Kernel debugging with Qemu and gdb
 

Hi,

I am trying to debug a panic problem observed in linux 3.x kernels.
I have seen it with 3.4.48,3.8.8 and 3.9.4.

I have trouble debugging the kernel with gdb.

STEPS TRIED
--------------------------

These are my command-line :


qemu-system-x86_64 -m 256
-kernel vmlinuz-3.x
-initrd initramfs-3.x
-s
-S
-hda rootfs.img
-append "root=/dev/sda1"




To debug :
I do
gdb vmlinux-3.x
(gdb) target remote:1234
(gdb) c

I run the qemu and the gdb on the same machine. I am root and have
set iptables -F. I have also set CONFIG_DEBGUG_INFO.

OBSERVATION
----------------------------

My problem is that gdb hangs after 'c' . It does not proceed.
When I do Ctrl-C, I observe at the prompt "interrupted while
waiting for the program". I feel I m missing something.

The qemu output is relayed through vnc. But after 'c' in gdb.
I do not see anything in the vncviewer console. I assume that the
kernel is not being executed.

SUMMARY
--------------------

In short, my main problem is debugging with gdb. I am not able to run the kernel using gdb so that I can do a trace analysis at the required breakpoint. I have seen in gdb-tutorials for kernel where they proceed with kernel execution with cont. But that too do not work in my case.

Help is appreciated.

Let's first address "I assume that the kernel is not being executed."

After 'c' what do you get from doing the qemu command 'info cpus' a few times? Are the cpus halted? Is the instruction pointer changing?

The prompt does not come after pressing 'c'. gdb terminal hangs. Have to Ctrl-D to terminate gdb

After a 'c' nothing should be happening on the gdb terminal because the system is running. You can only type gdb commands when you are in a breakpoint or Ctl-C.

The qemu prompt is what you get in the term where you issued the qemu-system-x86_64 command.