How to debug a double fault? (Re: Could MSGBUF_SIZE be made a loader tunable?)

[perryh at pluto.rain.com]

Andriy Gapon <avg at freebsd.org> wrote:
> on 15/12/2010 12:37 perryh at pluto.rain.com said the following:
> > Fatal double fault:
> > eip = 0xc07feb98
> > esp = 0xc101e000
> > ebp = 0xc101e004
> > cpuid = 0; apic id = 00
> > panic: double fault
> > cpuid = 0
> > 
> > How do I go about tracking this down?
>
> Do you have the standard debugging options in your kernel?

No, it is 8.1-RELEASE GENERIC with only the name changed and the
(first attempt) msgbufsize patches applied.  I was trying to
minimize changes to GENERIC, so as to minimize the opportunity
to screw something up, and I had this silly idea that something
this simple might "just work."

It does occur to me to wonder whether any debugger would be
functional this early, before even the first line of the signon
message has been displayed.  Is it possible, given the loader
messages, to come up with a base address which could be used to
compare the eip value with the kernel symbol table?  Granted this
won't provide a traceback, but even knowing in which function it
crashed would be a start.

> BTW, are you sure that you correctly placed initialization of
> msgbufsize ?

I am not at all sure of that, and am not sufficiently familiar with
the sequence of events early in intiialization to know how to find
out -- although I suppose the observed crash might not be altogether
surprising if the kernel message buffer got allocated with a zero
size :(

Apart from the name, msgbufsize is set up in exactly the same
way and place -- in init_param1() -- as maxswzone and maxbcache.
Perhaps that is not early enough; any idea what would be a better
example?