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Date: Mon, 2 Dec 2019 17:06:17 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Andreas Gruenbacher <agruenba@...hat.com> Cc: Christoph Hellwig <hch@...radead.org>, "Darrick J. Wong" <darrick.wong@...cle.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Alexander Viro <viro@...iv.linux.org.uk>, Jeff Layton <jlayton@...nel.org>, Sage Weil <sage@...hat.com>, Ilya Dryomov <idryomov@...il.com>, "Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>, linux-xfs <linux-xfs@...r.kernel.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Richard Weinberger <richard@....at>, Artem Bityutskiy <dedekind1@...il.com>, Adrian Hunter <adrian.hunter@...el.com>, ceph-devel@...r.kernel.org, Ext4 Developers List <linux-ext4@...r.kernel.org>, linux-f2fs-devel@...ts.sourceforge.net, linux-mtd@...ts.infradead.org, Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org Subject: Re: [PATCH v2] fs: Fix page_mkwrite off-by-one errors On Fri, Nov 29, 2019 at 6:21 AM Andreas Gruenbacher <agruenba@...hat.com> wrote: > > +/** > + * page_mkwrite_check_truncate - check if page was truncated > + * @page: the page to check > + * @inode: the inode to check the page against > + * > + * Returns the number of bytes in the page up to EOF, > + * or -EFAULT if the page was truncated. > + */ > +static inline int page_mkwrite_check_truncate(struct page *page, > + struct inode *inode) > +{ > + loff_t size = i_size_read(inode); > + pgoff_t end_index = (size + PAGE_SIZE - 1) >> PAGE_SHIFT; This special end_index calculation seems to be redundant. You later want "size >> PAGE_SHIFT" for another test, and that's actually the important part. The "+ PAGE_SIZE - 1" case is purely to handle the "AT the page boundary is special" case, but since you have to calculate "offset_in_page(size)" anyway, that's entirely redundant - the answer is part of that. So I think it would be better to write the logic as loff_t size = i_size_read(inode); pgoff_t index = size >> PAGE_SHIFT; int offset = offset_in_page(size); if (page->mapping != inode->i_mapping) return -EFAULT; /* Page is wholly past the EOF page */ if (page->index > index) return -EFAULT; /* page is wholly inside EOF */ if (page->index < index) return PAGE_SIZE; /* bytes in a page? If 0, it's past EOF */ return offset ? offset : -PAGE_SIZE; instead. That avoids the unnecessary "round up" part, and simply uses the same EOF index for everything. Linus
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