linux-kernel - Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter

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Date:   Fri, 27 Aug 2021 19:37:25 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

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@...iv.linux.org.uk> 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.