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. > > 2. Is there a minimum version of btrfs-progs that is required to use btrfs > verity? With ext4 and f2fs, the fsck tools had to be updated, so there were > minimum versions of the userspace tools required. Hmm. I didn't update fsck, but now that you mention it, I think I need to... I'll test it right away and get back to you, but I suspect I need to hurry up and implement it. Boris > > Thanks, > > - Eric
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