How to get anything useful out of kgdb?

[Sean Bruno]

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On 05/15/15 08:50, John Baldwin wrote:
> On Saturday, May 09, 2015 10:20:46 AM Sean Bruno wrote:
>> tl;dr  What are the kernel config options to get good output of
>> kgdb?
>> 
>> 
>> I'm trying to get the ability to debug and display internal
>> variables, as one does, with kgdb.  I'm *must* be doing this
>> wrong as I cannot get any useful output from accessing variables
>> that were JUST accessed in order to invoke a panic() that I have
>> inserted.  This is a GENERIC kernel without INVARIANTS and
>> without WITNESS:
>> 
>> https://people.freebsd.org/~sbruno/wtf_kgdb.txt
>> 
>> I seem to have debug enabled and am able to browse source, but I 
>> obviously haven't compiled correctly as things are optimized
>> out.
> 
> 1) gdb7 does a better job.  I hope to get the kgdb patches into
> the port soon.  If you are feeling brave:
> 
> # add texinfo for HEAD % pkg install gmake bison
> 
> % git clone https://github.com/bsdjhb/gdb.git % cd gdb % git
> checkout freebsd-7.9.0-kgdb % fetch
> http://people.freebsd.org/~jhb/gdb/build % sh ./build % cd obj   #
> or obj.i386 for i386 % gmake
> 
> Then use /path/to/git/gdb/obj/gdb/kgdb
> 
> 2) Even with gdb7 it can't figure out variables that it should
> figure out sometimes.  Other options are either to find the
> variable in a higher frame (e.g. if it is something like 'td' or a
> driver softc) or to start poking around in the dissassembly to work
> out which register it is in (or which register points to a
> structure that contains it) and go from there.
> 

So one non-obvious thing that I'd like an explanation about:  If I
manually break to the debbugger and cause a dump (doadump), how do I
poke around in the crash dump later on to find a thread that I'm
looking for, e.g. I want to poke at various bits inside em(4).

sean
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