Re: [PATCH v2] fs: Fix page_mkwrite off-by-one errors

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Nov 29, 2019 at 6:21 AM Andreas Gruenbacher <agruenba@redhat.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