Re: [RFC PATCH -tip 0/5] kprobes: Abolish jprobe APIs

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 5 Oct 2017 16:35:22 -0700
Kees Cook <keescook@chromium.org> wrote:

> > As far as I can see, tcp probe, dccp probe, sctp probe and lkdtm
> > are using jprobe to probe function. Please consider to migrate.  
> 
> I'm happy to do so, but I'm quite unfamiliar with how to do this (I
> didn't write lkdtm's jprobe code originally). lkdtm just wants to hook
> function entry and call it's own function before.

That can be done with ftrace. That's how live kernel patching works. It
registers a callback via register_ftrace_function(), and with fentry
(gcc 4.6 and later on x86), you can "hijack" the function. If you don't
modify the regs->ip, then the function you hooked to will be called.

> 
> It uses struct jprobe like this:
> 
>                 .jprobe = {                                     \
>                         .kp.symbol_name = _symbol,              \
>                         .entry = (kprobe_opcode_t *)_entry,     \
>                 },                                              \
> 
> and defines a bunch of handlers like this for the _symbol and _entry pairs:
> 
>                    "do_IRQ",                    jp_do_irq),
> ...
>                    "tasklet_action",            jp_tasklet_action),
> 
> where all the handlers look exactly the same (and don't care about arguments):

Hell, this is really easy then!

> 
> static unsigned int jp_do_irq(unsigned int irq)
> {
>         lkdtm_handler();
>         jprobe_return();
>         return 0;
> }
> 
> What's the right way to migrate away from jprobe for lkdtm?

Perhaps something like:

#include <linux/ftrace.h>

static void lkdtm_callback(unsigned long ip, unsigned long parent_ip,
			struct ftrace_ops *ops, struct pt_regs *regs)
{
	lkdt_handler();
}


static struct ftrace_ops ops = {
	.func		= lkdtm_callback,
};

[..]
	ftrace_set_filter(&ops, "do_IRQ", strlen("do_IRQ"), 0);
	ftrace_set_filter(&ops, "tasklet_action", strlen("tasklet_action"), 0);
	[..]

	/* to add the hook */

	register_ftrace_function(&ops);

Now all functions you set the filter for will be traced.

Oh you may want to check the return status of ftrace_set_filter()
otherwise, if they all fail, you will be tracing all functions.

-- Steve