Calling kernel functions from within online kernel GDB

[Tom Alsberg]

Hi there.

I'm fooling around a bit with kernel hacking recently, and got a bit
to work with online kernel debugging with GDB (over a serial port).

I built the kernel (FreeBSD-4.9) with DDB and -g compile option, and
set the flag on sio0, as described in section 10.6 of the FreeBSD
Developer's Handbook at:

http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/kerneldebug-online-gdb.html

Using gdb -k I can open the kernel image, and doing 'target remote
/dev/cuaa0' I can attach to the debugged host (having switched to gdb
mode from DDB on it, and entered 's' to generate a trap).

Trying to figure out some stuff I can do with it, I noticed that for
some reason I cannot call any kernel functions from kgdb...  When
entering, e.g. the command:

(kgdb) call print_uptime()

gdb points it caught of a SIGSEGV, which actually is a panic on the
host - I see a fatal trap 12 (page fault while in kernel mode), with
fault code 'supervisor read, page not present'...

Am I in some special context when inside the kernel debugger, from
which other kernel functions cannot be called? (no access to their
page?)  What needs to be done to setup the paging and memory
protection to be able to do so?

The even more weird thing yet is that from the information in the
panic, during the panic the instruction pointer, frame pointer (but
not the stack pointer), and panic virtual address are zero:

fault virtual address	= 0x0
instruction pointer	= 0x8:0x0
frame pointer		= 0x10:0x0

I thought maybe there is some zero at the time of return from the
function when ret is executed (so the CPU tries to jump to address 0),
but having put some breakpoints inside the function, I see that the
panic is even before the first instruction of it executes...

I found very little documentation on kernel debugging, except on how
to get into DDB and use it a bit, how to debug offline with gdb with
crash dumps, and how to start a remote online GDB session through the
serial port on a running kernel...  In the Developer's Handbook there
are examples on usage of DDB online, and GDB offline on a crash dump,
but no examples of online remote GDB usage.

I am not sure how exactly the contexts of the kernel should affect
debugging which (as far as I understand) is done completely within the
kernel, and how the kernel debugger/GDB interferes...  I'll try to
learn more and perhaps understand it...  But can someone provide some
ideas as to what is the cause of the problem, what is exactly
happening and why, and how to overcome it?

  Thanks, any help appreciated,
  -- Tom

-- 
  Tom Alsberg - hacker (being the best description fitting this space)
  Web page:	http://www.cs.huji.ac.il/~alsbergt/
DISCLAIMER:  The above message does not even necessarily represent what
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