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

["Andreas Grünbacher" <andreas.gruenbacher@gmail.com>]

Am Di., 3. Dez. 2019 um 02:09 Uhr schrieb 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.

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