to save kernel crash dumps. The dumpon(8) program must be called after the swap partition has been configured with swapon(8). This is normally handled by setting the dumpdev variable in rc.conf(5) to the path of the swap device (the recommended way to extract a kernel dump) or AUTO to use the first configured swap device. The default for dumpdev is AUTO in HEAD, and changed to NO on RELENG_* branches (except for RELENG_7, which was left set to AUTO). On FreeBSD 9.0-RELEASE and later versions, bsdinstall will ask whether crash dumps should be enabled on the target system during the install process.


Check /etc/fstab or swapinfo(8) for a list of swap devices.


Make sure the dumpdir specified in rc.conf(5) exists before a kernel crash!

# mkdir /var/crash
# chmod 700 /var/crash

Also, remember that the contents of /var/crash is sensitive and very likely contains confidential information such as passwords.

10.1.2. Extracting a Kernel Dump

Once a dump has been written to a dump device, the dump must be extracted before the swap device is mounted. To extract a dump from a dump device, use the savecore(8) program. If dumpdev has been set in rc.conf(5), savecore(8) will be called automatically on the first multi-user boot after the crash and before the swap device is mounted. The location of the extracted core is placed in the rc.conf(5) value dumpdir, by default /var/crash and will be named vmcore.0.

In the event that there is already a file called vmcore.0 in /var/crash (or whatever dumpdir is set to), the kernel will increment the trailing number for every crash to avoid overwriting an existing vmcore (e.g., vmcore.1). savecore(8) will always create a symbolic link to named vmcore.last in /var/crash after a dump is saved. This symbolic link can be used to locate the name of the most recent dump.

The crashinfo(8) utility generates a text file containing a summary of information from a full memory dump or minidump. If dumpdev has been set in rc.conf(5), crashinfo(8) will be invoked automatically after savecore(8). The output is saved to a file in dumpdir named core.txt.N.


If you are testing a new kernel but need to boot a different one in order to get your system up and running again, boot it only into single user mode using the -s flag at the boot prompt, and then perform the following steps:

# fsck -p
# mount -a -t ufs       # make sure /var/crash is writable
# savecore /var/crash /dev/ad0s1b
# exit                  # exit to multi-user

This instructs savecore(8) to extract a kernel dump from /dev/ad0s1b and place the contents in /var/crash. Do not forget to make sure the destination directory /var/crash has enough space for the dump. Also, do not forget to specify the correct path to your swap device as it is likely different than /dev/ad0s1b!

10.1.3. Testing Kernel Dump Configuration

The kernel includes a sysctl(8) node that requests a kernel panic. This can be used to verify that your system is properly configured to save kernel crash dumps. You may wish to remount existing file systems as read-only in single user mode before triggering the crash to avoid data loss.

# shutdown now
Enter full pathname of shell or RETURN for /bin/sh:
# mount -a -u -r
# sysctl debug.kdb.panic=1
debug.kdb.panic:panic: kdb_sysctl_panic

After rebooting, your system should save a dump in /var/crash along with a matching summary from crashinfo(8).

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