Re: [gentoo-dev] [RFC] GLEP 74 post-Council review update [v5]

[Fabian Groffen]

Hi,

While trying to implement full tree Manifests for the Prefix tree, I ran
into the following:

Would it be possible to add a section to define what directories receive
what kind of Manifest?

I mean in particular what is encoded in gemato/profile.py, the metadata
directory is an interesting mix and match of subdirectories that have a
Manifest of their own, and subdirectories whose content is included in
the Manifest at the metadata level.

More specifically, it seems like in the current GLEP it doesn't mention
what directories should have their own Manifest or not.  It would be
good to know if for instance adding Manifest(.gz) to
metadata/install-qa-check.d is ok as per GLEP or not (and if so, the
consumer of that directory should be fixed to ignore the Manifest*
files, instead of barking it can't source the gz file or doesn't get
it).  Also, what if someone would want to include all entries in the
top-level Manifest, would that be OK (albeit stupid I guess)?

I think it would be a good addition to specify (for a Gentoo tree) what
directories receive a Manifest file and what their content is.

In addition to this, because it is related, it would be nice to also
document the IGNORE entries that seem present at the top-level and
metadata-level, or specify where they would come from for the Gentoo
case.

Thanks!
Fabian

On 23-11-2017 21:53:57 +0100, Michał Górny wrote:
> W dniu czw, 16.11.2017 o godzinie 11∶19 +0100, użytkownik Michał Górny
> napisał:
> > Hi, everyone.
> >
> > Here's the updated version of GLEP 74 taking into consideration
> > the points made during the Council pre-review.
> >
> > ReST: https://dev.gentoo.org/~mgorny/tmp/glep-0074.rst
> > HTML: https://dev.gentoo.org/~mgorny/tmp/glep-0074.html
> >
> > Changes:
>
> 27c2a9e glep-0074: Grammar corrections from Ulrich Müller
> d39f865 glep-0074: Make extended filename encoding optional
> ed111f8 glep-0074: Always exclude control characters
>
> ---
> GLEP: 74
> Title: Full-tree verification using Manifest files
> Author: Michał Górny <mgorny@g.o>,
>         Robin Hugh Johnson <robbat2@g.o>,
>         Ulrich Müller <ulm@g.o>
> Type: Standards Track
> Status: Draft
> Version: 1
> Created: 2017-10-21
> Last-Modified: 2017-11-23
> Post-History: 2017-10-26, 2017-11-16
> Content-Type: text/x-rst
> Requires: 59, 61
> Replaces: 44, 58, 60
> ---
>
> Abstract
> ========
>
> This GLEP extends the Manifest file format to cover full-tree file
> integrity and authenticity checks. The format aims to be future-proof,
> efficient and provide means of backwards compatibility.
>
>
> Motivation
> ==========
>
> The Manifest files as defined by GLEP 44 [#GLEP44]_ provide the current
> means of verifying the integrity of distfiles and package files
> in Gentoo. Combined with OpenPGP signatures, they provide means to
> ensure the authenticity of the covered files. However, as noted
> in GLEP 57 [#GLEP57]_ they lack the ability to provide full-tree
> authenticity verification as they do not cover any files outside
> the package directory. In particular, they provide multiple ways
> for a third party to inject malicious code into the ebuild environment.
>
> Historically, the topic of providing authenticity coverage for the whole
> repository has been mentioned multiple times. The most noteworthy effort
> are GLEPs 58 [#GLEP58]_ and 60 [#GLEP60]_ by Robin H. Johnson from 2008.
> They were accepted by the Council in 2010 but have never been
> implemented. When potential implementation work started in 2017, a new
> discussion about the specification arose. It prompted the creation
> of a competing GLEP that would provide a redesigned alternative to
> the old GLEPs.
>
> This specification is designed with the following goals in mind:
>
> 1. It should provide means to ensure the authenticity of the complete
>    repository, including preventing the injection of additional files.
>
> 2. The format should be universal enough to work both for the Gentoo
>    repository and third-party repositories of different characteristics.
>
> 3. The Manifest files should be verifiable stand-alone, that is without
>    knowing any details about the underlying repository format.
>
>
> Specification
> =============
>
> Manifest file format
> --------------------
>
> This specification reuses and extends the Manifest file format defined
> in GLEP 44 [#GLEP44]_. For the purpose of it, the *file type* field is
> repurposed as a generic *tag* that could also indicate additional
> (non-checksum) metadata. Appropriately, those tags can be followed by
> other space-separated values.
>
> Unless specified otherwise, the paths used in the Manifest files
> are relative to the directory containing the Manifest file. The paths
> must not reference the parent directory (``..``). Forward slash (``/``)
> is used as path component separator.
>
> The Manifest files use UTF-8 encoding.
>
>
> Manifest file locations and nesting
> -----------------------------------
>
> The ``Manifest`` file located in the root directory of the repository
> is called top-level Manifest, and it is used to perform the full-tree
> verification. In order to verify the authenticity, it must be signed
> using OpenPGP, using the armored cleartext format.
