Hi -

I've been trying to track down a problem I've had with a USB CD-Burner
locking up.  In the course of my investigations I ported the i386 remote
gdb stuff to the Linux kernel, because I'm used to using gdb on the
kernel (it works on SPARC and PPC) instead of trying to read oopses.

For those not familiar with the remote debug feature, you use two
computers, connected together with a null modem serial line.  One
computer has a complete Linux kernel tree on it, compiled with debugging
information (-g); the other computer is the one running the kernel under
test.  You can breakpoint and halt the kernel, which puts it in a tight
little loop reading packets (gdb, not IP) from the serial port and
responding to the debugger.  You get almost all the features you're used
to with gdb - stack backtraces, single stepping, source-based variable
names, intelligent structure decodes, etc.

Anyway, I'm attaching the patch (against 2.4.6).  After installing, a
menu option appears under "Kernel hacking" for remote debugging. 
Recompile the whole kernel (make clean) so that it compiles with
debugging info.  Then supply the "kgdb" switch to the kernel command
line, make sure the debugging computer is attached on COM1 (or whatever
you want to call it), and run "target remote /dev/whatever" on the
debugging computer.  See arch/i386/kernel/stub-i386.c for more info.

Known problems:

- only runs on COM1.  Shouldn't be hard to fix this

- doesn't switch stacks, so you can't use gdb's "call" feature, which
scribbles on the stack.  Other than that, no problem.

- doesn't support SMP, since I don't have an Intel SMP box.  I'd guess
what you'd want it to do is an smp_call_function that would halt all the
processors and put them into some tight little loop while gdb fiddles
things.  ideas?

- doesn't support any concept of multiple tasks/threads, though GDB can
do this with it's remote protocol, and I've discovered that it'd be
really nice to switch to another task within the kernel.  Lacking this,
you have to do stack backtraces by hand for other tasks.

- we have to compile Linux with -O2 to get inline functions, and this
can confuse GDB sometimes.  When in doubt, study the assembly.

And it still sometimes does some strange things, so might need some
tweaks here and there, but works 95% of the time.  Please try it out and
let me know what you think.


-- 
                                        -bwb

                                        Brent Baccala
                                        baccala@freesoft.org

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