(2014/12/09 18:14), Jon Medhurst (Tixy) wrote:
[...]
>>> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
>>> index 3a58db4..a4ec240 100644
>>> --- a/arch/arm/probes/kprobes/core.c
>>> +++ b/arch/arm/probes/kprobes/core.c
>>> @@ -163,19 +163,31 @@ void __kprobes arch_arm_kprobe(struct kprobe *p)
>>> * memory. It is also needed to atomically set the two half-words of a
>>> 32-bit
>>> * Thumb breakpoint.
>>> */
>>> -int __kprobes __arch_disarm_kprobe(void *p)
>>> -{
>>> - struct kprobe *kp = p;
>>> - void *addr = (void *)((uintptr_t)kp->addr & ~1);
>>> -
>>> - __patch_text(addr, kp->opcode);
>>> +struct patch {
>>> + void *addr;
>>> + unsigned int insn;
>>> +};
>>>
>>> +static int __kprobes_remove_breakpoint(void *data)
>>> +{
>>> + struct patch *p = data;
>>> + __patch_text(p->addr, p->insn);
>>> return 0;
>>> }
>>>
>>> +void __kprobes kprobes_remove_breakpoint(void *addr, unsigned int insn)
>>> +{
>>> + struct patch p = {
>>> + .addr = addr,
>>> + .insn = insn,
>>> + };
>>> + stop_machine(__kprobes_remove_breakpoint, &p, cpu_online_mask);
>>> +}
>>
>> Hmm, I think finally we should fix patch_text() in patch.c to forcibly use
>> stop_machine
>> by adding "bool stop" parameter, instead of introducing new another
>> patch_text()
>> implementation, because we'd better avoid two private "patch" data
>> structures.
>
> That was my first thought too, then I realised that breaks encapsulation
> of the patch_text implementation, because its use of stop_machine is an
> implementation detail and it could be rewritten to not use stop machine.
> (That is sort of on my long term todo list
> https://lkml.org/lkml/2014/9/4/188)
Indeed. OK, now let it goes. :)
> Whereas stop machine is used by kprobes to avoid race conditions with
> the undefined instruction exception handler and something like that
> would be needed even if patch_text didn't use stop_machine.
At this point, it's OK.
However, I'm not convinced completely. Perhaps, it depends on cache-coherent bus
implementation, but there may be some implementations which can allow us to
change one instruction atomically without stop_machine.
I'm actually interested in PREEMPT_RT on arm32, and this stop_machine() is a
barrier
to apply kprobes on real-time systems.
Thank you,
--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu...@hitachi.com
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