Re: [PATCH v4 14/16] ext4: add basic fs-verity support

On Sat, Jun 15, 2019 at 11:31:12AM -0400, Theodore Ts'o wrote:

On Thu, Jun 06, 2019 at 08:52:03AM -0700, Eric Biggers wrote:

quoted

+/*
+ * Format of ext4 verity xattr.  This points to the location of the verity
+ * descriptor within the file data rather than containing it directly because
+ * the verity descriptor *must* be encrypted when ext4 encryption is used.  But,
+ * ext4 encryption does not encrypt xattrs.
+ */
+struct fsverity_descriptor_location {
+	__le32 version;
+	__le32 size;
+	__le64 pos;
+};

What's the benefit of storing the location in an xattr as opposed to
just keying it off the end of i_size, rounded up to next page size (or
64k) as I had suggested earlier?

Using an xattr burns xattr space, which is a limited resource, and it
adds some additional code complexity.  Does the benefits outweigh the
added complexity?

						- Ted

It means that only the fs/verity/ support layer has to be aware of the format of
the fsverity_descriptor, and the filesystem can just treat it an as opaque blob.

Otherwise the filesystem would need to read the first 'sizeof(struct
fsverity_descriptor)' bytes and use those to calculate the size as
'sizeof(struct fsverity_descriptor) + le32_to_cpu(desc.sig_size)', then read the
rest.  Is this what you have in mind?

Alternatively the filesystem could prepend the fsverity_descriptor with its
size, similar to how in the v1 and v2 patchsets there was an fsverity_footer
appended to the fsverity_descriptor.  But an xattr seems a cleaner approach to
store a few bytes that don't need to be encrypted.

Putting the verity descriptor before the Merkle tree also means that we'd have
to pass the desc_size to ->begin_enable_verity(), ->read_merkle_tree_page(), and
->write_merkle_tree_block(), versus just passing the merkle_tree_size to
->end_enable_verity().  This would be easy, but it would still add a bit of
complexity in the fsverity_operations rather than reduce it.

It's also somewhat nice to have the version number in the xattr, in case we ever
introduce a new fs-verity format for ext4 or f2fs.

So to me, it doesn't seem like the other possible solutions are better.

- Eric