On Fri, Mar 20, 2020 at 11:24:52AM -0700, Eric Biggers wrote: > On Fri, Mar 20, 2020 at 11:11:32AM -0700, Matthew Wilcox wrote: > > On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote: > > > 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) > > The max file size is ((loff_t)ULONG_MAX << PAGE_SHIFT) which means the maximum > page *index* is ULONG_MAX - 1, not ULONG_MAX. I see where we set that for _files_ ... I can't find anywhere that we prevent i_size getting that big for block devices. Maybe I'm missing something. > Anyway, I think we may be making this much too complicated. How about just: > > pgoff_t i_nrpages = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE); > > if (index >= i_nrpages) > return; > /* Don't read past the end of the file */ > nr_to_read = min(nr_to_read, i_nrpages - index); > > That's 2 branches instead of 4. (Note that assigning to i_nrpages can't > overflow, since the max number of pages is ULONG_MAX not ULONG_MAX + 1.) I like where you're going with this. Just to be on the safe side, I'd prefer to do this: @@ -266,11 +266,8 @@ void __do_page_cache_readahead(struct address_space *mapping, end_index = (isize - 1) >> PAGE_SHIFT; if (index > end_index) return; - /* 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) + if (nr_to_read > end_index - index) nr_to_read = end_index - index + 1; page_cache_readahead_unbounded(mapping, file, index, nr_to_read, end_index - index + 1 could only overflow if end_index is ULONG_MAX and index is 0. But if end_index is ULONG_MAX and index is 0, then nr_to_read is necessarily <= ULONG_MAX, so the condition is false. And if nr_to_read is 0, then the condition is also false, so it won't increase nr_to_read from 0 to 1. It might assign x to nr_to_read when nr_to_read is already x, but that's harmless. Thanks!
