linux-kernel - Re: [PATCH -tip v8 05/13] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe

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Date:   Mon, 5 Jul 2021 10:02:47 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, 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, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@...com,
        yhs@...com, linux-ia64@...r.kernel.org,
        Abhishek Sagar <sagar.abhishek@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH -tip v8 05/13] x86/kprobes: Add UNWIND_HINT_FUNC on
 kretprobe_trampoline code


* 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.

What is a 'kretporbe'?

> Note that when the CONFIG_FRAME_POINTER=y, since the
> kretprobe_trampoline skips updating frame pointer, the stack frame
> of the kretprobe_trampoline seems non-standard. So this marks it
> is STACK_FRAME_NON_STANDARD() and undefine UNWIND_HINT_FUNC.

What does 'marks it is' mean?

'undefine' UNWIND_HINT_FUNC?

Doesn't the patch do the exact opposite:

  > +#define UNWIND_HINT_FUNC \
  > +	UNWIND_HINT(ORC_REG_SP, 8, UNWIND_HINT_TYPE_FUNC, 0)

But it does undefine it in a specific spot:



> Anyway, with the frame pointer, FP unwinder can unwind the stack
> frame correctly without that hint.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> Tested-by: Andrii Nakryik <andrii@...nel.org>

I have to say these changelogs are very careless.

> +#else
> +

In headers, in longer CPP blocks, please always mark the '#else' branch 
with what it is the else branch of.

See the output of:

   kepler:~/tip> git grep '#else' arch/x86/include/asm/ | head

> +#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. Thus the kretprobe_trampoline doesn't seems to have a
> + * standard stack frame with CONFIG_FRAME_POINTER=y.
> + * Let's mark it non-standard function. Anyway, FP unwinder can correctly
> + * unwind without the hint.

s/doesn't seems to have a standard stack frame
 /doesn't have a standard stack frame

There's nothing 'seems' about the situation - it's a non-standard function 
entry and stack frame situation, and the unwinder needs to know about it.

> +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?

Thanks,

	Ingo