Re: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2014/05/08 21:48), Josh Poimboeuf wrote:
>> No, I was not demanding that at all, my suggestion was:
>>
>>    > My claim is that if a patch is correct/safe in the old fashioned 
>>    > way, then a fundamental principle is that a live patching 
>>    > subsystem must either safely apply, or safely reject the live 
>>    > patching attempt, independently from any user input.
>>
>> Note the 'if'. It could start simple and stupid, and only allow cases 
>> where we know the patch must be trivially safe (because it does not do 
>> much in terms of disturbing globally visible state). That needs some 
>> tooling help, but apparently tooling help is in place already.
>>
>> And then we can complicate it from there - but have a reasonably 
>> robust starting point that we _know_ works (as long as the 
>> implementation is correct).
> 
> I really wonder if detecting a "trivially safe" patch is even possible.
> 
> Where do you draw the line with the following patches?
> 
> - add a call to another function which modifies global data

This depends on what global data and how. For example, the global data
is used only from the replaced functions, it's a kind of local data.
And also, the global data modification is as designed (e.g. acquiring/
releasing a spinlock), that is also safe.

I think, the bad case is modifying shared global data to new state which
unexpected by other data holders.

> - add an early return or a goto which changes the way the function
>   modifies (or no longer modifies) global data

Ditto, if it is unexpected at other parts, that will be unacceptable.

> - touch a local stack variable which results in global data being
>   modified later in the function
> 
> - return a different value which causes the function's caller to modify
>   data

I think if the local variable or return value change is correctly handled
by the caller (as expected), that is good too.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com