FreeBSD kernel Debugging tools for Virtual Memory Module

[Bakul Shah]

>    ddb and kgdb are two useful and often indispensable tools for kernel
> debugging on FBSD. ddb won't allow you source level debugging, kgdb will,
> but you'll need an extra machine. 

If the code you are debugging doesn't depend on specific
hardware, one option is to run FreeBSD (with the kernel being
debugged) under qemu and run kgdb on the host FreeBSD.
Something like

In Window1
$ qemu -s freebsd-disk-img ...

In Window2
$ cd <where the kernel under test was built>
$ kgdb kernel.debug
(gdb) target remote localhost:1234
	<do your debugging>
(gdb) detach
Ending remote debugging.
(gdb) q
$

Note: I have not tried this recently but it should work.

> AFAIK, if you are modifying the kernel source directly  there is no option
> but to recompile all the changed and dependent files.

Well... there used to be a debugger called ups with a builtin
C interpreter. It allowed you to add code at run time.  This
was quite handy when you wanted to temporarily patch things
up and continue debugging or set conditional breakpoints or
insert assertion verification code on the fly.  The C
interpreter is worth adding to gdb but I am not sure if any
of ups code can be reused.  See http://ups.sourceforge.net/