From: "Darrick J. Wong" Subject: Re: [PATCH 17/28] ext4: Calculate and verify checksums for htree nodes Date: Thu, 13 Oct 2011 00:21:58 -0700 Message-ID: <20111013072158.GR12447@tux1.beaverton.ibm.com> References: <20111008075343.20506.23155.stgit@elm3c44.beaverton.ibm.com> <20111008075536.20506.13681.stgit@elm3c44.beaverton.ibm.com> Reply-To: djwong@us.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Theodore Tso , Sunil Mushran , Martin K Petersen , Greg Freemyer , Amir Goldstein , linux-kernel , Andi Kleen , Mingming Cao , Joel Becker , linux-fsdevel , linux-ext4@vger.kernel.org, Coly Li To: Andreas Dilger Return-path: Received: from e37.co.us.ibm.com ([32.97.110.158]:55118 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751453Ab1JMHVs (ORCPT ); Thu, 13 Oct 2011 03:21:48 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Oct 12, 2011 at 05:27:29PM -0600, Andreas Dilger wrote: > On 2011-10-08, at 1:55 AM, Darrick J. Wong wrote: > > Calculate and verify the checksum for directory index tree (htree) node blocks. The checksum is stored in the last 4 bytes of the htree block and requires the dx_entry array to stop 1 dx_entry short of the end of the block. > > > > +/* > > + * This goes at the end of each htree block. If you want to use the > > + * reserved field, you'll have to update the checksum code to include it. > > + */ > > +struct dx_tail { > > + u32 reserved; > > + u32 checksum; /* crc32c(uuid+inum+dirblock) */ > > +}; > > Why exclude the reserved field from the checksum? That would mean that > the checksum value will depend on whether the other feature is in use > or not, which will make everything more complicated in the future. > > Better to always set it to zero for now, and if it is used in the future > then it can be set to whatever value is needed and the checksum code > will remain the same in both the kernel and e2fsprogs. As a minor speed optimization, the htree checksum only covers the fake dirent structures, the htree block header, and header.count dx_entry structs, which means that in the common case it won't come anywhere close to checksumming all 4096 bytes. But you do make a compelling case to cover that reserved field even if it's zero now. > > +/* checksumming functions */ > > +static struct dx_countlimit *get_dx_countlimit(struct inode *inode, > > + struct ext4_dir_entry *dirent, > > + int *offset) > > +{ > > + if (le16_to_cpu(dirent->rec_len) == EXT4_BLOCK_SIZE(inode->i_sb)) > > + count_offset = 8; > > + else if (le16_to_cpu(dirent->rec_len) == 12) { > > + dp = (struct ext4_dir_entry *)(((void *)dirent) + 12); > > + if (le16_to_cpu(dp->rec_len) != > > + EXT4_BLOCK_SIZE(inode->i_sb) - 12) > > + return NULL; > > + root = (struct dx_root_info *)(((void *)dp + 12)); > > + if (root->reserved_zero || > > + root->info_length != sizeof(struct dx_root_info)) > > + return NULL; > > + count_offset = 32; > > + } else > > + return NULL; > > (style) if one branch of an if-else has braces, they all should Ok. > > +static void ext4_dx_csum_set(struct inode *inode, struct ext4_dir_entry *dirent) > > +{ > > + struct dx_countlimit *c; > > + struct dx_tail *t; > > + int count_offset, limit, count; > > + > > + if (!EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb, > > + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM)) > > It would be nice to add some macros to clean up the feature flag checks > (in a separate patch, but this long line reminded me of it): > > #define EXT4_ROCOMPAT(sb, feature) \ > EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_ ## feature) > > Then the code can be changed to use: > > if (!EXT4_ROCOMPAT(inode->i_sb, METADATA_CSUM)) > > which is not only shorter and has a chance of fitting on one line, but > also avoids the occasional hard-to-find bug that uses a mismatched mask > and flag word, like: > > if (!EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb, > EXT4_FEATURE_INCOMPAT_FLEX_BG) > > With the helper macros, this would fail at compile time, because > EXT4_FEATURE_ROCOMPAT_FLEX_BG does not exist. Yes, that would make a nice (separate) cleanup patch. --D