linux-kernel - Re: [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in

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Date:   Sat, 24 Jul 2021 23:38:20 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper

On Sat, Jul 24, 2021 at 10:24 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sat, Jul 24, 2021 at 12:52:34PM -0700, Linus Torvalds wrote:
> > ...
> > > +                       if (fault_in_user_pages(start, len, true) != len)
> > > +                               return -EFAULT;
> >
> > Looking at this once more, I think this is likely wrong.
> >
> > Why?
> >
> > Because any user can/should only care about at least *part* of the
> > area being writable.
> >
> > Imagine that you're doing a large read. If the *first* page is
> > writable, you should still return the partial read, not -EFAULT.
>
> Agreed.
>
> > So I think the code needs to return 0 if _any_ fault was successful.
>
> s/any/the first/...
>
> The same goes for fault-in for read, of course; I've a half-baked conversion
> to such semantics (-EFAULT vs. 0; precise length is unreliable anyway,
> especially if you have sub-page failure areas), need to finish and post
> it...

Hmm, how could we have sub-page failure areas when this is about if
and how pages are mapped? If we return the number of bytes that are
accessible, then users will know if they got nothing, something, or
everything, and they can act accordingly.

Thanks,
Andreas