[v2,01/12] fs-verity: add a documentation file

Message ID	20181101225230.88058-2-ebiggers@kernel.org (mailing list archive)
State	Superseded
Headers	show Return-Path: <linux-fscrypt-owner@kernel.org> From: Eric Biggers <ebiggers@kernel.org> To: linux-fscrypt@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, "Theodore Y . Ts'o" <tytso@mit.edu>, Jaegeuk Kim <jaegeuk@kernel.org>, Victor Hsieh <victorhsieh@google.com>, Chandan Rajendra <chandan@linux.vnet.ibm.com> Subject: [PATCH v2 01/12] fs-verity: add a documentation file Date: Thu, 1 Nov 2018 15:52:19 -0700 Message-Id: <20181101225230.88058-2-ebiggers@kernel.org> In-Reply-To: <20181101225230.88058-1-ebiggers@kernel.org> References: <20181101225230.88058-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-fscrypt-owner@vger.kernel.org Precedence: bulk
Series	fs-verity: read-only file-based authenticity protection \| expand [v2,00/12] fs-verity: read-only file-based authenticity protection [v2,01/12] fs-verity: add a documentation file [v2,02/12] fs-verity: add setup code, UAPI, and Kconfig [v2,03/12] fs-verity: add MAINTAINERS file entry [v2,04/12] fs-verity: add data verification hooks for ->readpages() [v2,05/12] fs-verity: implement FS_IOC_ENABLE_VERITY ioctl [v2,06/12] fs-verity: implement FS_IOC_MEASURE_VERITY ioctl [v2,07/12] fs-verity: add SHA-512 support [v2,08/12] fs-verity: add CRC-32C support [v2,09/12] fs-verity: support builtin file signatures [v2,10/12] ext4: add basic fs-verity support [v2,11/12] ext4: add fs-verity read support [v2,12/12] f2fs: fs-verity support

diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst new file mode 100644 index 0000000000000..d633fc0567bd5 --- /dev/null +++ b/Documentation/filesystems/fsverity.rst @@ -0,0 +1,583 @@ +======================================================== +Read-only file-based authenticity protection (fs-verity) +======================================================== + +Introduction +============ + +fs-verity (``fs/verity/``) is a library 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. Similar to 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>`_ +but works on files rather than block devices. On supported +filesystems, userspace can append a Merkle tree (hash tree) to a file, +then use an ioctl to enable fs-verity on it. Then, the filesystem +transparently verifies all data read from the file against the Merkle +tree; reads that fail verification will fail. The filesystem also +hides or moves the Merkle tree, and forbids changes to the file's +contents via the syscall interface. + +Essentially, fs-verity is a way of efficiently hashing a file, subject +to the caveat that the enforcement of that hash happens on-demand as +reads occur. The file hash that fs-verity computes is called the +"file measurement"; this is the hash of the Merkle tree's root hash +and certain other fs-verity metadata, and it takes constant time to +compute regardless of the size of the file. Note: the value of the +fs-verity file measurement will differ from a regular hash of the +file, even when they use the same hash algorithm, e.g. SHA-256; +however, they achieve the same purpose. + +Use cases +========= + +In general, fs-verity does not replace or obsolete dm-verity. +dm-verity should still be used when it is possible to authenticate the +full block device, i.e. when the device is read-only. fs-verity is +intended for use on read-write filesystems where dm-verity cannot be +used. + +fs-verity is most useful for hashing large files where only a small +portion may be accessed. For example, it's useful on Android +application package (APK) files, which typically contain many +translations, classes, and other resources that are infrequently or +even never accessed on a particular device. It would be wasteful to +hash the entire file before starting the application. + +Unlike an ahead-of-time hash, fs-verity also re-verifies data each +time it's paged in, which ensures the file measurement remains +correctly enforced even if the file contents are modified from +underneath the filesystem, e.g. by malicious disk firmware. + +fs-verity can support various use cases, such as: + +- Integrity protection (detecting accidental corruption) +- Auditing (logging file hashes before use) +- Authenticity protection (detecting malicious modification) + +Note that the latter two are not features of fs-verity per se, but +rather fs-verity is a tool for supporting these use cases. For +example, for the overall system to actually provide authenticity +protection, the