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SubjectRe: [PATCH -tip v3 05/11] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe_trampoline code
On Sat, Mar 20, 2021 at 10:05:43PM +0900, Masami Hiramatsu wrote:
> On Sat, 20 Mar 2021 21:16:16 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > On Fri, 19 Mar 2021 21:22:39 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > > From: Josh Poimboeuf <jpoimboe@redhat.com>
> > >
> > > Add UNWIND_HINT_FUNC on kretporbe_trampoline code so that ORC
> > > information is generated on the kretprobe_trampoline correctly.
> > >
> >
> > Test bot also found a new warning for this.
> >
> > > >> arch/x86/kernel/kprobes/core.o: warning: objtool: kretprobe_trampoline()+0x25: call without frame pointer save/setup
> >
> > With CONFIG_FRAME_POINTER=y.
> >
> > Of course this can be fixed with additional "push %bp; mov %sp, %bp" before calling
> > trampoline_handler. But actually we know that this function has a bit special
> > stack frame too.
> >
> > Can I recover STACK_FRAME_NON_STANDARD(kretprobe_trampoline) when CONFIG_FRAME_POINTER=y ?
>
> So something like this. Does it work?
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index b31058a152b6..651f337dc880 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -760,6 +760,10 @@ int kprobe_int3_handler(struct pt_regs *regs)
> }
> NOKPROBE_SYMBOL(kprobe_int3_handler);
>
> +#ifdef CONFIG_FRAME_POINTER
> +#undef UNWIND_HINT_FUNC
> +#define UNWIND_HINT_FUNC
> +#endif

This hunk isn't necessary. The unwind hints don't actually have an
effect with frame pointers.

> /*
> * When a retprobed function returns, this code saves registers and
> * calls trampoline_handler() runs, which calls the kretprobe's handler.
> @@ -797,7 +801,14 @@ asm(
> ".size kretprobe_trampoline, .-kretprobe_trampoline\n"
> );
> NOKPROBE_SYMBOL(kretprobe_trampoline);
> -
> +#ifdef CONFIG_FRAME_POINTER
> +/*
> + * kretprobe_trampoline skips updating frame pointer. The frame pointer
> + * saved in trampoline_handler points to the real caller function's
> + * frame pointer.
> + */
> +STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
> +#endif
>
> /*
> * Called from kretprobe_trampoline

Ack.

--
Josh

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