On Thu, Mar 11, 2021 at 10:54:38AM +0900, Masami Hiramatsu wrote:
> On Wed, 10 Mar 2021 19:06:15 -0600
> Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.
When unwinding from trampoline_handler(), state->ip will point to the
instruction after the call:
call trampoline_handler
movq %rax, 19*8(%rsp) <-- state->ip points to this insn
But then, the above version of is_kretprobe_trampoline_address() is
true, so state->ip gets immediately replaced with the real return
address:
if (is_kretprobe_trampoline_address(state->ip))
state->ip = orc_kretprobe_correct_ip(state);
so the unwinder skips over the kretprobe_trampoline() frame and goes
straight to the frame of the real return address. Thus it never reads
this bogus return value into state->ip:
pushq %rsp
which is why the weird 'state->ip == sp' check is no longer needed.
The only "downside" is that the unwinder skips the
kretprobe_trampoline() frame. (note that downside wouldn't exist in
the case of UNWIND_HINT_REGS + valid regs->ip).
> > > > 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?
ORC is 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)
That sounds better to me, though I don't have an understanding of what
it would break.
--
Josh
