[PATCH 15/51] ARM: kprobes: Don't trigger probes on conditional instructions when condition is false

[Nicolas Pitre]

On Sat, 9 Jul 2011, Tixy wrote:

> From: Jon Medhurst <tixy at yxit.co.uk>
> 
> This patch changes the behavior of kprobes on ARM so that:
> 
>     Kprobes on conditional instructions don't trigger when the
>     condition is false. For conditional branches, this means that
>     they don't trigger in the branch not taken case.
> 
> Rationale:
> 
> When probes are placed onto conditionally executed instructions in a
> Thumb IT block, they may not fire if the condition is not met. This
> is because we use invalid instructions for breakpoints and "it is
> IMPLEMENTATION DEFINED whether the instruction executes as a NOP or
> causes an Undefined Instruction exception". Therefore, for consistency,
> we will ignore all probes on any conditional instructions when the
> condition is false. Alternative solutions seem to be too complex to
> implement or inconsistent.
> 
> This issue was discussed on linux.arm.kernel in the thread titled
> "[RFC] kprobes with thumb2 conditional code" See
> http://comments.gmane.org/gmane.linux.linaro.devel/2985
> 
> Signed-off-by: Jon Medhurst <tixy at yxit.co.uk>

Acked-by: Nicolas Pitre <nicolas.pitre at linaro.org>

> ---
>  arch/arm/kernel/kprobes.c |   24 +++++++++++++++++++++++-
>  1 files changed, 23 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/arm/kernel/kprobes.c b/arch/arm/kernel/kprobes.c
> index b8f5b3b..7a9be60 100644
> --- a/arch/arm/kernel/kprobes.c
> +++ b/arch/arm/kernel/kprobes.c
> @@ -207,6 +207,20 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
>  	__get_cpu_var(current_kprobe) = p;
>  }
>  
> +static void __kprobes
> +singlestep_skip(struct kprobe *p, struct pt_regs *regs)
> +{
> +#ifdef CONFIG_THUMB2_KERNEL
> +	regs->ARM_cpsr = it_advance(regs->ARM_cpsr);
> +	if (is_wide_instruction(p->opcode))
> +		regs->ARM_pc += 4;
> +	else
> +		regs->ARM_pc += 2;
> +#else
> +	regs->ARM_pc += 4;
> +#endif
> +}
> +
>  static void __kprobes singlestep(struct kprobe *p, struct pt_regs *regs,
>  				 struct kprobe_ctlblk *kcb)
>  {
> @@ -262,7 +276,8 @@ void __kprobes kprobe_handler(struct pt_regs *regs)
>  				/* impossible cases */
>  				BUG();
>  			}
> -		} else {
> +		} else if (p->ainsn.insn_check_cc(regs->ARM_cpsr)) {
> +			/* Probe hit and conditional execution check ok. */
>  			set_current_kprobe(p);
>  			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>  
> @@ -282,6 +297,13 @@ void __kprobes kprobe_handler(struct pt_regs *regs)
>  				}
>  				reset_current_kprobe();
>  			}
> +		} else {
> +			/*
> +			 * Probe hit but conditional execution check failed,
> +			 * so just skip the instruction and continue as if
> +			 * nothing had happened.
> +			 */
> +			singlestep_skip(p, regs);
>  		}
>  	} else if (cur) {
>  		/* We probably hit a jprobe.  Call its break handler. */
> -- 
> 1.7.2.5
> 
> 
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