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Loading symbols when debugging the kernel and kernel modules

Recently I received some comments from a friend about a previous article on linux kernel debugging using kgdb. What he asked me was how could he load symbols from a kernel or a kernel module. So I wrote a quick guide to help you start with kernel debugging. After each step I will show you the gdb output.

First of all you should start gdb!

Then you should load all kernel symbols from the vmlinux file. This can be found at the directory where you compiled the kernel, most probably /usr/src/linux. Remember to compile the kernel using debug information by setting the appropriate option, it will help you a lot!

You’re ready to start debugging! Set the target and use the Alt-SysRq-G sequence as it was described at the previous post. You can now set breakpoints, watch anything you want in memory, step or continue running the kernel!

Now let’s see how we can debug kernel modules. I will test the l2cap bluetooth kernel module.

You first need to find the object file which contains the module. For l2cap this is net/bluetooth/l2cap.o in the kernel source tree. Transfer this to the host (or the machine running gdb if you’re not using a virtual machine). Then load the module in the virtual machine. This creates a new directory in /sys/module named after the module name, i.e. l2cap. Inside this directory, there is another one named sections which contains the addresses where all sections are loaded. We are interested in the .text section so we read the file /sys/module/l2cap/sections/.text.

We know where the .text section is loaded so we can now load the symbols from l2cap.o using the add-symbol-file gdb command.

If you need to load other sections too, in case they are not contiguous with the text in memory, you need to read their addresses. For example we’ll load both the .text and the .data sections (you should do .bss too but it’s omitted since I wanted to write a quick and dirty guide and it’s already very big!)

Find where both .text and .data are loaded.

Then you load apart from the .text section the .data too.

You’re now ready to start debugging your kernel module!