On Thu, Jan 24, 2019 at 06:22:37PM -0500, Theodore Y. Ts'o wrote:

The main issue is that for a 129 MB file, the Merkle data is going to
be a Megabyte.

127MB ... I pointed out this error the last time the documentation
was posted.

We could store the metadata somewhere else --- for example, we could
store it in another inode.  But inodes have overhead, and that would
mean using two inodes for every fs-verity protected files --- and we
don't need all of the other metadata (mtime, ctime, etc.) for the
Merkle tree.  So that's how we got to where we were.  I think the
approach of storing it using the same extent tree where we map logical
block numbers to physical block numbers make a lot of sense for ext4
and f2fs.

It seems that some file system (which may never even implement
fs-verity) their developers hate that particular approach.  So that's
where the suggestion of using a separate file descriptor to convey the
Merkle tree data to the file system came from.  It wasn't my first
choice.

I'll reiterate an API I suggested on December 21st:

: verity_fd = ioctl(fd, FS_IOC_VERITY_FD);
: write(verity_fd, &merkle_tree);
: close(verity_fd);
: 
: At final close of that verity_fd, the filesystem behaves in the same way
: that it does on receipt of this FS_IOC_ENABLE_VERITY ioctl today.