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? 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. Thanks, - Eric
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