Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patchingwithout stop_machine

From: Masami Hiramatsu
Date: Mon Jan 18 2010 - 11:00:12 EST

Arjan van de Ven wrote:
> On Sun, 17 Jan 2010 13:55:39 -0500
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
>> * H. Peter Anvin (hpa@xxxxxxxxx) wrote:
>>> On 01/14/2010 07:32 AM, Steven Rostedt wrote:
>>>>> +
>>>>> + /* Replacing 1 byte can be done atomically. */
>>>>> + if (unlikely(len <= 1))
>>>>> + return text_poke(addr, opcode, len);
>>>> This part bothers me. The text_poke just writes over the text
>>>> directly (using a separate mapping). But if that memory is in the
>>>> pipeline of another CPU, I think this could cause a GPF.
>>> Could you clarify why you think that?
>> Basically, what Steven and I were concerned about in this particular
>> patch version is the fact that this code took a "shortcut" for
>> single-byte text modification, thus bypassing the int3-bypass scheme
>> altogether.
> single byte instruction updates are likely 100x safer than any scheme
> of multi-byte instruction scheme that I have seen, other than a full
> stop_machine().
> That does not mean it is safe, it just means it's an order of
> complexity less to analyze ;-)

Yeah, so in the latest patch, I updated it to use int3 even if
len == 1. :-)

Thank you,

Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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