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

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