Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

[Linus Torvalds]

On Fri, Aug 27, 2021 at 12:23 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> Could you show the cases where "partial copy, so it's OK" behaviour would
> break anything?

Absolutely.

For example, i t would cause an infinite loop in
restore_fpregs_from_user() if the "buf" argument is a situation where
the first page is fine, but the next page is not.

Why? Because __restore_fpregs_from_user() would take a fault, but then
fault_in_pages_readable() (renamed) would succeed, so you'd just do
that "retry" forever and ever.

Probably there are a number of other places too. That was literally
the *first* place I looked at.

Seriously. The current semantics are "check the whole area".

THOSE MUST NOT CHANGE.

              Linus