On Fri, Apr 25, 2014 at 09:39:57PM +0530, Kumar Gaurav wrote:
> I have found some codes in the driver which use put_user() in loop.
> Can we avoid the overhead of checking the same memory area( where
> put_user() writes) again n again using __put_user() in side loop and
> checking permission using access_ok before entering the loop?
> if (put_user(type, dst) ||
> put_user(chs_bytes, dst + 1))
> return -EFAULT;
> dst += 2;
^^^^^^^^^
Note that increment. It's *not* "the same memory area" next time
around. Sure, you can check the whole range once before the loop
and switch the stuff inside to __put_user()/__copy_to_user(), but
it's not guaranteed to buy you any speedup.
BTW, you might be a bit confused about the work done by access_ok() - e.g.
on an architectures with separate kernel and userland MMU contexts it might
very well be a no-op (always return true). It's *not* checking if user has
permissions of some sort.
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