Re: [PATCH v6 2/3] btrfs: initial fsverity support

[Eric Biggers <ebiggers@kernel.org>]

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