Re: [PATCH -tip v5 07/10] kprobes/x86: Support kprobes jump optimization on x86

Masami Hiramatsu
2009-11-24T17:01:49Z

Hi Peter,

H. Peter Anvin wrote:
> On 11/23/2009 03:22 PM, Masami Hiramatsu wrote:
>>
>> This uses stop_machine() for corss modifying code from int3 to jump.
>> It doesn't allow us to modify code on NMI/SMI path. However, since
>> kprobes itself doesn't support NMI/SMI code probing, it's not a
>> problem.
>>
>
> I'm a bit confused by the above statement... does that mean you're
> poking int3 and *then* do stop_machine()?

Yes, as I said in http://lkml.org/lkml/2009/11/24/310,
there are two separated issues.

----
We have to separate below issues:
- int3-based multi-bytes code replacement
- multi-instruction replacement with int3-detour code

The former is implemented on patch 9/10 and 10/10. As you can see,
these patches are RFC status, because I'd like to wait for official
reply of safeness from processor architects.
And it may be able to use a dummy IPI for 2nd IPI because it
just for waiting int3 interrupts. But again, it is just estimated that
replacing with/recovering from int3 is automatically synchronized...

However, at least stop_machine() method is officially described
at "7.1.3 Handling Self- and Cross-Modifying Code" on the intel's
software developer's manual 3A . So currently we can use it.

For the latter issue, as I explained on previous reply, we need
to wait all running interrupts including hardware interrupts.
Thus I used synchronize_sched().
----

So that the previous "x86 generic jump patching" patch is
basically for single-instruction replacement. For multi-instructions
replacement, we need to make detour code and wait for all running
interruption. (of course, there are other static code limitations,
as I described at "Safety check" section in patch 0/10.)

Thank you,