How can the ftrace support hook function

Question

Is it possible to hook/callback kernel function to my kernel module by ftrace or anyway.

for example: I want hook each time the function net_tx_action() called to my module, included arguments.

regards peter

Answer 1

You can set the IP register to the desired hook function in your ftrace callback:

static void notrace ftrace_callback(unsigned long ip, unsigned long parent_ip,
                struct ftrace_ops *ops, struct pt_regs *regs)
{
        regs->ip = (unsigned long) hook_function;
}

This will make the hook function to execute instead of the hooked function. The hook function should have the same argument types and return value as the original one.

You will also need to save the address of the original hooked function somewhere so that you can call it from your hook function:

static void (*original_function)(void);

static void hook_function(void)
{
        /* some code before */
        original_function();
        /* some code after */
}

However, calling the original function has a caveat: it will recursively call the ftrace callback again, so you need to somehow avoid resetting IP register in this case. One option is to track the recursion manually (count the function activations, set IP only on odd activations).

Here's a working example with the code. It should work for non-IRQ functions. Functions which can be called from interrupt context are trickier because there is no particular task to associate function activations with. I do not have a robust solution for that case yet. Maybe per-CPU variables could help if interrupt handlers cannot be rescheduled to another CPU in mid-execution.