>
> The top-level Manifest may reference sub-Manifests contained
> in subdirectories of the repository. The sub-Manifests are traditionally
> named ``Manifest``; however, the implementation must support arbitrary
> names, including the possibility of multiple (split) Manifests
> for a single directory. The sub-Manifest can only cover the files inside
> the directory tree where it resides.
>
> The sub-Manifest can also be signed using OpenPGP armored cleartext
> format. However, the signature verification can be omitted since it
> already is covered by the signed top-level Manifest.
>
>
> Directory tree coverage
> -----------------------
>
> The specification provides three ways of skipping Manifest verification
> of specific files and directories (recursively):
>
> 1. explicit ``IGNORE`` entries in Manifest files,
>
> 2. injected ignore paths via package manager configuration,
>
> 3. using names starting with a dot (``.``) which are always skipped.
>
> All files that are not ignored must be covered by at least one
> of the Manifests.
>
> A single file may be matched by multiple identical or equivalent
> Manifest entries, if and only if the entries have the same semantics,
> specify the same size and the checksums common to both entries match.
> It is an error for a single file to be matched by multiple entries
> of different semantics, file size or checksum values. It is an error
> to specify another entry for a file that matches ``IGNORE``, or that
> is located inside an ignored directory.
>
> The file entries (except for ``IGNORE``) can be specified for regular
> files only. Symbolic links are followed when opening files
> and traversing directories. It is an error to specify an entry for
> a different file type. If the tree contain files of other types
> that are not otherwise ignored, they need to be covered by an explicit
> ``IGNORE``.
>
> All the local (non-``DIST``) files covered by a Manifest tree must
> reside on the same filesystem. It is an error to specify entries
> applying to files on another filesystem. If files or directories that
> are not otherwise ignored reside on a different filesystem, or symbolic
> links point to targets on a different filesystem, they must
> be explicitly excluded via ``IGNORE``.
>
>
> Path and filename encoding
> --------------------------
>
> The path fields in the Manifest file must consist of characters
> corresponding to valid UTF-8 code points excluding the backwards slash
> (``\``) and characters classified as control characters or as whitespace
> in the current version of the Unicode standard [#UNICODE]_.
>
> The implementation can optionally support extended filename encoding
> to support those paths. If encoding is not supported, the implementation
> must reject directories containing any files using non-compliant names,
> as well as Manifest files whose filename field contains such filenames.
>
> If encoding is supported, then all of the excluded characters that
> are present in paths must be encoded using one of the following escape
> sequences:
>
> - characters in the ``U+0000`` to ``U+007F`` range can be encoded
>   as ``\xHH`` where ``HH`` specifies the zero-padded, hexadecimal
>   character code,
>
> - characters in the ``U+0000`` to ``U+FFFF`` range can be encoded
>   as ``\uHHHH`` where ``HHHH`` specifies the zero-padded, hexadecimal
>   character code,
>
> - characters in the UCS-4 range can be encoded as ``\UHHHHHHHH``
>   where ``HHHHHHHH`` specifies the zero-padded, hexadecimal character
>   code.
>
> It is invalid for the backwards slash to be used in any other context,
> and a backwards slash present in filename must be encoded. A backwards
> slash used as a path component separator should be replaced by a forward
> slash instead.
>
> The encoding can be used for other characters as well. In particular,
> escaping non-printable characters might be desirable.
>
>
> File verification
> -----------------
>
> When verifying a file against the Manifest, the following rules are
> used:
>
> 1. If the file is covered directly or indirectly by an entry
>    of the ``IGNORE`` type, the verification always succeeds.
>
> 2. If the file is covered by an entry of the ``MANIFEST``, ``DATA``,
>    ``MISC``, ``EBUILD`` or ``AUX`` type:
>
>    a. if the file is not present, then the verification fails,
>
>    b. if the file is present but has a different size or one
>       of the checksums does not match, the verification fails,
>
>    c. otherwise, the verification succeeds.
>
> 3. If the file is present but not listed in Manifest, the verification
>    fails.
>
> Unless specified otherwise, the package manager must not allow using
> any files for which the verification failed. The package manager may
> reject any package or even the whole repository if it may refer to files
> for which the verification failed.
>
>
> Timestamp verification
> ----------------------
>
> The top-level Manifest file can contain a ``TIMESTAMP`` entry to account
> for attacks against tree update distribution. If such an entry
> is present, it should be updated every time at least one
> of the Manifests changes. Every unique timestamp value must correspond
> to a single tree state.
>
> During the verification process, the client should compare the timestamp
> against the update time obtained from a local clock or a trusted time
> source. If the comparison result indicates that the Manifest at the time
> of receiving was already significantly outdated, the client should
> either fail the verification or require manual confirmation from
> the user.
>
> Furthermore, the Manifest provider may employ additional methods
> of distributing the timestamps of recently generated Manifests
> using a secure channel from a trusted source for exact comparison.
> The exact details of such a solution are outside the scope of this
> specification.
>
> ``TIMESTAMP`` entries may also be present in sub-Manifests. Those
> timestamps must not be newer than the timestamp of the top-level
> Manifest (if present). This specification does not define any specific
> use for them.
>
>
> Modern Manifest tags
> --------------------
>
> The Manifest files can specify the following tags:
>
> ``TIMESTAMP <iso8601>``
>   Specifies a timestamp of when the Manifest file was last updated.