file measurement itself must still be authenticated, +e.g. by comparing it with a known good value or by verifying a digital +signature of it. + +This can be userspace driven, in which case fs-verity will only be +used (essentially) as a fast way of hashing the file contents, via the +`FS_IOC_MEASURE_VERITY`_ ioctl. For authenticity protection, trusted +userspace code [#]_ still must verify the relevant portions of the +untrusted filesystem state before it is used in a security-critical +way, such as executing code from it. + +For example, the trusted userspace code might verify that the file +located at ``/foo/bar/baz`` has an fs-verity file measurement of +``sha256:a83d5cd722ef0d070b23353c2d9f316c38425114da8bd007cb9e8499371a97b3``, +or that all security-critical files (e.g. executable code) have stored +alongside them a valid digital signature (signed by a known, trusted +public key) of their fs-verity file measurement, potentially combined +with other important file metadata such as path and SELinux label. + +However, for ease of use, a subset of this policy logic (but not all +of it!) is also supported in the kernel by the `Built-in signature +verification`_ mechanism. Support for fs-verity file hashes in IMA +(Integrity Measurement Architecture) policies is also planned. + +.. [#] For example, on Android, "trusted userspace code" would be code + running from the system or vendor partitions, which are + read-only partitions authenticated by dm-verity tied into + Verified Boot, as opposed to the userdata partition which is + read-write. + +Metadata format +=============== + +Merkle tree +----------- + +fs-verity uses the same Merkle tree (hash tree) format as dm-verity; +the only difference is that fs-verity's Merkle tree is built over the +contents of a regular file rather than a block device. + +Briefly, the file contents is divided into blocks, where the blocksize +is configurable but usually 4096 bytes. The last block is zero-padded +if needed. Each block is then hashed, producing the first level of +hashes. Then, the hashes in this first level are grouped into +'blocksize'-byte blocks (zero-padding the ends as needed) and these +blocks are hashed, producing the second level of hashes. This +proceeds up the tree until only a single block remains. The hash of +this block is called the "Merkle tree root hash". Note: if the entire +file contents fit in one block, then there are no hash blocks and the +"Merkle tree root hash" is simply the hash of the data block. + +The blocks of the Merkle tree are stored on-disk starting from the +root level and then proceeding to store each level down to the "first" +(the level that gives the hashes of the data blocks). + +The hash algorithm is configurable. The default is SHA-256, but +SHA-512 is also supported. The non-cryptographic checksum CRC-32C is +also supported for integrity-only use cases such as detecting bit +errors in read-only backup files. A non-cryptographic checksum must +not be used if authenticity protection is desired. + +In the recommended configuration of SHA-256 and 4K blocks, 128 hash +values fit in each block. Thus, each level of the hash tree is 128 +times smaller than the previous, and for large files the Merkle tree's +size converges to approximately 1/129 of the original file size. +However, for small files, the padding to a block boundary is +significant, making the space overhead proportionally more. + +fs-verity descriptor +-------------------- + +For each file, fs-verity also uses an additional on-disk metadata +structure called the *fs-verity descriptor*. This contains the +properties of the Merkle tree and some other information. It begins +with a header in the following format:: + + struct fsverity_descriptor { + __u8 magic[8]; + __u8 major_version; + __u8 minor_version; + __u8 log_data_blocksize; + __u8 log_tree_blocksize; + __le16 data_algorithm; + __le16 tree_algorithm; + __le32 flags; + __le32 reserved1; + __le64 orig_file_size; + __le16 auth_ext_count; + __u8 reserved2[30]; + }; + +This structure contains: + +- ``magic`` is the ASCII bytes "FSVerity". +- ``major_version`` is 1. +- ``minor_version`` is 0. +- ``log_data_blocksize`` and ``log_tree_blocksize`` are the log base 2 + of the block size (in bytes) of data blocks and Merkle tree blocks, + respectively. Currently, in both cases the kernel only supports + page-sized blocks, i.e. on most architectures, 4096-byte blocks. + Thus, usually both of these fields must be 12. +- ``data_algorithm`` and ``tree_algorithm`` are