Re: [PATCH v9 12/25] mm: Move end_index check out of readahead loop

Eric Biggers
2020-03-20T18:00:23Z

On Fri, Mar 20, 2020 at 10:30:40AM -0700, Matthew Wilcox wrote:
> On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers wrote:
> > On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote:
> > > + /* Avoid wrapping to the beginning of the file */
> > > + if (index + nr_to_read < index)
> > > + nr_to_read = ULONG_MAX - index + 1;
> > > + /* Don't read past the page containing the last byte of the file */
> > > + if (index + nr_to_read >= end_index)
> > > + nr_to_read = end_index - index + 1;
> >
> > There seem to be a couple off-by-one errors here. Shouldn't it be:
> >
> > /* Avoid wrapping to the beginning of the file */
> > if (index + nr_to_read < index)
> > nr_to_read = ULONG_MAX - index;
>
> I think it's right. Imagine that index is ULONG_MAX. We should read one
> page (the one at ULONG_MAX). That would be ULONG_MAX - ULONG_MAX + 1.
>
> > /* Don't read past the page containing the last byte of the file */
> > if (index + nr_to_read > end_index)
> > nr_to_read = end_index - index + 1;
> >
> > I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that
> > 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0.
> >
> > Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index',
> > since otherwise nr_to_read can be increased by 1 rather than decreased or stay
> > the same as expected.
>
> Ooh, I missed the overflow case here. It should be:
>
> + if (index + nr_to_read - 1 > end_index)
> + nr_to_read = end_index - index + 1;
>

But then if someone passes index=0 and nr_to_read=0, this underflows and the
entire file gets read.

The page cache isn't actually supposed to contain a page at index ULONG_MAX,
since MAX_LFS_FILESIZE is at most ((loff_t)ULONG_MAX << PAGE_SHIFT), right? So
I don't think we need to worry about reading the page with index ULONG_MAX.
I.e. I think it's fine to limit nr_to_read to 'ULONG_MAX - index', if that makes
it easier to avoid an overflow or underflow in the next check.

- Eric