How to debug problems in Linux kernel module `init()`?

I am using remote (k)gdb to debug a problem in a module that causes a panic when loaded e.g. when init() is called.

The stack trace just shows that do_one_initcall(mod->init) causes the crash. In order to get the symbol file loaded in the gdb, I need to get the address of the module text section, and to get that I need to get the module loaded.

Because the insmod in busybox (1.16.1) doesn’t support -m so I’m stuck to grep modulename /proc/modules + adding the offset from nm to figure out the address.

So I’m facing a sort a of a chicken and an egg problem here – to be able to debug the module loading, I need to get the module loaded – but in order to get the module loaded, I need to debug the problem…

So I am currently thinking about two options – is there a way to get the address information either:

1. by printk() in the module init code
2. by printk() somewhere in the kernel code

all this prior to calling the mod->init() – so I could place a breakpoint there, load the symbol file, hit c and see it crash and burn…

Answer

Can you build your code into the kernel rather than as a module? That might simplify debugging the init() call.

You could also set a breakpoint at do_one_initcall() and look at the address of mod->init to get the load address.