linux-kernel - Re: [PATCH -tip v3 05/11] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe

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Date:   Sun, 21 Mar 2021 12:52:03 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kuba@...nel.org, mingo@...hat.com,
        ast@...nel.org, tglx@...utronix.de, kernel-team@...com, yhs@...com,
        linux-ia64@...r.kernel.org
Subject: Re: [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@...nel.org> wrote:
> 
> > On Fri, 19 Mar 2021 21:22:39 +0900
> > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > 
> > > From: Josh Poimboeuf <jpoimboe@...hat.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