the hash algorithms + used to hash data blocks and Merkle tree blocks, respectively. + Currently the kernel requires these to have the same value. The + recommended value is FS_VERITY_ALG_SHA256. See + ``include/uapi/linux/fsverity.h`` for the list of allowed values. +- ``orig_file_size`` is the original size of the file in bytes. This + means the size excluding the verity metadata and padding. +- ``auth_ext_count`` is the number of authenticated extensions that + follow. +- All other fields are zeroed. + +Following the ``struct fsverity_descriptor``, there is a list of +"authenticated extensions". Each extension is a variable-length +structure that begins with the following header:: + + struct fsverity_extension { + __le32 length; + __le16 type; + __le16 reserved; + }; + +This structure contains: + +- ``length`` is the length of this extension in bytes, including the + header. +- ``type`` is the extension number. See + ``include/uapi/linux/fsverity.h`` for the allowed values. +- ``reserved`` must be 0. + +Each extension begins on an 8-byte aligned boundary. When an +extension's length is not a multiple of 8, it must be zero-padded to +the next 8-byte boundary, even if it is the last extension. This zero +padding is not counted in the ``length`` field. + +This first list of extensions is "authenticated", meaning that they +are included in the file measurement. Currently, the following +authenticated extensions are supported. Except where otherwise +indicated, extensions are optional and cannot be given multiple times: + +- FS_VERITY_EXT_ROOT_HASH: This is mandatory. It gives the root hash + of the Merkle tree, as a byte array. +- FS_VERITY_EXT_SALT: A salt to salt the hashes with, given as a byte + array. The salt is prepended to every block that is hashed. Any + length salt is supported. Note that using a unique salt for every + file should make it more difficult for fs-verity to be attacked + across many files. However, in principle this is unnecessary since + simply choosing a strong cryptographic hash algorithm such as + SHA-256 or SHA-512 should be sufficient. + +Following the authenticated extensions, there is a list of +unauthenticated extensions. These are *not* included in the file +measurement. This list begins with a header:: + + __le16 unauth_ext_count; + __le16 padding[3]; + +``unauth_ext_count`` is the number of unauthenticated extensions. +This may be 0. + +Like authenticated extensions, each unauthenticated extension begins +with the header ``struct fsverity_extension`` from above. + +The following types of unauthenticated extensions are supported: + +- FS_VERITY_EXT_PKCS7_SIGNATURE. This is a DER-encoded PKCS#7 message + containing the signed file measurement. See `Built-in signature + verification`_ for details. + +fsveritysetup format +-------------------- + +When enabling fs-verity on a file via the `FS_IOC_ENABLE_VERITY`_ +ioctl, the kernel requires that the verity metadata has been appended +to the file contents. Specifically, the file must be arranged as: + +#. Original file contents +#. Zero-padding to next block boundary +#. `Merkle tree`_ +#. `fs-verity descriptor`_ +#. fs-verity footer + +We call this file format the "fsveritysetup format". It is not +necessarily the on-disk format actually used by the filesystem, since +the filesystem is free to move things around during the ioctl. +However, the easiest way to implement fs-verity is to just keep this +arrangement in-place, as ext4 and f2fs do; see `Filesystem support`_. + +Note that "block" here means the fs-verity block size, which is not +necessarily the same as the filesystem's block size. For example, on +ext4, fs-verity can use 4K blocks on top of a filesystem formatted to +use a 1K block size. + +The fs-verity footer is a structure of the following format:: + + struct fsverity_footer { + __le32 desc_reverse_offset; + __u8 magic[8]; + }; + +``desc_reverse_offset`` is the distance in bytes from the end of the +fs-verity footer to the beginning of the fs-verity descriptor; this +allows software to find the fs-verity descriptor. ``magic`` is the +ASCII bytes "FSVerity"; this allows software to quickly identify a +file as being in the "fsveritysetup" format as well as find the +fs-verity footer if zeroes have been appended. + +The kernel cannot handle fs-verity footers that cross a page boundary. +Padding must be prepended as needed to meet this constaint. + +Filesystem support +================== + +ext4 +---- + +ext4 supports fs-verity since kernel version TODO. + +CONFIG_EXT4_FS_VERITY