linux-kernel - Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page

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Date:   Mon, 05 Jun 2017 20:49:30 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe LEROY <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault()

Christophe LEROY <christophe.leroy@....fr> writes:

> Le 02/06/2017 à 14:11, Benjamin Herrenschmidt a écrit :
>> On Fri, 2017-06-02 at 11:39 +0200, Christophe LEROY wrote:
>>> The difference between get_user() and __get_user() is that get_user()
>>> performs an access_ok() in addition.
>>>
>>> Doesn't access_ok() only checks whether addr is below TASK_SIZE to
>>> ensure it is a valid user address ?
>> 
>> Do you have a measurable improvement by skipping that check ? I agree
>> with your reasoning but I'm also paranoid and so I wouldn't change it
>> unless it's really worth it.
>> 
>
> No I don't have. Taking into account the patch following this serie 
> which limits even more the calls to get_user(), it is probably not worth 
> it anymore (see https://patchwork.ozlabs.org/patch/757564/)
>
> I will then have to resubmit the entire serie (including that additional 
> one), but there is no get_user_inatomic() so will have to either:
> - do the access_ok() verification inside the function

I think open coding the access_ok() check is probably the best option.

cheers