Remote Debugging Using gdbserver

There are many articles on Remote debugging using gdb-gdbserver setup. Sadly many of them are not comprehensive and some of them are even outright wrong.

I will try and explain the setup here :-

1. You need a gdbserver running on the target. This is target specific. E.g if you are compiling for a ARM target , make sure you have a gdbserver compiled for ARM.

2. A gdb debugger running on the host. This is where many of the tutorials on the net are not clear. The gdb running on the host has to debug programs built for the target architecture but still run on the host machine ( Just like gcc cross compiler).

3. The program to be debugged has to be the same --both on the target and the host. However you can run a stripped version of the program on the host. Apart from this they should be exactly the same.

steps:-

Get the iptables mess out of the way

#iptables -P INPUT ACCEPT

#iptables -P OUTPUT ACCEPT

#iptables -P FORWARD ACCEPT

#iptables --flush

on the board start the gdbserver

#gdbserver :1234 debug-program

here it means thats gdbserver would listen for connection on port 1234. The program which is to be debugged is "debug-program". The above command is same as using :-

#gdbserver localhost:1234 debug-program 

or even

#gdbserver 127.0.0.1:1234 debug-program

on the host start the gdb

#gdb debug-program

At the gdb prompt type the following

(gdb) target remote 192.168.1.1:1234 

This instructs the gdb to connect to gdbserver on the machine with IP 192.168.1.1 on port 1234. 

On-Target source Level Debugging Using NFS

Many a times we would like to have to have a luxury of source level debugging but do not have fancy tools like a JTAG/ICE. But thanks to NFS, we do have a solution. Using NFS one can mount the entire source directory ( or any other directory for that matter) on to the target and run a debuggger on the target.

Besides aiding source level debugging NFS can help save crucial development time by circumventing the need to flash the board for every little change. If only a small portion of the code changes, selectively compile that particular program and replace the existing program on the board with the newly built program. Copy-And-Paste instead of Flash-And-Burn!!!

1. Build the Kernel Image for the target with NFS support.

2. Install NFS server package on the host. On debian machines it is as simple as

sudo apt-get install nfs-kernel-server

3. A file called /etc/exports is created. Open this file for editing.

add the following line :-

/home/kernelexploit 192.168.1.1(rw,all_squash,anonuid=kernelexploit,anongid=1000,insecure,no_subtree_check)

where /home/kernelexploit is the path on the host. This will be mounted on the target. [192.168.1.1] is the IP address of the target in my system. Replace it with your board specific parameters.

4. On the target type the following command

mkdir -p /home/kernelexploit-target [ This is where the remote directory would be mapped]

mount -t nfs -o async,noatime,exec,rw,nfsvers=2,nolock 192.168.1.11:/home/kernelexploit /home/kernelexploit-target

Where, 192.168.1.11 is the IP address of the host machine and /home/kernelexploit is the path on the host. The trailing /home/kernelexploit-target is a directory we created on the target where the remote directory is mounted.