This feature has been supported since FreeBSD 2.2, and it is actually a very neat
one.
GDB has already supported remote
debugging for a long time. This is done using a very simple protocol along a
serial line. Unlike the other methods described above, you will need two machines for
doing this. One is the host providing the debugging environment, including all the
sources, and a copy of the kernel binary with all the symbols in it, and the other one is
the target machine that simply runs a similar copy of the very same kernel (but stripped
of the debugging information).
You should configure the kernel in question with config -g,
include DDB into the configuration, and compile it as usual.
This gives a large binary, due to the debugging information. Copy this kernel to the
target machine, strip the debugging symbols off with strip -x,
and boot it using the -d boot option. Connect the serial line
of the target machine that has "flags 080" set on its sio device to any serial line of
the debugging host. Now, on the debugging machine, go to the compile directory of the
target kernel, and start gdb:
% gdb -k kernel
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-freebsd),
Copyright 1996 Free Software Foundation, Inc...
(kgdb)
Initialize the remote debugging session (assuming the first serial port is being used)
by:
(kgdb) target remote /dev/cuaa0
Now, on the target host (the one that entered DDB right before even starting the
device probe), type:
Debugger("Boot flags requested debugger")
Stopped at Debugger+0x35: movb $0, edata+0x51bc
db> gdb
DDB will respond with:
Next trap will enter GDB remote protocol mode
Every time you type gdb, the mode will be toggled between
remote GDB and local DDB. In order to force a next trap immediately, simply type s (step). Your hosting GDB will now gain control over the target
kernel:
Remote debugging using /dev/cuaa0
Debugger (msg=0xf01b0383 "Boot flags requested debugger")
at ../../i386/i386/db_interface.c:257
(kgdb)
You can use this session almost as any other GDB session, including full access to the
source, running it in gud-mode inside an Emacs window (which gives you an automatic
source code display in another Emacs window), etc.