must be enabled in the kernel config. Also, the +filesystem must have been formatted with ``-O verity``, or had +``tune2fs -O verity`` run on it. These require e2fsprogs v1.44.4-2 or +later. This e2fsprogs version is also required for e2fsck to +understand the verity feature. Since "verity" is an RO_COMPAT +feature, once enabled earlier kernels will be unable to mount the +filesystem for writing, and earlier versions of e2fsck will be unable +to check the filesystem. + +ext4 only allows fs-verity on extent-based files. + +The EXT4_VERITY_FL flag in the inode is used to indicate that the +inode uses fs-verity. This bit cannot be set directly; it can only be +set indirectly via `FS_IOC_ENABLE_VERITY`_. + +When enabling verity on an inode, ext4 leaves the verity metadata +in-place in the `fsveritysetup format`_. However, it changes the +on-disk i_size to the original file size, which allows the verity +feature to be RO_COMPAT rather than INCOMPAT. Later, the fs-verity +footer is found by scanning backwards from the end of the last extent +rather than from i_size. + +f2fs +---- + +f2fs supports fs-verity since kernel version TODO. + +CONFIG_F2FS_FS_VERITY must be enabled in the kernel config. Also, the +filesystem must have been formatted with ``-O verity``. This requires +f2fs-tools v1.11.0 or later. + +The FADVISE_VERITY_BIT flag in the inode is used to indicate that the +inode uses fs-verity. This bit cannot be set directly; it can only be +set indirectly via `FS_IOC_ENABLE_VERITY`_. + +When enabling verity on an inode, f2fs leaves the verity metadata +in-place in the `fsveritysetup format`_. It leaves the on-disk i_size +as the full file size; however, the in-memory i_size is overridden +with the original size. + +User API +======== + +FS_IOC_ENABLE_VERITY +-------------------- + +The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a regular file. +Userspace must have already appended verity metadata to the file, +using the file format described in `fsveritysetup format`_. +Additionally, the filesystem must support fs-verity. + +The argument parameter for this ioctl is reserved and must be NULL. + +This ioctl checks for write access to the inode; no capability is +required. However, it must be executed on an O_RDONLY file +descriptor, and no processes may have the file open for writing. +(This is necessary to prevent various race conditions.) + +On success, this ioctl returns 0, and the file becomes a verity file. +This means that: + +- The filesystem marks the file as a verity file both in-memory and + on-disk, e.g. by setting a bit in the inode. +- All later reads from the file are verified against the Merkle tree. +- The verity metadata at the end of the file is hidden or moved. +- Opening the file for writing or truncating it is no longer allowed. +- There is no way to disable verity on the file, other than by + deleting it and replacing it with a copy. + +If this ioctl fails, then no changes are made to the file. The +reasons it might fail include: + +- ``EACCES``: the process does not have write access to the file +- ``EBADMSG``: the file's fs-verity metadata is invalid +- ``EEXIST``: the file already has fs-verity enabled +- ``EINVAL``: a value was specified for the reserved argument + parameter, or the file descriptor refers to neither a regular file + nor a directory +- ``EIO``: an I/O error occurred +- ``EISDIR``: the file descriptor refers to a directory, not a regular + file +- ``ENOTTY``: this type of filesystem does not implement fs-verity +- ``EOPNOTSUPP``: the kernel was not configured with fs-verity support + for this filesystem, or the filesystem superblock has not had the + 'verity' feature enabled on it. (See `Filesystem support`_.) +- ``EPERM``: the file is append-only +- ``EROFS``: the filesystem is read-only +- ``ETXTBSY``: the file is open for writing. Note that this can be + the caller's file descriptor, or another open file descriptor, or + the file reference held by a writable memory map. + +FS_IOC_MEASURE_VERITY +--------------------- + +The FS_IOC_MEASURE_VERITY ioctl retrieves the fs-verity measurement of +a regular file. This is a digest that cryptographically summarizes +the file contents that are being enforced on reads. The file must +have fs-verity enabled. + +This ioctl takes in a pointer to a variable-length structure:: + + struct fsverity_digest { + __u16 digest_algorithm; + __u16 digest_size; /* input/output */ + __u8 digest[]; + }; + +``digest_size`` is an input/output field. On input, it must be +initialized to the number of bytes allocated for the variable-length +``digest`` field. + +On success, 0 is returned and the kernel fills in the structure as +follows: + +- ``digest_algorithm`` will be the hash algorithm used for the file + measurement. It will match the algorithm used in the Merkle tree, + e.g. FS_VERITY_ALG_SHA256. See ``include/uapi/linux/fsverity.h`` + for the list of possible values. +- ``digest_size`` will be the size of the digest in bytes, e.g. 32 + for SHA-256. (This can be redundant with ``digest_algorithm``.) +- ``digest`` will be the actual bytes of the digest. + +This ioctl is guaranteed to be very fast. Due to fs-verity's use of a +Merkle tree, its running time is independent of the file size. + +This ioctl can fail with the following errors: + +- ``EFAULT``: invalid buffer was specified +- ``ENODATA``: the file is not a verity file +- ``ENOTTY``: this type of filesystem does not implement fs-verity +- ``EOPNOTSUPP``: the kernel was not configured with fs-verity support + for this filesystem, or the filesystem superblock has not had the + 'verity' feature enabled on it. (See `Filesystem support`_.) +- ``EOVERFLOW``: the file measurement is longer than the specified + ``digest_size`` bytes. Try providing a larger buffer. + +Access semantics +================ + +fs-verity only implements reads, not writes. Therefore, after it is +enabled on a given file, regardless of the mode bits filesystems will +forbid opening the file for writing as well as changing the size of +the file via truncate(). The error code received for this is EPERM. + +However, fs-verity does not measure metadata such as owner, mode, +timestamps, and xattrs. Therefore, changes to these are still +allowed. + +For read-only access, fs-verity is intended to be transparent; no +changes to userspace applications should be needed. However, astute +users may notice some slight differences in behavior: + +- Direct I/O is not supported on verity files. Attempts to use direct + I/O on such files will fall back to buffered I/O. + +- DAX (Direct Access) is not supported on verity files. + +Note: read-only mmaps are supported, as is combining fs-verity and +fscrypt. + +Verity files can be sparse; holes are still verified. + +In-kernel policies +================== + +Built-in signature verification +------------------------------- + +With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting +a portion of an authentication policy (see `Use cases`_) in the +kernel. Specifically, it adds support for: + +1. At fs-verity module initialization time, a keyring ".fs-verity" is + created. The root user can add trusted X.509 certificates to this + keyring using the add_key() system call, then (when done) + optionally use keyctl_restrict_keyring() to prevent additional + certificates from being added. + +2. When a PKCS7_SIGNATURE extension containing a signed file + measurement is found in a file's verity metadata, the kernel will + verify this signature against the certificates in the ".fs-verity" + keyring, and verify that it matches the actual file measurement. + The extension must contain the PKCS#7 formatted signature in DER + format, where the signed data is the file measurement as a ``struct + fsverity_digest`` as described for `FS_IOC_MEASURE_VERITY`_ except + that all fields must be little-endian rather than native endian. + +3. A new sysctl "fs.verity.require_signatures" is made available. + When set to 1, the kernel requires that all fs-verity files have a + correctly signed file measurement as described in (2). + +This is meant as a relatively simple mechanism that can be used to +provide some level of authenticity protection for fs-verity files, as +an alternative to doing the signature verification in userspace or +using IMA-appraisal. However, with this mechanism, userspace programs +still need to check that the fs-verity bit is set, and there is no +protection against fs-verity files being swapped around. + +Implementation details +====================== + +I/O path design +--------------- + +To support fs-verity, the filesystem's ``->readpage()`` and +``->readpages()`` methods are modified to verify the data pages before +they are marked Uptodate. Merely hooking ``->read_iter()`` would be +insufficient, since ``->read_iter()`` is not used for memory maps. +fs-verity exposes functions to verify data: + +- ``fsverity_verify_page()`` verifies an individual page +- ``fsverity_verify_bio()`` verifies all pages in a bio + +Currently, fs-verity only supports the case