There are four categories of trouble that can occur when building a custom kernel. They are:
If the config(8) command fails when you give it your kernel description, you have probably made a simple error somewhere. Fortunately, config(8) will print the line number that it had trouble with, so that you can quickly locate the line containing the error. For example, if you see:
config: line 17: syntax error
Make sure the keyword is typed correctly by comparing it to the GENERIC kernel or another reference.
If the make command fails, it usually signals an error in your kernel description which is not severe enough for config(8) to catch. Again, look over your configuration, and if you still cannot resolve the problem, send mail to the FreeBSD general questions mailing list with your kernel configuration, and it should be diagnosed quickly.
If your new kernel does not boot, or fails to recognize your devices, do not panic! Fortunately, FreeBSD has an excellent mechanism for recovering from incompatible kernels. Simply choose the kernel you want to boot from at the FreeBSD boot loader. You can access this when the system boot menu appears. Select the “Escape to a loader prompt” option, number six. At the prompt, type unload kernel and then type boot /boot/kernel.old/kernel, or the filename of any other kernel that will boot properly. When reconfiguring a kernel, it is always a good idea to keep a kernel that is known to work on hand.
After booting with a good kernel you can check over your configuration file and try to build it again. One helpful resource is the /var/log/messages file which records, among other things, all of the kernel messages from every successful boot. Also, the dmesg(8) command will print the kernel messages from the current boot.
Note: If you are having trouble building a kernel, make sure to keep a GENERIC, or some other kernel that is known to work on hand as a different name that will not get erased on the next build. You cannot rely on kernel.old because when installing a new kernel, kernel.old is overwritten with the last installed kernel which may be non-functional. Also, as soon as possible, move the working kernel to the proper /boot/kernel location or commands such as ps(1) may not work properly. To do this, simply rename the directory containing the good kernel:
# mv /boot/kernel /boot/kernel.bad # mv /boot/kernel.good /boot/kernel
If you have installed a different version of the kernel from the one that the system utilities have been built with, for example, a -CURRENT kernel on a -RELEASE, many system-status commands like ps(1) and vmstat(8) will not work any more. You should recompile and install a world built with the same version of the source tree as your kernel. This is one reason it is not normally a good idea to use a different version of the kernel from the rest of the operating system.