Subject: Re: [PATCH 2/2] powerpc/kprobes: Reduce depth of a test

From: "Naveen N. Rao" <[email-masked]>
Date: Tue, 18 Feb 2020 20:10:52 +0530

Christophe Leroy wrote:
> 	if (a) {
> 		if (b)
> 			do_something();
> 	}
> 
> Is equivalent to
> 
> 	if (a & b)
> 		do_something();
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/kernel/kprobes.c | 58 +++++++++++++++++------------------
>  1 file changed, 28 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 7a925eb76ec0..d7c80a078c1e 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -277,38 +277,36 @@ int kprobe_handler(struct pt_regs *regs)
>  
>  	/* Check we're not actually recursing */
>  	p = get_kprobe(addr);
> -	if (kprobe_running()) {
> -		if (p) {
> -			kprobe_opcode_t insn = *p->ainsn.insn;
> -			if (kcb->kprobe_status == KPROBE_HIT_SS &&
> -					is_trap(insn)) {
> -				/* Turn off 'trace' bits */
> -				regs->msr &= ~MSR_SINGLESTEP;
> -				regs->msr |= kcb->kprobe_saved_msr;
> -				goto no_kprobe;
> -			}
> -			/* We have reentered the kprobe_handler(), since
> -			 * another probe was hit while within the handler.
> -			 * We here save the original kprobes variables and
> -			 * just single step on the instruction of the new probe
> -			 * without calling any user handlers.
> -			 */
> -			save_previous_kprobe(kcb);
> -			set_current_kprobe(p, regs, kcb);
> -			kprobes_inc_nmissed_count(p);
> -			kcb->kprobe_status = KPROBE_REENTER;
> -			if (p->ainsn.boostable >= 0) {
> -				ret = try_to_emulate(p, regs);
> -
> -				if (ret > 0) {
> -					restore_previous_kprobe(kcb);
> -					preempt_enable_no_resched();
> -					return 1;
> -				}
> +	if (kprobe_running() && p) {
> +		kprobe_opcode_t insn = *p->ainsn.insn;
> +
> +		if (kcb->kprobe_status == KPROBE_HIT_SS && is_trap(insn)) {
> +			/* Turn off 'trace' bits */
> +			regs->msr &= ~MSR_SINGLESTEP;
> +			regs->msr |= kcb->kprobe_saved_msr;
> +			goto no_kprobe;
> +		}
> +		/* We have reentered the kprobe_handler(), since
> +		 * another probe was hit while within the handler.
> +		 * We here save the original kprobes variables and
> +		 * just single step on the instruction of the new probe
> +		 * without calling any user handlers.
> +		 */
> +		save_previous_kprobe(kcb);
> +		set_current_kprobe(p, regs, kcb);
> +		kprobes_inc_nmissed_count(p);
> +		kcb->kprobe_status = KPROBE_REENTER;
> +		if (p->ainsn.boostable >= 0) {
> +			ret = try_to_emulate(p, regs);
> +
> +			if (ret > 0) {
> +				restore_previous_kprobe(kcb);
> +				preempt_enable_no_resched();
> +				return 1;
>  			}
> -			prepare_singlestep(p, regs);
> -			return 1;
>  		}
> +		prepare_singlestep(p, regs);
> +		return 1;
>  	}
>  

If we move the below !p case before the check for kprobe_running() right 
after get_kprobe(), we won't need to check for (p) above and we won't 
have any change in logic from Patch 1.

>  	if (!p) {

- Naveen