[PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

[David Laight]

...
> The use of alternatives allows to return right away if the buffer is
> beyond the usable user address space, and it's not just "slightly
> faster" for some cases (a very large buffer with only a few bytes being
> beyond the limit or someone could fault-in all the user pages and fail
> very late...etc). access_ok() is here to guarantee that such situations
> don't happen, so actually it makes more sense to use an alternative to
> avoid that.

Is it really worth doing ANY optimisations for the -EFAULT path?
They really don't happen.

The only fault path that matters is the one that has to page in
data from somewhere.

Provided there is a gap between the highest valid user address and the
lowest valid kernel address (may not be true on some 32bit systems)
and copy_to/from_user() do 'increasing address' copies then the
access_ok() check they do can almost certainly ignore the length.

This may be true for pretty much all access_ok() tests?
It would certainly simplify the test.

	David

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