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

[Masami Hiramatsu <mhiramat@kernel.org>]

On Wed, 10 Mar 2021 19:06:15 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> 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.

Hmm, I couldn't get your point. Since sp is the address of stack,
it always out of text address.

> 
> > > 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.

OK, but this is not arch independent specification. I can make a hack
only for x86, but that is not clean implementation, hmm.

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

What about the ORC implementation in other architecture? Is that for x86 only?

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

Usually, the legacy handlers will ignore it, the official way to access
the correct return address is kretprobe_instance.ret_addr. Because it is
arch independent.

Nowadays there are instruction_pointer() and instruction_pointer_set() APIs
in many (not all) architecutre, so I can try to replace to use it instead
of the kretprobe_instance.ret_addr.
(and it will break the out-of-tree codes)


Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>