kgdbWhen 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, 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.
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.
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.
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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