84fb7ca4b3
Document the format of verity files on ext4, and the corresponding inode and superblock flags. Reviewed-by: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Eric Biggers <ebiggers@google.com>
42 lines
1.6 KiB
ReStructuredText
42 lines
1.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Verity files
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------------
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ext4 supports fs-verity, which is a filesystem feature that provides
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Merkle tree based hashing for individual readonly files. Most of
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fs-verity is common to all filesystems that support it; see
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:ref:`Documentation/filesystems/fsverity.rst <fsverity>` for the
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fs-verity documentation. However, the on-disk layout of the verity
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metadata is filesystem-specific. On ext4, the verity metadata is
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stored after the end of the file data itself, in the following format:
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- Zero-padding to the next 65536-byte boundary. This padding need not
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actually be allocated on-disk, i.e. it may be a hole.
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- The Merkle tree, as documented in
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:ref:`Documentation/filesystems/fsverity.rst
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<fsverity_merkle_tree>`, with the tree levels stored in order from
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root to leaf, and the tree blocks within each level stored in their
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natural order.
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- Zero-padding to the next filesystem block boundary.
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- The verity descriptor, as documented in
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:ref:`Documentation/filesystems/fsverity.rst <fsverity_descriptor>`,
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with optionally appended signature blob.
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- Zero-padding to the next offset that is 4 bytes before a filesystem
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block boundary.
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- The size of the verity descriptor in bytes, as a 4-byte little
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endian integer.
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Verity inodes have EXT4_VERITY_FL set, and they must use extents, i.e.
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EXT4_EXTENTS_FL must be set and EXT4_INLINE_DATA_FL must be clear.
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They can have EXT4_ENCRYPT_FL set, in which case the verity metadata
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is encrypted as well as the data itself.
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Verity files cannot have blocks allocated past the end of the verity
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metadata.
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