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

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Message-ID: <YPx28cEvrVl6YrDk@zeniv-ca.linux.org.uk>
Date:   Sat, 24 Jul 2021 20:24:17 +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 v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable
 helper

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...