On Jan 28, 2018, at 6:03 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> 
> So if there as a solution which enapculated the information needed to
> create the fs-verity header and the PKCS7 signature in an xattr ---
> which is how you carry it around in the tar image --- and when the
> tarfile is unpacked, the software which does the unpacking calls a
> library which checks for the xattr, removes it, writes out the
> fsverity header and Merkle tree, and then calls the ioctl which sets
> the "verity" bit, thus instantiating the data integrity protection,
> would this meet your requirements?
> 
> In other words, the xattr in the tar file is just the method for
> carrying the information; and (a) it is not how the information would
> be stored in the underlying file system when it is actually used, and
> (b) it requires the userspace code to do this transformation, so we
> don't have to build the Merkle tree in the kernel.  Is this sufficient
> for your container use case?

Why would you throw away the xattr signature at that point?  That would
still be useful to store back into a new tarball or other backup of the
file, and is also useful for external tools to verify (e.g. bittorrent,
maybe rsync or others in the future), especially if it is a common format.

That doesn't mean storing the whole fs-verity tree in an xattr (though
that could also be done with the ext4 xattr inode feature), nor does it
imply that this is a "magic" xattr that causes the fs-verity tree to
be created when written.  It would just be an xattr that stores the
top-level hash of the Merkle tree to expose to userspace.

Also, the Merkle tree should be computed with fixed-size leaf blocks
like 1KB or 4KB, so that userspace doesn't need to know details like the
underlying fs blocksize.  The actual on-disk fs-verity tree can be stored
in fs blocksize units, and if the blocksize is larger than the leaf blocks
it would just store a higher-level node in the Merkle tree.

We actually worked on a design for this years ago with Lustre+ZFS, with
sending higher-level nodes in the Merkle tree over the network for the
RPC checksum so that it is independent of the client/server PAGE_SIZE,
the RPC size and the underlying fs blocksize, but this would also be
useful for storage in tarballs or for read/write requests with fs-verity.

For more details see:
http://wiki.old.lustre.org/images/c/c1/End-to-End-Integrity-2009-06-15.pdf

Cheers, Andreas