Re: [patch V4 part 4 02/24] x86/int3: Avoid atomic instrumentation

[Andy Lutomirski]

> On May 14, 2020, at 8:06 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> ïPeter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>>> On Thu, May 14, 2020 at 02:51:32PM +0200, Thomas Gleixner wrote:
>>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>>>> On Wed, May 13, 2020 at 09:57:52PM -0700, Andy Lutomirski wrote:
>>>>> On Tue, May 5, 2020 at 7:15 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>>>>> 
>>>>>> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>>>>> 
>>>>>> Use arch_atomic_*() and READ_ONCE_NOCHECK() to ensure nothing untoward
>>>>>> creeps in and ruins things.
>>>>>> 
>>>>>> That is; this is the INT3 text poke handler, strictly limit the code
>>>>>> that runs in it, lest it inadvertenly hits yet another INT3.
>>>>> 
>>>>> 
>>>>> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
>>>>> 
>>>>> Does objtool catch this error?
>>>> 
>>>> It does not. I'll put it on the (endless) todo list..
>>> 
>>> Well, at least it detects when that code calls out into something which
>>> is not in the non-instrumentable section.
>> 
>> True, but the more specific problem is that noinstr code can use
>> jump_label/static_call just fine.
>> 
>> So a more specific test is validating none of that happens in the INT3
>> handler before poke_int3_handler(). Which is what I think Andy was
>> after.
> 
> Indeed. Forgot about that one.
> 
> Hmm, alternatives and jumplabel patch locations in entry.text and
> noinstr.text can be valid at least during early boot where we know that
> we don't run those code pathes...

Alternatives should be valid regardless. Isnât the world essentially stopped while we apply them?

> 
> Thanks,
> 
>        tglx