linux-kernel - Re: [PATCH v2] fs: Fix page

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Date:   Tue, 3 Dec 2019 02:52:13 +0100
From:   Andreas Grünbacher <andreas.gruenbacher@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>, Sage Weil <sage@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Richard Weinberger <richard@....at>,
        Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ceph Development <ceph-devel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v2] fs: Fix page_mkwrite off-by-one errors

Am Di., 3. Dez. 2019 um 02:09 Uhr schrieb Linus Torvalds
<torvalds@...ux-foundation.org>:
>
> On Fri, Nov 29, 2019 at 6:21 AM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> >
> > +/**
> > + * page_mkwrite_check_truncate - check if page was truncated
> > + * @page: the page to check
> > + * @inode: the inode to check the page against
> > + *
> > + * Returns the number of bytes in the page up to EOF,
> > + * or -EFAULT if the page was truncated.
> > + */
> > +static inline int page_mkwrite_check_truncate(struct page *page,
> > +                                             struct inode *inode)
> > +{
> > +       loff_t size = i_size_read(inode);
> > +       pgoff_t end_index = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
>
> This special end_index calculation seems to be redundant.
>
> You later want "size >> PAGE_SHIFT" for another test, and that's
> actually the important part.
>
> The "+ PAGE_SIZE - 1" case is purely to handle the "AT the page
> boundary is special" case, but since you have to calculate
> "offset_in_page(size)" anyway, that's entirely redundant - the answer
> is part of that.
>
> So I think it would be better to write the logic as
>
>         loff_t size = i_size_read(inode);
>         pgoff_t index = size >> PAGE_SHIFT;
>         int offset = offset_in_page(size);
>
>         if (page->mapping != inode->i_mapping)
>                 return -EFAULT;
>
>         /* Page is wholly past the EOF page */
>         if (page->index > index)
>                 return -EFAULT;
>         /* page is wholly inside EOF */
>         if (page->index < index)
>                 return PAGE_SIZE;
>         /* bytes in a page? If 0, it's past EOF */
>         return offset ? offset : -PAGE_SIZE;
>
> instead. That avoids the unnecessary "round up" part, and simply uses
> the same EOF index for everything.

And if we rearrange things slightly, we end up with:

        /* page is wholly inside EOF */
        if (page->index < index)
                return PAGE_SIZE;
        /* page is wholly past EOF */
        if (page->index > index || !offset)
                return -EFAULT;
        /* page is partially inside EOF */
        return offset;

Thanks,
Andreas