>   The timestamp must be a valid second-precision ISO 8601 extended
>   format combined date and time in UTC timezone, i.e. using
>   the following ``strftime()`` format string: ``%Y-%m-%dT%H:%M:%SZ``.
>   Optional. The package manager can use it to detect an outdated
>   repository checkout as described in `Timestamp verification`_.
>
> ``MANIFEST <path> <size> <checksums>...``
>   Specifies a sub-Manifest. The sub-Manifest must be verified like
>   a regular file. If the verification succeeds, the entries from
>   the sub-Manifest are included for verification as described
>   in `Manifest file locations and nesting`_.
>
> ``IGNORE <path>``
>   Ignores a subdirectory or file from Manifest checks. If the specified
>   path is present, it and its contents are omitted from the Manifest
>   verification (always pass). *Path* must be a plain file or directory
>   path without a trailing slash. Wildcards are not supported
>   and wildcard characters are interpreted literally.
>
> ``DATA <path> <size> <checksums>...``
>   Specifies a regular file subject to Manifest verification. The file
>   is required to pass verification. Used for all files that do not match
>   any other type.
>
> ``DIST <filename> <size> <checksums>...``
>   Specifies a distfile entry used to verify files fetched as part
>   of ``SRC_URI``. The filename must match the filename used to store
>   the fetched file as specified in the PMS [#PMS-FETCH]_. The package
>   manager must reject the fetched file if it fails verification.
>   ``DIST`` entries apply to all packages below the Manifest file
>   specifying them.
>
>
> Deprecated Manifest tags
> ------------------------
>
> For backwards compatibility, the following tags are additionally
> allowed at the package directory level:
>
> ``EBUILD <filename> <size> <checksums>...``
>   Equivalent to the ``DATA`` type.
>
> ``MISC <path> <size> <checksums>...``
>   Equivalent to the ``DATA`` type. Historically indicated that
>   the package manager may ignore a verification failure if operating
>   in non-strict mode. However, that behavior is deprecated.
>
> ``AUX <filename> <size> <checksums>...``
>   Equivalent to the ``DATA`` type, except that the filename is relative
>   to the ``files/`` subdirectory.
>
>
> Algorithm for full-tree verification
> ------------------------------------
>
> In order to perform full-tree verification, the following algorithm
> can be used:
>
> 1. Collect all files present in the repository into *present* set.
>
> 2. Start at the top-level Manifest file. Verify its OpenPGP signature.
>    Optionally verify the ``TIMESTAMP`` entry if present as specified
>    in `timestamp verification`. Remove the top-level Manifest
>    from the *present* set.
>
> 3. Process all ``MANIFEST`` entries, recursively. Verify the Manifest
>    files according to the `file verification`_ section, and include
>    their entries in the current Manifest entry list (using paths
>    relative to directories containing the Manifests).
>
> 4. Process all ``IGNORE`` entries. Remove any paths matching them
>    from the *present* set.
>
> 5. Collect all files covered by ``DATA``, ``MISC``, ``EBUILD``
>    and ``AUX`` entries into the *covered* set.
>
> 6. Verify the entries in the *covered* set for incompatible duplicates
>    and collisions with ignored files as explained in `Manifest file
>    locations and nesting`_.
>
> 7. Verify all the files in the union of the *present* and *covered*
>    sets, according to the `file verification`_ section.
>
>
> Algorithm for finding parent Manifests
> --------------------------------------
>
> In order to find the top-level Manifest from the current directory
> the following algorithm can be used:
>
> 1. Store the current directory as *original* and the device ID
>    of the containing filesystem (``st_dev``) as *startdev*,
>
> 2. If the device ID of the containing filesystem (``st_dev``)
>    of the current directory is different than *startdev*, stop.
>
> 3. If the current directory contains a ``Manifest`` file:
>
>    a. If an ``IGNORE`` entry in the ``Manifest`` file covers
>       the *original* directory (or one of the parent directories), stop.
>
>    b. Otherwise, store the current directory as *last_found*.
>
> 4. If the current directory is the root system directory (``/``), stop.
>
> 5. Otherwise, enter the parent directory and jump to step 2.
>
> Once the algorithm stops, *last_found* will contain the relevant
> top-level Manifest. If *last_found* is null, then the directory tree
> does not contain any valid top-level Manifest candidates and one should
> be created in the *original* directory.
>
> Once the top-level Manifest is found, its ``MANIFEST`` entries should
> be used to find any sub-Manifests below the top-level Manifest,
> up to and including the *original* directory. Note that those
> sub-Manifests can use different filenames than ``Manifest``.
>
>
> Checksum algorithms
> -------------------
>
> This section is informational only. Specifying the exact set
> of supported algorithms is outside the scope of this specification.
>
> The algorithm names reserved at the time of writing are:
>
> - ``MD5`` [#MD5]_,
> - ``RMD160`` -- RIPEMD-160 [#RIPEMD160]_,
> - ``SHA1`` [#SHS]_,
> - ``SHA256`` and ``SHA512`` -- SHA-2 family of hashes [#SHS]_,
> - ``WHIRLPOOL`` [#WHIRLPOOL]_,
> - ``BLAKE2B`` and ``BLAKE2S`` -- BLAKE2 family of hashes [#BLAKE2]_,
> - ``SHA3_256`` and ``SHA3_512`` -- SHA-3 family of hashes [#SHA3]_,
> - ``STREEBOG256`` and ``STREEBOG512`` -- Streebog family of hashes
>   [#STREEBOG]_.
