On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote: > 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. nr_to_read == 0 doesn't make sense ... I thought we filtered that out earlier, but I can't find anywhere that does that right now. I'd rather return early from __do_page_cache_readahead() to fix that. > 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. I think we can get a page at ULONG_MAX on 32-bit systems? I mean, we can buy hard drives which are larger than 16TiB these days: https://www.pcmag.com/news/seagate-will-ship-18tb-and-20tb-hard-drives-in-2020 (even ignoring RAID devices)
