linux-kernel - Re: [PATCH V2 4/4] arch/powerpc: Optimize kprobe in kretprobe

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Message-Id: <20161216230914.1d5817b0e7f2813b108ac8ee@kernel.org>
Date:   Fri, 16 Dec 2016 23:09:14 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Anju T Sudhakar <anju@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        ananth@...ibm.com, naveen.n.rao@...ux.vnet.ibm.com,
        paulus@...ba.org, srikar@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        mahesh@...ux.vnet.ibm.com, mhiramat@...nel.org
Subject: Re: [PATCH V2 4/4] arch/powerpc: Optimize kprobe in
 kretprobe_trampoline

On Wed, 14 Dec 2016 21:48:28 +0530
Anju T Sudhakar <anju@...ux.vnet.ibm.com> wrote:

> Kprobe placed on the  kretprobe_trampoline during boot time can be 
> optimized, since the instruction at probe point is a 'nop'.
> 

How simple & clever way! ;)

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

Thank you!

> Signed-off-by: Anju T Sudhakar <anju@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/kprobes.c   | 8 ++++++++
>  arch/powerpc/kernel/optprobes.c | 7 +++----
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index e785cc9..5b0fd07 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -282,6 +282,7 @@ asm(".global kretprobe_trampoline\n"
>  	".type kretprobe_trampoline, @function\n"
>  	"kretprobe_trampoline:\n"
>  	"nop\n"
> +	"blr\n"
>  	".size kretprobe_trampoline, .-kretprobe_trampoline\n");
>  
>  /*
> @@ -334,6 +335,13 @@ static int __kprobes trampoline_probe_handler(struct kprobe *p,
>  
>  	kretprobe_assert(ri, orig_ret_address, trampoline_address);
>  	regs->nip = orig_ret_address;
> +	/*
> +	 * Make LR point to the orig_ret_address.
> +	 * When the 'nop' inside the kretprobe_trampoline
> +	 * is optimized, we can do a 'blr' after executing the
> +	 * detour buffer code.
> +	 */
> +	regs->link = orig_ret_address;
>  
>  	reset_current_kprobe();
>  	kretprobe_hash_unlock(current, &flags);
> diff --git a/arch/powerpc/kernel/optprobes.c b/arch/powerpc/kernel/optprobes.c
> index ecba221..5e4c254 100644
> --- a/arch/powerpc/kernel/optprobes.c
> +++ b/arch/powerpc/kernel/optprobes.c
> @@ -72,12 +72,11 @@ static unsigned long can_optimize(struct kprobe *p)
>  
>  	/*
>  	 * kprobe placed for kretprobe during boot time
> -	 * is not optimizing now.
> -	 *
> -	 * TODO: Optimize kprobe in kretprobe_trampoline
> +	 * has a 'nop' instruction, which can be emulated.
> +	 * So further checks can be skipped.
>  	 */
>  	if (p->addr == (kprobe_opcode_t *)&kretprobe_trampoline)
> -		return 0;
> +		return (unsigned long)p->addr + sizeof(kprobe_opcode_t);
>  
>  	/*
>  	 * We only support optimizing kernel addresses, but not
> -- 
> 2.7.4
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>