There are four categories of trouble that can occur when building a custom kernel:
config
failsIf config
fails, it will print the
line number that is incorrect. As an example, for the
following message, make sure that line 17 is typed
correctly by comparing it to GENERIC
or NOTES
:
config: line 17: syntax error
make
failsIf make
fails, it is usually due to
an error in the kernel configuration file which is not
severe enough for config
to catch.
Review the configuration, and if the problem is not
apparent, send an email to the FreeBSD general questions mailing list which
contains the kernel configuration file.
If the new kernel does not boot or fails to recognize
devices, do not panic! Fortunately, FreeBSD has an excellent
mechanism for recovering from incompatible kernels.
Simply choose the kernel to boot from at the FreeBSD boot
loader. This can be accessed when the system boot menu
appears by selecting the “Escape to a loader
prompt” option. At the prompt, type
boot
, or the
name of any other kernel that is known to boot
properly.kernel.old
After booting with a good kernel, check over the
configuration file and try to build it again. One helpful
resource is /var/log/messages
which
records the kernel messages from every successful boot.
Also, dmesg(8) will print the kernel messages from
the current boot.
When troubleshooting a kernel, make sure to keep
a copy of GENERIC
, or some other
kernel that is known to work, as a different name that
will not get erased on the next build. This is
important because every time a new kernel is installed,
kernel.old
is overwritten with the
last installed kernel, which may or may not be bootable.
As soon as possible, move the working kernel by renaming
the directory containing the good kernel:
#
mv /boot/kernel
/boot/kernel.bad
#
mv /boot/
kernel.good
/boot/kernel
If the kernel version differs from the one that the system utilities have been built with, for example, a kernel built from -CURRENT sources is installed on a -RELEASE system, many system status commands like ps(1) and vmstat(8) will not work. To fix this, recompile and install a world built with the same version of the source tree as the kernel. It is never a good idea to use a different version of the kernel than the rest of the operating system.
All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.