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Date:   Mon, 2 Dec 2019 17:06:17 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     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-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

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.

              Linus

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