From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD971C433F5 for ; Tue, 14 Sep 2021 17:56:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BC43A610F9 for ; Tue, 14 Sep 2021 17:56:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230012AbhINR5r (ORCPT ); Tue, 14 Sep 2021 13:57:47 -0400 Received: from mail.kernel.org ([198.145.29.99]:47066 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229617AbhINR5r (ORCPT ); Tue, 14 Sep 2021 13:57:47 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id A002860FED; Tue, 14 Sep 2021 17:56:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1631642189; bh=FUgtlBLWyFRID8ToWCNWzOoRplAIHq4T/+tM3YN4HX0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H0YbxapIJl1IFk1UA9oHiymBBucKlTmg97i2ok/tT20btjWIgUYmCPoGGiMsl8S7Y +lBvC52luAAQgAbHrsUv9wNMGDJkvCyo4L9CwRthiKoCP24NFOJ/zIVwnAZy/KT3Sl c5/5WVaQiMFK9g+1BMegbZdkFWFQwWOCBCc886qUFHJCxCZRRNv5Le/fdd5eMrH/zR aSpAhHf62px6gxxIwN+z9SmLyEEyMB9hXIHsFHO8ZuFVY6TamIol8uAT5gLQ0QePfR ibokFf/qGJ4qpMuITHaPGYZqNA2C/39+P9Y9ghvqxSJD/ScaUVOgBSo6m2uHFr4616 8XOveadKithtw== Date: Tue, 14 Sep 2021 10:56:28 -0700 From: Eric Biggers To: Boris Burkov Cc: linux-btrfs@vger.kernel.org, linux-fscrypt@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH v6 2/3] btrfs: initial fsverity support Message-ID: References: <797d6524e4e6386fc343cd5d0bcdd53878a6593e.1625083099.git.boris@bur.io> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fscrypt@vger.kernel.org On Tue, Sep 14, 2021 at 10:49:33AM -0700, Boris Burkov wrote: > On Tue, Sep 14, 2021 at 10:32:59AM -0700, Eric Biggers wrote: > > Hi Boris, > > > > On Wed, Jun 30, 2021 at 01:01:49PM -0700, Boris Burkov wrote: > > > Add support for fsverity in btrfs. To support the generic interface in > > > fs/verity, we add two new item types in the fs tree for inodes with > > > verity enabled. One stores the per-file verity descriptor and btrfs > > > verity item and the other stores the Merkle tree data itself. > > > > > > Verity checking is done in end_page_read just before a page is marked > > > uptodate. This naturally handles a variety of edge cases like holes, > > > preallocated extents, and inline extents. Some care needs to be taken to > > > not try to verity pages past the end of the file, which are accessed by > > > the generic buffered file reading code under some circumstances like > > > reading to the end of the last page and trying to read again. Direct IO > > > on a verity file falls back to buffered reads. > > > > > > Verity relies on PageChecked for the Merkle tree data itself to avoid > > > re-walking up shared paths in the tree. For this reason, we need to > > > cache the Merkle tree data. Since the file is immutable after verity is > > > turned on, we can cache it at an index past EOF. > > > > > > Use the new inode ro_flags to store verity on the inode item, so that we > > > can enable verity on a file, then rollback to an older kernel and still > > > mount the file system and read the file. Since we can't safely write the > > > file anymore without ruining the invariants of the Merkle tree, we mark > > > a ro_compat flag on the file system when a file has verity enabled. > > > > I want to mention the btrfs verity support in > > Documentation/filesystems/fsverity.rst, and I have a couple questions: > > > > 1. Is the ro_compat filesystem flag still a thing? The commit message claims it > > is, and BTRFS_FEATURE_COMPAT_RO_VERITY is defined in the code, but it doesn't > > seem to actually be used. It's not needed since you found a way to make the > > inode flags ro_compat instead, right? > > I believe it is still being used, unless I messed up the patch I sent in > the end. Taking a quick look, I think it's set at fs/btrfs/verity.c:558. > > btrfs_set_fs_compat_ro(root->fs_info, VERITY); > > I believe I still needed it because the tree checker doesn't scan every > inode on the filesystem when you mount, so it would only freak out about > a ro-compat inode later on if the inode didn't happen to be in a leaf > that was being checked at mount time. > Okay, so it is used. (Due to the macro, it didn't show up when grepping.) Doesn't it defeat the purpose of a ro_compat inode flag if the whole filesystem is marked with a ro_compat feature flag, though? I thought that the point of the ro_compat inode flag is to allow old kernels to mount the filesystem read-write, with only verity files being forced to read-only. That would be more flexible than ext4's implementation of fs-verity which forces the whole filesystem to read-only. But it seems you're forcing the whole filesystem to read-only anyway? - Eric