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

[Al Viro]

On Fri, Aug 27, 2021 at 12:33:00PM -0700, Linus Torvalds wrote:
> 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.

OK...

Let me dig out the notes from the last time I looked through that area
and grep around a bit.  Should be about an hour or two.