From 3cca6c243568d355c1ccecaaa71bf490f014d729 Mon Sep 17 00:00:00 2001 From: Jon Medhurst Date: Thu, 16 Jun 2011 15:54:00 +0100 Subject: ARM: kprobes: Don't trigger probes on conditional instructions when condition is false 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 Acked-by: Nicolas Pitre --- arch/arm/kernel/kprobes.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) (limited to 'arch/arm/kernel/kprobes.c') diff --git a/arch/arm/kernel/kprobes.c b/arch/arm/kernel/kprobes.c index a9050bad4434..b6e9a1cc1c55 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. */ -- cgit v1.2.3