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!
