Re: [PATCH -tip v8 11/13] x86/unwind: Recover kretprobe trampoline entry

[Masami Hiramatsu]

On Mon, 5 Jul 2021 13:36:14 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, Jun 18, 2021 at 04:07:06PM +0900, Masami Hiramatsu wrote:
> > @@ -549,7 +548,15 @@ bool unwind_next_frame(struct unwind_state *state)
> >  					 (void *)orig_ip);
> >  			goto err;
> >  		}
> > -
> > +		/*
> > +		 * There is a small chance to interrupt at the entry of
> > +		 * kretprobe_trampoline where the ORC info doesn't exist.
> > +		 * That point is right after the RET to kretprobe_trampoline
> > +		 * which was modified return address. So the @addr_p must
> > +		 * be right before the regs->sp.
> > +		 */
> > +		state->ip = unwind_recover_kretprobe(state, state->ip,
> > +				(unsigned long *)(state->sp - sizeof(long)));
> >  		state->regs = (struct pt_regs *)sp;
> >  		state->prev_regs = NULL;
> >  		state->full_regs = true;
> > @@ -562,6 +569,9 @@ bool unwind_next_frame(struct unwind_state *state)
> >  					 (void *)orig_ip);
> >  			goto err;
> >  		}
> > +		/* See UNWIND_HINT_TYPE_REGS case comment. */
> > +		state->ip = unwind_recover_kretprobe(state, state->ip,
> > +				(unsigned long *)(state->sp - sizeof(long)));
> >  
> >  		if (state->full_regs)
> >  			state->prev_regs = state->regs;
> 
> Why doesn't the ftrace case have this? That is, why aren't both return
> trampolines having the same general shape?

Ah, this strongly depends what the trampoline code does.
For the kretprobe case, the PUSHQ at the entry of the kretprobe_trampoline()
does not covered by UNWIND_HINT_FUNC. Thus it needs to find 'correct_ret_addr'
by the frame pointer (which is next to the sp).

        "kretprobe_trampoline:\n"
#ifdef CONFIG_X86_64
        /* Push fake return address to tell the unwinder it's a kretprobe */
        "       pushq $kretprobe_trampoline\n"
        UNWIND_HINT_FUNC

But I'm not so sure how ftrace treat it. It seems that the return_to_handler()
doesn't care such case. (anyway, return_to_handler() does not return but jump
to the original call-site, in that case, the information will be lost.)

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>