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

Ingo Molnar
2021-07-05T08:02:47Z

* 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.

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@redhat.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Tested-by: Andrii Nakryik <andrii@kernel.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