Re: [PATCH v12 7/7] ARM: kprobes: enable OPTPROBES for ARM 32
From: Wang Nan
Date: Mon Dec 08 2014 - 01:35:13 EST
On 2014/12/5 22:59, Jon Medhurst (Tixy) wrote:
> On Fri, 2014-12-05 at 10:10 +0000, Jon Medhurst (Tixy) wrote:
>> I'm worried because this whole optimised kprobes has some rather
>> complicated interactions, e.g. can the background thread that changes
>> breakpoints to jumps (or back again?) could occur at the same time
>> another CPU is processing a kprobe that's been hit, or is in the process
>> of removing a probe.
> I think that is a plausible theory. We can have this situation...
> 1. CPU A executes a probe's 'breakpoint' instruction and the undefined
> instruction exception handler is triggered.
> 2. CPU B is executing the kprobes optimisation thread and replaces the
> 'breakpoint' with a branch instruction.
> 3. CPU A reads the invalid instruction from memory and because this is
> now the branch instruction it doesn't match
> KPROBE_ARM_BREAKPOINT_INSTRUCTION which kprobes registered to handle.
> This means the undefined instruction exception is treated as just that,
> execution of an undefined instruction.
I confirmed your theory by printing the buggy instruction:
[ 474.824206] subls r9, r9, r14, lsr r7 @ 9049973e
[ 476.954206] subge r10, r11, r14, asr r7 @ a04ba75e
[ 479.014206] sublt r11, r11, r14, asr r7 @ b04bb75e
[ 479.194212] undefined instruction: pc=bf001bbc, instruction=ea01187f
[ 479.290190] Internal error: Oops - undefined instruction: 0 [#1] SMP ARM
[ 479.370533] Modules linked in: test_kprobes(+)
[ 479.423990] CPU: 10 PID: 1410 Comm: insmod Not tainted 3.10.53-HULK2+ #31
[ 479.505377] task: c42b72c0 ti: ed4f8000 task.ti: ed4f8000
[ 479.570189] PC is at kprobe_arm_test_cases+0x122c/0xfeed [test_kprobes]
ea01187f is a branch instruction.
Please help me to review my v14 patch series:
In which I fix it by wrapping __arch_optimize_kprobes() using stop_machine().
> The above scenario is the exact reason why arch_disarm_kprobe is
> implemented to always use stop_machine to modify the code and we need to
> ensure the same happens with arch_optimize_kprobes.
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