Ocfs2 devel - Mar 2020 - [PATCH v9 12/25] mm: Move end_index check out of readahead loop

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;

Let's say index comes in at ULONG_MAX - 2, end_index is ULONG_MAX - 1
and nr_to_read is 8.  The first condition triggers and nr_to_read is
reduced to 3.  But then the second condition wouldn't trigger because
ULONG_MAX - 2 + 3 is 0.

With the rewrite I have in this message, ULONG_MAX - 2 + 3 - 1 is ULONG_MAX,
which is > ULONG_MAX - 1.  So the condition triggers and nr_to_read becomes
(ULONG_MAX - 1) - (ULONG_MAX - 2) + 1.  Which is -1 + 2 + 1, which is 2.
Which is the right answer because we want to read two pages; the one
at ULONG_MAX - 2 and the one at ULONG_MAX - 1.

Thank you!

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