fs-verity: support reading Merkle tree with ioctl

Add support for FS_VERITY_METADATA_TYPE_MERKLE_TREE to
FS_IOC_READ_VERITY_METADATA.  This allows a userspace server program to
retrieve the Merkle tree of a verity file for serving to a client which
implements fs-verity compatible verification.  See the patch which
introduced FS_IOC_READ_VERITY_METADATA for more details.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-5-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst
index 9ef7a7de6008..50b47a6d9ea1 100644
--- a/Documentation/filesystems/fsverity.rst
+++ b/Documentation/filesystems/fsverity.rst
@@ -234,6 +234,8 @@ need this ioctl.
 
 This ioctl takes in a pointer to the following structure::
 
+   #define FS_VERITY_METADATA_TYPE_MERKLE_TREE     1
+
    struct fsverity_read_metadata_arg {
            __u64 metadata_type;
            __u64 offset;
@@ -242,7 +244,13 @@ This ioctl takes in a pointer to the following structure::
            __u64 __reserved;
    };
 
-``metadata_type`` specifies the type of metadata to read.
+``metadata_type`` specifies the type of metadata to read:
+
+- ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
+  Merkle tree.  The blocks are returned in order from the root level
+  to the leaf level.  Within each level, the blocks are returned in
+  the same order that their hashes are themselves hashed.
+  See `Merkle tree`_ for more information.
 
 The semantics are similar to those of ``pread()``.  ``offset``
 specifies the offset in bytes into the metadata item to read from, and