From: Eric Biggers <ebiggers@xxxxxxxxxx> btrfs supports fs-verity since Linux v5.15. Document this. Acked-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- [resent to add linux-doc to CC] Documentation/filesystems/fsverity.rst | 53 +++++++++++++++----------- fs/verity/Kconfig | 10 ++--- 2 files changed, 35 insertions(+), 28 deletions(-) diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst index 756f2c215ba13..cb8e7573882a1 100644 --- a/Documentation/filesystems/fsverity.rst +++ b/Documentation/filesystems/fsverity.rst @@ -11,9 +11,9 @@ Introduction fs-verity (``fs/verity/``) is a support layer that filesystems can hook into to support transparent integrity and authenticity protection -of read-only files. Currently, it is supported by the ext4 and f2fs -filesystems. Like fscrypt, not too much filesystem-specific code is -needed to support fs-verity. +of read-only files. Currently, it is supported by the ext4, f2fs, and +btrfs filesystems. Like fscrypt, not too much filesystem-specific +code is needed to support fs-verity. fs-verity is similar to `dm-verity <https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_ @@ -473,9 +473,9 @@ files being swapped around. Filesystem support ================== -fs-verity is currently supported by the ext4 and f2fs filesystems. -The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity -on either filesystem. +fs-verity is supported by several filesystems, described below. The +CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on +any of these filesystems. ``include/linux/fsverity.h`` declares the interface between the ``fs/verity/`` support layer and filesystems. Briefly, filesystems @@ -544,6 +544,13 @@ Currently, f2fs verity only supports a Merkle tree block size of 4096. Also, f2fs doesn't support enabling verity on files that currently have atomic or volatile writes pending. +btrfs +----- + +btrfs supports fs-verity since Linux v5.15. Verity-enabled inodes are +marked with a RO_COMPAT inode flag, and the verity metadata is stored +in separate btree items. + Implementation details ====================== @@ -622,14 +629,14 @@ workqueue, and then the workqueue work does the decryption or verification. Finally, pages where no decryption or verity error occurred are marked Uptodate, and the pages are unlocked. -Files on ext4 and f2fs may contain holes. Normally, ``->readahead()`` -simply zeroes holes and sets the corresponding pages Uptodate; no bios -are issued. To prevent this case from bypassing fs-verity, these -filesystems use fsverity_verify_page() to verify hole pages. +On many filesystems, files can contain holes. Normally, +``->readahead()`` simply zeroes holes and sets the corresponding pages +Uptodate; no bios are issued. To prevent this case from bypassing +fs-verity, these filesystems use fsverity_verify_page() to verify hole +pages. -ext4 and f2fs disable direct I/O on verity files, since otherwise -direct I/O would bypass fs-verity. (They also do the same for -encrypted files.) +Filesystems also disable direct I/O on verity files, since otherwise +direct I/O would bypass fs-verity. Userspace utility ================= @@ -648,7 +655,7 @@ Tests To test fs-verity, use xfstests. For example, using `kvm-xfstests <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: - kvm-xfstests -c ext4,f2fs -g verity + kvm-xfstests -c ext4,f2fs,btrfs -g verity FAQ === @@ -771,15 +778,15 @@ weren't already directly answered in other parts of this document. e.g. magically trigger construction of a Merkle tree. :Q: Does fs-verity support remote filesystems? -:A: Only ext4 and f2fs support is implemented currently, but in - principle any filesystem that can store per-file verity metadata - can support fs-verity, regardless of whether it's local or remote. - Some filesystems may have fewer options of where to store the - verity metadata; one possibility is to store it past the end of - the file and "hide" it from userspace by manipulating i_size. The - data verification functions provided by ``fs/verity/`` also assume - that the filesystem uses the Linux pagecache, but both local and - remote filesystems normally do so. +:A: So far all filesystems that have implemented fs-verity support are + local filesystems, but in principle any filesystem that can store + per-file verity metadata can support fs-verity, regardless of + whether it's local or remote. Some filesystems may have fewer + options of where to store the verity metadata; one possibility is + to store it past the end of the file and "hide" it from userspace + by manipulating i_size. The data verification functions provided + by ``fs/verity/`` also assume that the filesystem uses the Linux + pagecache, but both local and remote filesystems normally do so. :Q: Why is anything filesystem-specific at all? Shouldn't fs-verity be implemented entirely at the VFS level? diff --git a/fs/verity/Kconfig b/fs/verity/Kconfig index 54598cd801457..aad1f1d998b9d 100644 --- a/fs/verity/Kconfig +++ b/fs/verity/Kconfig @@ -14,11 +14,11 @@ config FS_VERITY help This option enables fs-verity. fs-verity is the dm-verity mechanism implemented at the file level. On supported - filesystems (currently EXT4 and F2FS), userspace can use an - ioctl to enable verity for a file, which causes the filesystem - to build a Merkle tree for the file. The filesystem will then - transparently verify any data read from the file against the - Merkle tree. The file is also made read-only. + filesystems (currently ext4, f2fs, and btrfs), userspace can + use an ioctl to enable verity for a file, which causes the + filesystem to build a Merkle tree for the file. The filesystem + will then transparently verify any data read from the file + against the Merkle tree. The file is also made read-only. This serves as an integrity check, but the availability of the Merkle tree root hash also allows efficiently supporting base-commit: b13baccc3850ca8b8cccbf8ed9912dbaa0fdf7f3 -- 2.36.1
