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SubjectRe: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault()
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Le 02/06/2017 à 11:26, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@c-s.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

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