linux-kernel - Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in __kretprobe_trampoline

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Date:   Wed, 16 Jun 2021 23:39:09 -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,
        Abhishek Sagar <sagar.abhishek@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in
 __kretprobe_trampoline_handler

On Thu, May 27, 2021 at 03:40:29PM +0900, Masami Hiramatsu wrote:
> To simplify the stacktrace with pt_regs from kretprobe handler,
> set the correct return address to the instruction pointer in
> the pt_regs before calling kretprobe handlers.
> 
> Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> Tested-by: Andrii Nakryik <andrii@...nel.org>
> ---
>  Changes in v3:
>   - Cast the correct_ret_addr to unsigned long.
> ---
>  kernel/kprobes.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 54e5b89aad67..1598aca375c9 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1914,6 +1914,9 @@ unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
>  		BUG_ON(1);
>  	}
>  
> +	/* Set the instruction pointer to the correct address */
> +	instruction_pointer_set(regs, (unsigned long)correct_ret_addr);
> +
>  	/* Run them. */
>  	first = current->kretprobe_instances.first;
>  	while (first) {
> 

Hi Masami,

I know I suggested this patch, but I believe it would only be useful in
combination with the use of UNWIND_HINT_REGS in SAVE_REGS_STRING.  But I
think that would be tricky to pull off correctly.  Instead, we have
UNWIND_HINT_FUNC, which is working fine.

So I'd suggest dropping this patch, as the unwinder isn't actually
reading regs->ip after all.

(I apologize for the churn, and my lack of responsiveness in general.
I've been working on moving to California, which is no small task with a
family, and in this housing market...)

-- 
Josh