linux-kernel - Re: [RFC PATCH tip/master V2 4/8] kprobes/x86: Do not modify singlestep buffer while resuming

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Date:   Tue, 28 Mar 2017 09:04:19 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ye Xiaolong <xiaolong.ye@...el.com>
Subject: Re: [RFC PATCH tip/master V2 4/8] kprobes/x86: Do not modify
 singlestep buffer while resuming


* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Do not modify singlestep execution buffer (kprobe.ainsn.insn)
> while resuming from single-stepping, instead, modifies
> the buffer to add a jump back instruction at preparing
> buffer.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  arch/x86/kernel/kprobes/core.c |   42 +++++++++++++++++++---------------------
>  1 file changed, 20 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 6327f95..ea3b8e5 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -399,23 +399,36 @@ int __copy_instruction(u8 *dest, u8 *src)
>  	return length;
>  }
>  
> +/* Prepare reljump right after instruction to boost */
> +static void prepare_boost(struct kprobe *p, int length)
> +{
> +	if (can_boost(p->ainsn.insn, p->addr) &&
> +	    MAX_INSN_SIZE - length >= RELATIVEJUMP_SIZE) {
> +		/*
> +		 * These instructions can be executed directly if it
> +		 * jumps back to correct address.
> +		 */
> +		synthesize_reljump((void *)p->ainsn.insn + length,
> +				   (void *)p->addr + length);
> +		p->ainsn.boostable = 1;
> +	} else
> +		p->ainsn.boostable = -1;

Those imbalanced curly braces are not proper kernel style.

Also, is the (void *) cast required? arch.insns ought to be void * already, right? 
(I haven't checking whether that's true for all architectures.)

Btw., the original code had the curly braces right:

> -	if (p->ainsn.boostable == 0) {
> -		if ((regs->ip > copy_ip) &&
> -		    (regs->ip - copy_ip) + 5 < MAX_INSN_SIZE) {
> -			/*
> -			 * These instructions can be executed directly if it
> -			 * jumps back to correct address.
> -			 */
> -			synthesize_reljump((void *)regs->ip,
> -				(void *)orig_ip + (regs->ip - copy_ip));
> -			p->ainsn.boostable = 1;
> -		} else {
> -			p->ainsn.boostable = -1;
> -		}
> -	}

Thanks,

	Ingo