Re: [PATCH -tip 0/5] kprobes: Fix stacktrace in kretprobes

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Mar 11, 2021 at 09:20:18AM +0900, Masami Hiramatsu wrote:
> > >  bool unwind_next_frame(struct unwind_state *state)
> > >  {
> > >  	unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
> > > @@ -536,6 +561,18 @@ bool unwind_next_frame(struct unwind_state *state)
> > >  
> > >  		state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
> > >  						  state->ip, (void *)ip_p);
> > > +		/*
> > > +		 * There are special cases when the stack unwinder is called
> > > +		 * from the kretprobe handler or the interrupt handler which
> > > +		 * occurs in the kretprobe trampoline code. In those cases,
> > > +		 * %sp is shown on the stack instead of the return address.
> > > +		 * Or, when the unwinder find the return address is replaced
> > > +		 * by kretprobe_trampoline.
> > > +		 * In those cases, correct address can be found in kretprobe.
> > > +		 */
> > > +		if (state->ip == sp ||
> > 
> > Why is the 'state->ip == sp' needed?
> 
> As I commented above, until kretprobe_trampoline writes back the real
> address to the stack, sp value is there (which has been pushed by the
> 'pushq %rsp' at the entry of kretprobe_trampoline.)
> 
>         ".type kretprobe_trampoline, @function\n"
>         "kretprobe_trampoline:\n"
>         /* We don't bother saving the ss register */
>         "       pushq %rsp\n"				// THIS
>         "       pushfq\n"
> 
> Thus, from inside the kretprobe handler, like ftrace, you'll see
> the sp value instead of the real return address.

I see.  If you change is_kretprobe_trampoline_address() to include the
entire function, like:

static bool is_kretprobe_trampoline_address(unsigned long ip)
{
	return (void *)ip >= kretprobe_trampoline &&
	       (void *)ip < kretprobe_trampoline_end;
}

then the unwinder won't ever read the bogus %rsp value into state->ip,
and the 'state->ip == sp' check can be removed.

> > And it would make the unwinder just work automatically when unwinding
> > from the handler using the regs.
> > 
> > It would also work when unwinding from the handler's stack, if we put an
> > UNWIND_HINT_REGS after saving the regs.
> 
> At that moment, the real return address is not identified. So we can not
> put it.

True, at the time the regs are originally saved, the real return address
isn't available.  But by the time the user handler is called, the return
address *is* available.  So if the real return address were placed in
regs->ip before calling the handler, the unwinder could find it there,
when called from the handler.

Then we wouldn't need the call to orc_kretprobe_correct_ip() in
__unwind_start().

But maybe it's not possible due to the regs->ip expectations of legacy
handlers?

-- 
Josh