FreeBSD Developers' Handbook
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# Chapter 10 Kernel Debugging

Contributed by Paul Richards, Jörg Wunsch, and Robert Watson.

# 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.

> Note: 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, which hold the complete contents of physical memory, minidumps, which hold only memory pages in use by the kernel (FreeBSD 6.2 and higher), and textdumps, which 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. `AUTO` is the default as of FreeBSD 6.0.

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

> Important: 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`). While debugging, it is highly likely that you will want to use the highest version `vmcore` in `/var/crash` when searching for the right `vmcore`.

> Tip: 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`!

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