Date: Fri, 14 Mar 1997 15:53:12 -0500 From: Bakul Shah <bakul@torrentnet.com> To: hackers@freebsd.org Subject: kernel remote debugging using gdb Message-ID: <199703142053.PAA00554@chai.plexuscom.com>
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This seems to work mostly but there are a few problems.
What I did:
- recompiled the boot program to always use COM1 as console and
installed it on the primary disk on the test machine (called TEST).
- used the following options in the TEST config file:
options COMCONSOLE
options DDB
options BREAK_TO_DEBUGGER
- config -g TEST and compiled the test kernel on the development
machine (called DEVEL), and copied it to TEST.
- connected TEST's COM1 to DEVEL's COM2 with a `null modem' cable.
- started kermit on DEVEL and connected to TEST @ 9600 baud
- rebooted TEST. When the boot prompt came up in the kermit window,
booted the kernel single-user.
- once the kernel came up I hit ^\b in kermit to send a break to TEST.
When DDB gained control I typed gdb so that on the next trap it
will start the gdb remote protocol. I continued the kernel and
then I sent another break and the TEST kernel did spew out out gdb
remote stuff (which has a distinctive pattern).
- Next I stopped kermit on DEVEL and chdir'ed to $SRC/sys/compile/TEST
and started gdb.
- in gdb I typed
file kernel
target remote /dev/cuaa1
At this point gdb starts controlling the remote kernel. Now one can
add breakpoints, examine variables and so on.
There are two problems:
- Sendind a break in multiuser mode (before or after attaching gdb)
does not seem to drop the kernel in the debugger. This seems to
work occasionally but I haven't as yet figured out under what
conditions.
- In single user mode when gdb is attached, you can not type anything
on the console nor can you see any printf() output[1].
These problems make debugging the kernel with gdb somewhat
challenging. Am I doing something wrong or missing a crucial step?
If these problems can be solved may be the above description can be
added to the handbook or the FAQ.
I am debugging a 2.2-961006-SNAP, if it matters. Thanks for any
help! Any other hints/ideas on making this process less tedious are
also more than welcome.
-- bakul
[1] Note that passing console device IO `through gdb' would be
extremely useful as it will feel as if gdb is built in. Not
sure if this works at all for the standard gdb remote protocol.
At any rate adding it would not be hard.
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