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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 27 Aug 2021 12:33:00 -0700
Message-ID: <CAHk-=wjMyZLH+ta5SohAViSc10iPj-hRnHc-KPDoj1XZCmxdBg@mail.gmail.com>
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com
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On Fri, Aug 27, 2021 at 12:23 PM Al Viro <viro@zeniv.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.

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

THOSE MUST NOT CHANGE.

              Linus