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Fri, 20 Mar 2020 18:00:35 +0000 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 678A120739; Fri, 20 Mar 2020 18:00:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584727219; bh=AOHcFFqPmmUrKkBgj3LGw8anyo9Fi+nPIa1g3zqyqMs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=NZ9HU316DvAVNcUxlvzbNwg+y2txH2gpkS3rUJxhCWe7t2TNzS4cSz0aP9oXCmCeZ lA62gX2HF5PTlSZ1WRtHNFeEZvmAX/Jupq9oBbL1ohyHNdGKoAs/qCozdPyM6IUdHI QNBy8s3jas9smKOGMMh5TBeJx1mwdzpN9wtLUJa0= Date: Fri, 20 Mar 2020 11:00:17 -0700 From: Eric Biggers To: Matthew Wilcox Message-ID: <20200320180017.GE851@sol.localdomain> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> <20200320165828.GB851@sol.localdomain> <20200320173040.GB4971@bombadil.infradead.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200320173040.GB4971@bombadil.infradead.org> X-Headers-End: 1jFLwc-00APSa-9D Subject: Re: [f2fs-dev] [PATCH v9 12/25] mm: Move end_index check out of readahead loop X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: cluster-devel@redhat.com, linux-mm@kvack.org, John Hubbard , linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, William Kucharski , linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, ocfs2-devel@oss.oracle.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net 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 _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel