Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault()

[Christophe LEROY]

Le 02/06/2017 Ã 11:26, Michael Ellerman a Ãcrit :
> Christophe Leroy <christophe.leroy@xxxxxx> 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