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

[David Sterba <dsterba@suse.cz>]

On Sun, Jul 11, 2021 at 09:52:56AM -0500, Eric Biggers wrote:
> 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.
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Co-developed-by: Chris Mason <clm@fb.com>
> > Signed-off-by: Chris Mason <clm@fb.com>
> > Signed-off-by: Boris Burkov <boris@bur.io>
> 
> Generally looks good, feel free to add:
> 
> Acked-by: Eric Biggers <ebiggers@google.com>
> 
> A few minor comments below:

Thanks for the comments. Lots of them are minor fixups, I can do that
when applying the patch. There are some questions that I'll leave to
Boris to answer, I don't think they'd prevent merging the patches now
and fixing up later.