>
> The method of introducing new hashes is defined by GLEP 59 [#GLEP59]_.
> It is recommended that any new hashes are named after the Python
> ``hashlib`` module algorithm names, transformed into uppercase.
>
>
> Manifest compression
> --------------------
>
> The topic of Manifest file compression is covered by GLEP 61 [#GLEP61]_.
> This section merely addresses interoperability issues between Manifest
> compression and this specification.
>
> The compressed Manifest files are required to be suffixed for their
> compression algorithm. This suffix should be used to recognize
> the compression and decompress Manifests transparently. The exact list
> of algorithms and their corresponding suffixes are outside the scope
> of this specification.
>
> The top-level Manifest file must not be compressed. Since the OpenPGP
> signature covers the uncompressed text and is compressed itself,
> the data would have to be decompressed without any prior verification.
> This could expose users e.g. to zip bombs or exploits on decompressor
> vulnerabilities.
>
> Whenever this specification refers to sub-Manifests, they can use any
> names but are also required to use a specific compression suffix.
> The ``MANIFEST`` entries are required to specify the full name including
> compression suffix, and the verification is performed on the compressed
> file.
>
> The specification permits uncompressed Manifests to exist alongside
> their compressed counterparts, and multiple compressed formats
> to coexist. If that is the case, the files must have the same
> uncompressed content and the specification is free to choose either
> of the files using the same base name.
>
>
> Combining multiple Manifest trees (informational)
> -------------------------------------------------
>
> This specification permits nesting multiple hierarchical Manifest trees.
> In this layout, the specific directories of the Manifest tree can
> be verified both as a part of another top-level Manifest,
> and as an independent Manifest tree (when obtained without the parent
> directory).
>
> For this to work, the sub-Manifest file in the directory must also
> satisfy the requirements for the top-level Manifest file. That is:
>
> - it must be named ``Manifest`` and not compressed,
>
> - it must cover all the files in this directory and its subdirectories
>   (i.e. no files from the directory tree can be covered by parent
>   Manifest),
>
> - if authenticity verification is desired, it must be OpenPGP-signed.
>
> It should be noted that if such a directory is a subdirectory of a valid
> Manifest tree, the sub-Manifest needs to be valid according
> to the top-level Manifest and the OpenPGP signature is disregarded
> as detailed in `Manifest file locations and nesting`_. The top-level
> behavior is exhibited only when the directory is obtained without parent
> directories.
>
>
> An example Manifest file (informational)
> ----------------------------------------
>
> An example top-level Manifest file for the Gentoo repository would have
> the following content::
>
>     TIMESTAMP 2017-10-30T10:11:12Z
>     IGNORE distfiles
>     IGNORE local
>     IGNORE lost+found
>     IGNORE packages
>     MANIFEST app-accessibility/Manifest 14821 SHA256 1b5f.. SHA512 f7eb..
>     ...
>     MANIFEST eclass/Manifest.gz 50812 SHA256 8c55.. SHA512 2915..
>     ...
>
> An example modern Manifest (disregarding backwards compatibility)
> for a package directory would have the following content::
>
>     DATA SphinxTrain-0.9.1-r1.ebuild 932 SHA256 3d3b.. SHA512 be4d..
>     DATA SphinxTrain-1.0.8.ebuild 912 SHA256 f681.. SHA512 0749..
>     DATA metadata.xml 664 SHA256 97c6.. SHA512 1175..
>     DATA files/gcc.patch 816 SHA256 b56e.. SHA512 2468..
>     DATA files/gcc34.patch 333 SHA256 c107.. SHA512 9919..
>     DIST SphinxTrain-0.9.1-beta.tar.gz 469617 SHA256 c1a4.. SHA512 1b33..
>     DIST sphinxtrain-1.0.8.tar.gz 8925803 SHA256 548e.. SHA512 465d..
>
>
> Rationale
> =========
>
> Stand-alone format
> ------------------
>
> The first question that needed to be asked before proceeding with
> the design was whether the Manifest file format was supposed to be
> stand-alone, or tightly bound to the repository format.
>
> The stand-alone format has been selected because of its three
> advantages:
>
> 1. It is more future-proof. If an incompatible change to the repository
>    format is introduced, only developers need to upgrade the tools
>    they use to generate the Manifests. The tools used to verify
>    the updated Manifests will continue to work.
>
> 2. It is more flexible and universal. With a dedicated tool,
>    the Manifest files can be used to sign and verify arbitrary file
>    sets.
>
> 3. It keeps the verification tool simpler. In particular, we can easily
>    write an independent verification tool that could work on any
>    distribution without needing to depend on a package manager
>    implementation or rewrite parts of it.
>
> Designing a stand-alone format requires that the Manifest carries enough
> information to perform the verification following all the rules specific
> to the Gentoo repository.
>
>
> Tree design
> -----------
>
> The second important point of the design was determining whether
> the Manifest files should be structured hierarchically, or independent.
> Both options have their advantages.
>
> In the hierarchical model, each sub-Manifest file is covered by a higher
> level Manifest. As a result, only the top-level Manifest has to be
> OpenPGP-signed, and subsequent Manifests need to be only verified by
> checksum stored in the parent Manifest. This has the following
> implications:
>
> - Verifying any set of files in the repository requires using checksums
>   from the most relevant Manifests and the parent Manifests.
>
> - The OpenPGP signature of the top-level Manifest needs to be verified
>   only once per process.
>
> - Altering any set of files requires updating the relevant Manifests,
>   and their parent Manifests up to the top-level Manifest, and signing
>   the last one.
>
> - As a result, the top-level Manifest changes on every commit,
>   and various middle-level Manifests change (and need to be transferred)
>   frequently.
>
> In the independent model, each sub-Manifest file is independent
> of the parent Manifests. As a result, each of them needs to be signed
> and verified independently. However, the parent Manifests still need
> to list sub-Manifests (albeit without verification data) in order
> to detect removal or replacement of subdirectories. This has
> the following implications:
>
> - Verifying any set of files in the repository requires using checksums
>   and verifying signatures of the most relevant Manifest files.
>
> - Altering any set of files requires updating the relevant Manifests
>   and signing them again.
>
> - Parent Manifests are updated only when Manifests are added or removed
>   from subdirectories. As a result, they change infrequently.
>
> While both models have their advantages, the hierarchical model was
> selected because it reduces the number of OpenPGP operations
> (which are comparatively costly) to the minimum.
>
>
> Tree layout restrictions
> ------------------------
>
> The algorithm is meant to work primarily with ebuild repositories which
> normally contain only files and directories. Directories provide
> no useful metadata for verification, and specifying special entries
> for additional file types is purposeless. Therefore, the specification
> is restricted to dealing with regular files.
>
> The Gentoo repository does not use symbolic links. Some Gentoo
> repositories do, however. To provide a simple solution for dealing with
> symlinks without having to take care to implement special handling for
> them, the common behavior of implicitly resolving them is used.
> Therefore, symbolic links to files are stored as if they were regular
> files, and symbolic links to directories are followed as if they were
> regular directories.
>
> Dotfiles are implicitly ignored as that is a common notion used
> in software written for POSIX systems. All other filenames require
> explicit ``IGNORE`` lines.
>
> An ability to inject additional ignore entries is provided to account
> for site configuration affecting the repository tree -- placing
> additional files in it, skipping some of the categories from syncing.
> This configuration can extend beyond the limits of this GLEP,
> e.g. by allowing wildcards or regular expressions.
>
> The algorithm is restricted to work on a single filesystem. This is
> mostly relevant when scanning for top-level Manifest -- we do not want
> to cross filesystem boundaries then. However, to ensure consistent
> bidirectional behavior we need to also ban them when operating downwards
> the tree.
>
> The directories and files on different filesystems need to be ignored
> explicitly as implicitly skipping them would cause confusion.
> In particular, tools might then claim that a file does not exist when
> it clearly does because it was skipped due to filesystem boundaries.
>
>
> Filename character set restriction
> ----------------------------------
>
> The valid set of filename characters for the Gentoo repository
> is restricted by the devmanual 'File Naming Rules' section
> [#FILE-NAMING-RULES]_, and enforced via a git hook. The valid distfile
> names are not restricted explicitly -- however, the PMS dependency
> specification syntax [#PMS-FETCH]_ implicitly makes it impossible to use
> filenames containing whitespace.
>
> This specification aims to avoid arbitrary restrictions. For this
> reason, filename characters are only restricted by excluding three
> technically problematic groups:
>
> 1. The backwards slash character (``\``) is used as path separator
>    on Windows systems, so it's extremely unlikely to be used in real
>    filenames. For this reason it is used to implement character
>    encoding with minimal risk of breaking backwards compatibility.
>
> 2. The control characters can trigger special behavior in various
>    programs and confuse them from recognizing text files. In particular,
>    the NULL character (``U+0000``) is normally used to indicate the end
>    of a null-terminated string. Its use could therefore break
>    implementations written in the C language. Other control characters
>    could trigger various formatting routines, garbling text output.
>
> 3. Whitespace characters are used to separate Manifest fields
>    and entries. While technically it would be enough to restrict space
>    (``U+0020``) character that is normally used as the separator
>    and newline (``U+000A``) character that is used to separate lines,
>    all whitespace characters are forbidden to avoid confusion
>    and implementation errors.
>
> Historically, Portage attempted to overcome the whitespace limitation
> by attempting to locate the size field and take everything before it
> as filename. This was terribly fragile and even if it worked, it would
> solve the problem only partially.
>
> To preserve compatibility with the current implementations and given
> that all of the listed characters are not allowed for the foreseeable
> Gentoo uses, extended encoding support is optional. If such support
> is not provided, the implementation must unconditionally reject any
> such files. Ignoring them implicitly would be confusing, and it is
> not possible to use them in explicit ``IGNORE`` entries.
>
> The character encoding method provides means to overcome the character
> restrictions to extend the tool usability beyond immediate Gentoo uses.
> The backslash escape form based on Python unicode strings is used
> since it can encode all characters within the Unicode range, the syntax
> is familiar to many programmers and the backwards slash character
> is extremely unlikely to appear in real filenames.
>
> Syntax is limited to the minimum necessary to implement the encoding.
> Shorthand forms (e.g. ``\t`` or ``\\``) are omitted to avoid unnecessary
> complexity, and to reduce the risk of shell users using backslash
> to escape space directly. The ``\x`` form is limited to ``\x00..\x7F``
> range to avoid ambiguity of higher values which might be interpreted
> either as UCS-2 code points or part of a UTF-8 encoded character.
>
> Encoding stores UCS-2/UCS-4 characters directly rather than hex-encoded
> UTF-8 string to simplify the implementation. In particular, it makes it
> possible to process the Manifest file as UTF-8 encoded text without
> having to perform additional UTF-8 decoding (and verification)
> of the escaped data.
>
> URL-encoding was considered as an alternative. However, it could collide
> with ``DIST`` entries that are implicitly named after the URL filename
> part where URL-encoding is pretty common.
>
>
> File verification model
> -----------------------
>
> The verification model aims to provide full coverage against different
> forms of attack. In particular, three different kinds of manipulation
> are considered:
>
> 1. Alteration of the file content.
>
> 2. Removal of a file.
>
> 3. Addition of a new file.
>
> In order to prevent against all three, the system requires that all
> files in the repository are listed in Manifests and verified against
> them.
>
> As a special case, ignores are allowed to account for directories
> that are not part of the repository but were traditionally placed inside
> it. Those directories were ``distfiles``, ``local`` and ``packages``. It
> could be also used to ignore VCS directories such as ``CVS``.
>
>
> Non-strict Manifest verification
> --------------------------------
>
> Originally the Manifest2 format provided a special ``MISC`` tag that
> was used for ``metadata.xml`` and ``ChangeLog`` files. This tag
> indicated that the Manifest verification failures could be ignored for
> those files unless the package manager was working in strict mode.
>
> The first versions of this specification continued the use of this tag.
> However, after a long debate it was decided to deprecate it along with
> the non-strict behavior, and require all files to strictly match.
>
> Two arguments were mentioned for the usefulness of a ``MISC`` type:
>
> 1. being able to reduce the checkout size by stripping unnecessary
>    files out, and
>
> 2. being able to update automatically generated files locally
>    without causing unnecessary verification failures.
>
> However, the usefulness of ``MISC`` in both cases is doubtful.
>
> The cases for stripping unnecessary files mostly focused around space
> savings. For this purpose, stripping ``metadata.xml`` and similar files
> has little value. It is much more common for users to strip whole
> packages or categories. The ``MISC`` type is not suitable for that,
> and so a dedicated package manager mechanism needs to be developed
> instead. The same mechanism can also handle files that historically used
> the ``MISC`` type. As an example, the package manager may choose
> to generate both the rsync exclusion list and Manifest ignore list
> using a single source list.
>
> The cases for autogenerated files involve such cache files
> as ``use.local.desc``. However, we can not include ``md5-cache`` there
> due to security concerns which results in inconsistent cache handling.
> Furthermore, the tools were historically modified to provide stable
> output which means that their content can not change without
> a non-``MISC`` content being changed first. This practically defeats
> the purpose of using ``MISC``.
>
> Finally, the non-strict mode could be used as means to an attack.
> The allowance of missing or modified documentation file could be used
> to spread misinformation, resulting in bad decisions made by the user.
> A modified file could also be used, e.g. to exploit vulnerabilities
> of an XML parser.
>
>
> Timestamp field
> ---------------
>
> The top-level Manifest optionally allows using a ``TIMESTAMP`` tag
> to include a generation timestamp in the Manifest. A similar feature
> was originally proposed in GLEP 58 [#GLEP58]_.
>
> A malicious third-party may use the principles of exclusion or replay
> [#C08]_ to deny an update to clients, while at the same time recording
> the identity of clients to attack. The timestamp field can be used to
> detect that.
>
> In order to provide more complete protection, the Gentoo Infrastructure
> should provide an ability to obtain the timestamps of all Manifests
> from a recent timeframe over a secure channel from a trusted source
> for comparison.
>
> Strictly speaking, this information is provided by the various
> ``metadata/timestamp*`` files that are already present. However,
> including the value in the Manifest itself has a little cost
> and provides the ability to perform the verification stand-alone.
>
> Furthermore, some of the timestamp files are added very late
> in the distribution process, past the Manifest generation phase. Those
> files will most likely receive ``IGNORE`` entries and therefore
> be unsafe to use.
>
> The specification permits additional timestamps in sub-Manifest files
> for local use. A generic testing tool should ignore them.
>
>
> New vs deprecated tags
> ----------------------
>
> Out of the four types defined by Manifest2, only one is reused
> and the remaining three are replaced by a single, universal ``DATA``
> type.
>
> The ``DIST`` tag is reused since the specification does not change
> anything with regard to distfile handling.
>
> The ``EBUILD`` tag could potentially be reused for generic file
> verification data. However, it would be confusing if all the different
> data files were marked as ``EBUILD``. Therefore, an equivalent ``DATA``
> type was introduced as a replacement.
>
> The ``MISC`` tag and the relevant non-strict mode has been removed
> as being of little value, as detailed in the `Non-strict Manifest
> verification`_ section.
>
> The ``AUX`` tag is deprecated as it is redundant to ``DATA``, and has
> the limiting property of implicit ``files/`` path prefix.
>
>
> Finding top-level Manifest
> --------------------------
>
> The development of a reference implementation for this GLEP has brought
> the following problem: how to find all the relevant Manifests when
> the Manifest tool is run inside a subdirectory of the repository?
>
> One of the options would be to provide a bi-directional linking
> of Manifests via a ``PARENT`` tag. However, that would not solve
> the problem when a new Manifest file is being created.
>
> Instead, an algorithm for iterating over parent directories is proposed.
> Since there is no obligatory explicit indicator for the top-level
> Manifest, the algorithm assumes that the top-level Manifest
> is the highest ``Manifest`` in the directory hierarchy that can cover
> the current directory. This generally makes sense since the Manifest
> files are required to provide coverage for all subdirectories, so all
> Manifests starting from that one need to be updated.
>
> If independent Manifest trees are nested in the directory structure,
> then an ``IGNORE`` entry needs to be used to separate them.
>
> Since sub-Manifests can use any filenames, the Manifest finding
> algorithm must not short-cut the procedure by storing all ``Manifest``
> files along the parent directories. Instead, it needs to retrace
> the relevant sub-Manifest files along ``MANIFEST`` entries
> in the top-level Manifest.
>
>
> Injecting ChangeLogs into the checkout
> --------------------------------------
>
> One of the problems considered in the new Manifest format was injecting
> historical and autogenerated ChangeLog into the repository. We normally
> don't include those files, to reduce the checkout size. However, some
> users have shown interest in them and Infra is working on providing them
> via an additional rsync module.
>
> If such files were injected into the repository, they would cause
> verification failures of Manifests. To account for this, Infra could
> provide ``IGNORE`` entries to allow them to exist.
>
>
> Splitting distfile checksums from file checksums
> ------------------------------------------------
>
> Another problem with the current Manifest format is that the checksums
> for fetched files are combined with checksums for local files
> in a single file inside the package directory. It has been specifically
> pointed out that:
>
> - since distfiles are sometimes reused across different packages,
>   the repeating checksums are redundant [#DIST]_.
>
> - mirror admins were interested in the possibility of verifying all
>   the distfiles with a single tool.
>
> This specification does not provide a clean solution to this problem.
> It technically permits moving ``DIST`` entries to higher-level Manifests
> but the usefulness of such a solution is doubtful.
>
> However, for the second problem we will probably deliver a dedicated
> tool working with this Manifest format.
>
>
> Hash algorithms
> ---------------
>
> While maintaining a consistent supported hash set is important
> for interoperability, it is not a good fit for the generic layout
> of this GLEP. Furthermore, it would require updating the GLEP
> in the future every time the used algorithms change.
>
> Instead, the specification focuses on listing the currently used
> algorithm names for interoperability, and sets a recommendation
> for consistent naming of algorithms in the future. The Python
> ``hashlib`` module is used as a reference since it is used
> as the provider of hash functions for most of the Python software,
> including Portage and PkgCore.
>
> The basic rules for changing hash algorithms are defined in GLEP 59
> [#GLEP59]_. The implementations can focus only on those algorithms
> that are actually used or planned on being used. It may be feasible
> to devise a new GLEP that specifies the currently used hashes (or update
> GLEP 59 accordingly).
>
>
> Manifest compression
> --------------------
>
> The support for Manifest compression is introduced with minimal changes
> to the file format. The ``MANIFEST`` entries are required to provide
> the real (compressed) file path for compatibility with other file
> entries and to avoid confusion.
>
> The compression of top-level Manifest file has been prohibited
> as the specification currently does not provide any means of verifying
> the file prior to decompression. If the top-level Manifest is
> compressed, tooling will have to unpack the file before being able
> to verify the contents. This makes it possible for a malicious third
> party to attack the system by providing a compressed Manifest that
> exposes decompressor vulnerabilities, or a zip bomb.
>
> The OpenPGP cleartext signature covers the contents of the Manifest,
> and is therefore compressed along with them. The possibility of using
> a detached signature has been considered but it was rejected as
> unnecessary complexity for minor gain.
>
> Technically, a similar result could be effected via moving all the data
> into a compressed sub-Manifest in the top directory (e.g.
> ``Manifest.sub.gz``), and including a ``MANIFEST`` entry for this file
> in a signed, uncompressed top-level Manifest.
>
> The existence of additional entries for uncompressed Manifest checksums
> was debated. However, plain entries for the uncompressed file would
> be confusing if only the compressed file existed, and conflicting
> if both uncompressed and compressed variants existed. Furthermore,
> it has been pointed out that ``DIST`` entries do not have
> an uncompressed variant either.
>
>
> Performance considerations
> --------------------------
>
> Performing a full-tree verification on every sync raises some
> performance concerns for end-user systems. The initial testing has shown
> that a cold-cache verification on a btrfs file system can take up around
> 4 minutes, with the process being mostly I/O bound. On the other hand,
> it can be expected that the verification will be performed directly
> after syncing, taking advantage of a warm filesystem cache.
