ext4: update on-disk format documentation for fs-verity

Document the format of verity files on ext4, and the corresponding inode
and superblock flags.

Reviewed-by: Theodore Ts'o <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>

Documentation/filesystems/ext4/inodes.rst

···
      - This is a huge file (EXT4\_HUGE\_FILE\_FL).
    * - 0x80000
      - Inode uses extents (EXT4\_EXTENTS\_FL).
+   * - 0x100000
+     - Verity protected file (EXT4\_VERITY\_FL).
    * - 0x200000
      - Inode stores a large extended attribute value in its data blocks
        (EXT4\_EA\_INODE\_FL).
···
      - Reserved for ext4 library (EXT4\_RESERVED\_FL).
    * -
      - Aggregate flags:
-   * - 0x4BDFFF
+   * - 0x705BDFFF
      - User-visible flags.
-   * - 0x4B80FF
+   * - 0x604BC0FF
      - User-modifiable flags. Note that while EXT4\_JOURNAL\_DATA\_FL and
        EXT4\_EXTENTS\_FL can be set with setattr, they are not in the kernel's
        EXT4\_FL\_USER\_MODIFIABLE mask, since it needs to handle the setting of

Documentation/filesystems/ext4/overview.rst

···
 .. include:: bigalloc.rst
 .. include:: inlinedata.rst
 .. include:: eainode.rst
+.. include:: verity.rst

Documentation/filesystems/ext4/super.rst

···
        (RO\_COMPAT\_READONLY)
    * - 0x2000
      - Filesystem tracks project quotas. (RO\_COMPAT\_PROJECT)
+   * - 0x8000
+     - Verity inodes may be present on the filesystem. (RO\_COMPAT\_VERITY)
 
 .. _super_def_hash:
 

Documentation/filesystems/ext4/verity.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+Verity files
+------------
+
+ext4 supports fs-verity, which is a filesystem feature that provides
+Merkle tree based hashing for individual readonly files.  Most of
+fs-verity is common to all filesystems that support it; see
+:ref:`Documentation/filesystems/fsverity.rst <fsverity>` for the
+fs-verity documentation.  However, the on-disk layout of the verity
+metadata is filesystem-specific.  On ext4, the verity metadata is
+stored after the end of the file data itself, in the following format:
+
+- Zero-padding to the next 65536-byte boundary.  This padding need not
+  actually be allocated on-disk, i.e. it may be a hole.
+
+- The Merkle tree, as documented in
+  :ref:`Documentation/filesystems/fsverity.rst
+  <fsverity_merkle_tree>`, with the tree levels stored in order from
+  root to leaf, and the tree blocks within each level stored in their
+  natural order.
+
+- Zero-padding to the next filesystem block boundary.
+
+- The verity descriptor, as documented in
+  :ref:`Documentation/filesystems/fsverity.rst <fsverity_descriptor>`,
+  with optionally appended signature blob.
+
+- Zero-padding to the next offset that is 4 bytes before a filesystem
+  block boundary.
+
+- The size of the verity descriptor in bytes, as a 4-byte little
+  endian integer.
+
+Verity inodes have EXT4_VERITY_FL set, and they must use extents, i.e.
+EXT4_EXTENTS_FL must be set and EXT4_INLINE_DATA_FL must be clear.
+They can have EXT4_ENCRYPT_FL set, in which case the verity metadata
+is encrypted as well as the data itself.
+
+Verity files cannot have blocks allocated past the end of the verity
+metadata.