linux-kernel - Re: [RFC PATCH 5/9] powerpc/kprobes: Use ftrace to determine if a probe is at function entry

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Message-Id: <20231211143950.980b4ab4e11b71de04332540@kernel.org>
Date:   Mon, 11 Dec 2023 14:39:50 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Naveen N Rao <naveen@...nel.org>
Cc:     <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....com>,
        Florent Revest <revest@...omium.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC PATCH 5/9] powerpc/kprobes: Use ftrace to determine if a
 probe is at function entry

On Fri,  8 Dec 2023 22:00:44 +0530
Naveen N Rao <naveen@...nel.org> wrote:

> Rather than hard-coding the offset into a function to be used to
> determine if a kprobe is at function entry, use ftrace_location() to
> determine the ftrace location within the function and categorize all
> instructions till that offset to be function entry.
> 
> For functions that cannot be traced, we fall back to using a fixed
> offset of 8 (two instructions) to categorize a probe as being at
> function entry for 64-bit elfv2.
> 

OK, so this can cover both KPROBES_ON_FTRACE=y/n cases and the function
is traced by ftrace or not.

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks,

> Signed-off-by: Naveen N Rao <naveen@...nel.org>
> ---
>  arch/powerpc/kernel/kprobes.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index b20ee72e873a..42665dfab59e 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -105,24 +105,22 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>  	return addr;
>  }
>  
> -static bool arch_kprobe_on_func_entry(unsigned long offset)
> +static bool arch_kprobe_on_func_entry(unsigned long addr, unsigned long offset)
>  {
> -#ifdef CONFIG_PPC64_ELF_ABI_V2
> -#ifdef CONFIG_KPROBES_ON_FTRACE
> -	return offset <= 16;
> -#else
> -	return offset <= 8;
> -#endif
> -#else
> +	unsigned long ip = ftrace_location(addr);
> +
> +	if (ip)
> +		return offset <= (ip - addr);
> +	if (IS_ENABLED(CONFIG_PPC64_ELF_ABI_V2))
> +		return offset <= 8;
>  	return !offset;
> -#endif
>  }
>  
>  /* XXX try and fold the magic of kprobe_lookup_name() in this */
>  kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
>  					 bool *on_func_entry)
>  {
> -	*on_func_entry = arch_kprobe_on_func_entry(offset);
> +	*on_func_entry = arch_kprobe_on_func_entry(addr, offset);
>  	return (kprobe_opcode_t *)(addr + offset);
>  }
>  
> -- 
> 2.43.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>