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

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Message-ID: <6daf8f4e-9b39-d585-2c64-9b0348fef123@c-s.fr>
Date:   Fri, 2 Jun 2017 11:39:23 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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()



Le 02/06/2017 à 11:26, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
> 
>> Only the get_user() in store_updates_sp() has to be done outside
>> the mm semaphore. All the comparison can be done within the semaphore,
>> so only when really needed.
>>
>> As we got a DSI exception, the address pointed by regs->nip is
>> obviously valid, otherwise we would have had a instruction exception.
>> So __get_user() can be used instead of get_user()
> 
> I don't think that part is true.
> 
> You took a DSI so there *was* an instruction at NIP, but since then it
> may have been unmapped by another thread.
> 
> So I don't think you can assume the get_user() will succeed.
> 

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 ?

Christophe