Re: [PATCH -tip v8 05/13] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe_trampoline code

Josh Poimboeuf
2021-07-10T19:01:43Z

On Sat, Jul 10, 2021 at 10:41:04AM +0900, Masami Hiramatsu wrote:
> Hi Ingo and Josh,
>
> On Sat, 10 Jul 2021 00:31:40 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > > > +STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
> > > > +#undef UNWIND_HINT_FUNC
> > > > +#define UNWIND_HINT_FUNC
> > > > +#endif
> > > > /*
> > > > * When a retprobed function returns, this code saves registers and
> > > > * calls trampoline_handler() runs, which calls the kretprobe's handler.
> > > > @@ -1031,6 +1044,7 @@ asm(
> > > > /* We don't bother saving the ss register */
> > > > #ifdef CONFIG_X86_64
> > > > " pushq %rsp\n"
> > > > + UNWIND_HINT_FUNC
> > > > " pushfq\n"
> > > > SAVE_REGS_STRING
> > > > " movq %rsp, %rdi\n"
> > > > @@ -1041,6 +1055,7 @@ asm(
> > > > " popfq\n"
> > > > #else
> > > > " pushl %esp\n"
> > > > + UNWIND_HINT_FUNC
> > > > " pushfl\n"
> > > > SAVE_REGS_STRING
> > > > " movl %esp, %eax\n"
> > >
> > > Why not provide an appropriate annotation method in <asm/unwind_hints.h>,
> > > so that other future code can use it too instead of reinventing the wheel?
>
> I think I got what you meant. Let me summarize the issue.
>
> In case of CONFIG_FRAME_POINTER=n, it is OK just adding UNWIND_HINT_FUNC.
>
> In case of CONFIG_FRAME_POINTER=y, without STACK_FRAME_NON_STANDARD(),
> the objtool complains that a CALL instruction without the frame pointer.
> ---
> arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x25: call without frame pointer save/setup
> ---
>
> If we just add STACK_FRAME_NON_STANDARD() with UNWIND_HINT_FUNC macro,
> the objtool complains that non-standard function has unwind hint.
> ---
> arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x1: BUG: why am I validating an ignored function?

I'm thinking this latter warning indicates an objtool bug (as the BUG
warning claims). If a function is ignored with
STACK_FRAME_NON_STANDARD() then objtool should probably also ignore its
hints. Then we should be able to get rid of the #undef/#ifdef
UNWIND_HINT_FUNC silliness.

The other warning is correct and STACK_FRAME_NON_STANDARD() still needs
to be behind '#ifdef CONFIG_FRAME_POINTER' since the function is missing
a frame pointer. So maybe we can make a STACK_FRAME_NON_STANDARD_FP()
or similar.

I'll post a few patches.

--
Josh