where data blocks, hash +blocks, and pages all have the same size (usually 4096 bytes). + +Filesystems that use bios call ``fsverity_verify_bio()`` after each +read bio completes. To do this while also continuing to support +encryption (fscrypt), filesystems allocate a "post-read context" for +each bio and store it in ``->bi_private``:: + + struct bio_post_read_ctx { + struct bio *bio; + struct work_struct work; + unsigned int cur_step; + unsigned int enabled_steps; + }; + +``enabled_steps`` is a bitmask of the post-read steps that are +enabled. The available steps are STEP_DECRYPT and STEP_VERITY. These +steps can be enabled together, independently, or not at all. If both +are enabled, then decryption is done first. Since bio completion +callbacks cannot sleep, each post-read step is done by enqueueing the +struct on a workqueue, and then actual work happens in the work item. +Different workqueues are needed for encryption and verity because +verity work may require decrypting metadata pages from the file. + +The bio completion callback sets PG_error for each page if either +decryption or verification failed. Finally, after the work item(s) +complete, pages without PG_error are set Uptodate, and all pages are +unlocked. + +A data page being set Uptodate and unlocked implies that it has been +verified, and such pages become visible to userspace via read(), +mmap(), etc. Otherwise, the page is left in the PG_error && !Uptodate +state which results in the read() family of syscalls failing with EIO, +and accesses to the data via a memory map raising SIGBUS. Note that +even if some pages in a file fail verification, pages that pass +verification can still be read. + +To verify a data page, fs-verity reads the required hash page(s) +starting at the leaves and ascending to the root; then, the pages are +verified descending from the root. Filesystems that store the verity +metadata past EOF implement reading hash pages using their usual +``->readpage{,s}()`` methods, with modifications: + +- Verification is skipped for pages beyond ``i_size``. +- When checking whether a page is in the implicit hole beyond EOF, + the full file size (including the verity metadata) is used rather + than the original data i_size. Note that this does not allow + userspace to read or mmap the verity metadata. + +The hash pages are also cached in the inode's address_space, similar +to data pages. However, to simplify the verification logic, a hash +page being Uptodate doesn't imply that it has been verified; instead, +the PG_checked bit is used for this purpose. Hash pages aren't locked +while being verified, so multiple threads may race to set PG_checked, +but this doesn't matter. + +Thus, when ascending the tree reading hash pages, fs-verity can stop +as soon as it finds an already-checked hash page. This optimization, +which is also used by dm-verity, results in excellent sequential read +performance since usually the deepest needed hash page will already be +cached and checked. However, random reads perform worse. + +Files may contain holes. Normally, the filesystem's +``->readpage{,s}()`` methods will zero pages in holes and set them +Uptodate without issuing any bios. To prevent this from being abused +to bypass fs-verity, filesystems call ``fsverity_verify_page()`` on +hole pages. + +Like fscrypt, filesystems also disable direct I/O on verity files, +since direct I/O bypasses the normal read paths. + +Userspace utility +================= + +This document focuses on the kernel, but a userspace utility for +fs-verity can be found at: + + https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git + +See the README.md file in the fsverity-utils source tree for details, +including examples of setting up fs-verity protected files. + +Tests +===== + +To test fs-verity, use xfstests. For example, using `kvm-xfstests +<https://git.kernel.org/pub/scm/fs/ext2/xfstests-bld.git/tree/Documentation/kvm-quickstart.md>`_:: + + kvm-xfstests -c ext4,f2fs -g verity diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst index 46d1b1be3a510..818390c32be63 100644 --- a/Documentation/filesystems/index.rst +++ b/Documentation/filesystems/index.rst @@ -359,3 +359,14 @@ encryption of files and directories. :maxdepth: 2 fscrypt + +Verity API +========== + +A library which filesystems can hook into to support transparent +authentication of read-only files. + +.. toctree:: + :maxdepth: 2 + + fsverity

[v2,01/12] fs-verity: add a documentation file

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