Le 02/06/2017 à 11:26, Michael Ellerman a écrit :

Christophe Leroy [off-list ref] writes:

quoted

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