>
> To improve speed on I/O and/or CPU-restrained systems even further,
> the algorithms can be easily extended to perform incremental
> verification. Given that rsync does not preserve mtimes by default,
> the tool can take advantage of mtime and Manifest comparisons to recheck
> only the parts of the repository that have changed.
>
> Furthermore, the package manager implementations can restrict checking
> only to the parts of the repository that are actually being used.
>
>
> Backwards Compatibility
> =======================
>
> This GLEP provides optional means of preserving backwards compatibility.
> To preserve the backwards compatibility, the following needs to hold
> for the ``Manifest`` file in every package directory:
>
> - all files must be covered by the single ``Manifest`` file,
>
> - all distfiles used by the package must be included,
>
> - all files inside the ``files/`` subdirectory need to use
>   the ``AUX`` tag (rather than ``DATA``),
>
> - all ``.ebuild`` files need to use the ``EBUILD`` tag,
>
> - the ``metadata.xml`` and ``ChangeLog`` files need to use
>   the ``MISC`` tag,
>
> - the Manifest can be signed to provide authenticity verification,
>
> - an uncompressed Manifest must always exist, and a compressed Manifest
>   of identical content may be present.
>
> Once the backwards compatibility is no longer a concern, the above
> no longer needs to hold and the deprecated tags can be removed.
>
>
> Reference Implementation
> ========================
>
> The reference implementation for this GLEP is being developed
> as the gemato project [#GEMATO]_.
>
>
> Credits
> =======
>
> Thanks to all the people whose contributions were invaluable
> to the creation of this GLEP. This includes but is not limited to:
>
> - Robin Hugh Johnson,
> - Ulrich Müller.
>
> Additionally, thanks to Robin Hugh Johnson for the original
> MetaManifest GLEP series which served both as inspiration and source
> of many concepts used in this GLEP. Recursively, also thanks to all
> the people who contributed to the original GLEPs.
>
>
> References
> ==========
>
> .. [#GLEP44] GLEP 44: Manifest2 format
>    (https://www.gentoo.org/glep/glep-0044.html)
>
> .. [#GLEP57] GLEP 57: Security of distribution of Gentoo software
>    - Overview
>    (https://www.gentoo.org/glep/glep-0057.html)
>
> .. [#GLEP58] GLEP 58: Security of distribution of Gentoo software
>    - Infrastructure to User distribution - MetaManifest
>    (https://www.gentoo.org/glep/glep-0058.html)
>
> .. [#GLEP59] GLEP 59: Manifest2 hash policies and security implications
>    (https://www.gentoo.org/glep/glep-0059.html)
>
> .. [#GLEP60] GLEP 60: Manifest2 filetypes
>    (https://www.gentoo.org/glep/glep-0060.html)
>
> .. [#GLEP61] GLEP 61: Manifest2 compression
>    (https://www.gentoo.org/glep/glep-0061.html)
>
> .. [#UNICODE] The Unicode standard
>    (https://unicode.org/versions/latest/)
>
> .. [#PMS-FETCH] Package Manager Specification: Dependency Specification
>    Format - SRC_URI
>    (https://projects.gentoo.org/pms/6/pms.html#x1-940008.2.10)
>
> .. [#FILE-NAMING-RULES] Ebuild File Format -- Gentoo Development Guide
>    (https://devmanual.gentoo.org/ebuild-writing/file-format/#file-naming-rules)
>
> .. [#MD5] RFC1321: The MD5 Message-Digest Algorithm
>    (https://www.ietf.org/rfc/rfc1321.txt)
>
> .. [#RIPEMD160] The hash function RIPEMD-160
>    (https://homes.esat.kuleuven.be/~bosselae/ripemd160.html)
>
> .. [#SHS] FIPS PUB 180-4: Secure Hash Standard (SHS)
>    (http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf)
>
> .. [#WHIRLPOOL] The WHIRLPOOL Hash Function
>    (http://www.larc.usp.br/~pbarreto/WhirlpoolPage.html)
>
> .. [#BLAKE2] BLAKE2 -- fast secure hashing
>    (https://blake2.net/)
>
> .. [#SHA3] FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash
>    and Extendable-Output Functions
>    (http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf)
>
> .. [#STREEBOG] GOST R 34.11-2012: Streebog Hash Function
>    (https://www.streebog.net/)
>
> .. [#C08] Cappos, J et al. (2008). "Attacks on Package Managers"
>    (https://www2.cs.arizona.edu/stork/packagemanagersecurity/attacks-on-package-managers.html)
>
> .. [#DIST] According to Robin H. Johnson, 8.4% of all DIST entries
>    at the time of writing are duplicate, representing 2 MiB
>    out of 25 MiB of DIST entries altogether.
>
> .. [#GEMATO] gemato: Gentoo Manifest Tool
>    (https://github.com/mgorny/gemato/)
>
>
> Copyright
> =========
> This work is licensed under the Creative Commons Attribution-ShareAlike 3.0
> Unported License. To view a copy of this license, visit
> http://creativecommons.org/licenses/by-sa/3.0/.
>
> --
> Best regards,
> Michał Górny
>
>

-- 
Fabian Groffen
Gentoo on a different level