On 12/1/25 12:04, Alyssa Ross wrote:
> I tried to build this with Meson, but pathrs depends on thiserror,
> which tries to read OUT_DIR, and support for setting that isn't
> currently in a Meson release, and requires nightly Rust, so build with
> Cargo for now and we can revisit that later.
> 
> Link: https://github.com/mesonbuild/meson/pull/15148
> Link: https://github.com/rust-lang/rust/issues/118372
> Signed-off-by: Alyssa Ross <hi@alyssa.is>
> ---
> v5: • Extract mount_commit() function
>     • Expand safety comment.
> v4: https://spectrum-os.org/lists/archives/spectrum-devel/20251201044534.977524-5-hi@alyssa.is/
> 
>  LICENSES/MPL-2.0.txt                   | 373 +++++++++++++++++++++++++
>  pkgs/default.nix                       |   1 +
>  tools/mount-flatpak/Cargo.lock         | 238 ++++++++++++++++
>  tools/mount-flatpak/Cargo.lock.license |   2 +
>  tools/mount-flatpak/Cargo.toml         |  11 +
>  tools/mount-flatpak/config             |   7 +
>  tools/mount-flatpak/default.nix        |  25 ++
>  tools/mount-flatpak/src/keyfile.rs     | 189 +++++++++++++
>  tools/mount-flatpak/src/main.rs        | 288 +++++++++++++++++++
>  tools/mount-flatpak/src/metadata.rs    |  19 ++
>  10 files changed, 1153 insertions(+)
>  create mode 100644 LICENSES/MPL-2.0.txt
>  create mode 100644 tools/mount-flatpak/Cargo.lock
>  create mode 100644 tools/mount-flatpak/Cargo.lock.license
>  create mode 100644 tools/mount-flatpak/Cargo.toml
>  create mode 100644 tools/mount-flatpak/config
>  create mode 100644 tools/mount-flatpak/default.nix
>  create mode 100644 tools/mount-flatpak/src/keyfile.rs
>  create mode 100644 tools/mount-flatpak/src/main.rs
>  create mode 100644 tools/mount-flatpak/src/metadata.rs
> 
> diff --git a/LICENSES/MPL-2.0.txt b/LICENSES/MPL-2.0.txt
> new file mode 100644
> index 0000000..d0a1fa1
> --- /dev/null
> +++ b/LICENSES/MPL-2.0.txt
> @@ -0,0 +1,373 @@
> +Mozilla Public License Version 2.0
> +==================================
> +
> +1. Definitions
> +--------------
> +
> +1.1. "Contributor"
> +    means each individual or legal entity that creates, contributes to
> +    the creation of, or owns Covered Software.
> +
> +1.2. "Contributor Version"
> +    means the combination of the Contributions of others (if any) used
> +    by a Contributor and that particular Contributor's Contribution.
> +
> +1.3. "Contribution"
> +    means Covered Software of a particular Contributor.
> +
> +1.4. "Covered Software"
> +    means Source Code Form to which the initial Contributor has attached
> +    the notice in Exhibit A, the Executable Form of such Source Code
> +    Form, and Modifications of such Source Code Form, in each case
> +    including portions thereof.
> +
> +1.5. "Incompatible With Secondary Licenses"
> +    means
> +
> +    (a) that the initial Contributor has attached the notice described
> +        in Exhibit B to the Covered Software; or
> +
> +    (b) that the Covered Software was made available under the terms of
> +        version 1.1 or earlier of the License, but not also under the
> +        terms of a Secondary License.
> +
> +1.6. "Executable Form"
> +    means any form of the work other than Source Code Form.
> +
> +1.7. "Larger Work"
> +    means a work that combines Covered Software with other material, in
> +    a separate file or files, that is not Covered Software.
> +
> +1.8. "License"
> +    means this document.
> +
> +1.9. "Licensable"
> +    means having the right to grant, to the maximum extent possible,
> +    whether at the time of the initial grant or subsequently, any and
> +    all of the rights conveyed by this License.
> +
> +1.10. "Modifications"
> +    means any of the following:
> +
> +    (a) any file in Source Code Form that results from an addition to,
> +        deletion from, or modification of the contents of Covered
> +        Software; or
> +
> +    (b) any new file in Source Code Form that contains any Covered
> +        Software.
> +
> +1.11. "Patent Claims" of a Contributor
> +    means any patent claim(s), including without limitation, method,
> +    process, and apparatus claims, in any patent Licensable by such
> +    Contributor that would be infringed, but for the grant of the
> +    License, by the making, using, selling, offering for sale, having
> +    made, import, or transfer of either its Contributions or its
> +    Contributor Version.
> +
> +1.12. "Secondary License"
> +    means either the GNU General Public License, Version 2.0, the GNU
> +    Lesser General Public License, Version 2.1, the GNU Affero General
> +    Public License, Version 3.0, or any later versions of those
> +    licenses.
> +
> +1.13. "Source Code Form"
> +    means the form of the work preferred for making modifications.
> +
> +1.14. "You" (or "Your")
> +    means an individual or a legal entity exercising rights under this
> +    License. For legal entities, "You" includes any entity that
> +    controls, is controlled by, or is under common control with You. For
> +    purposes of this definition, "control" means (a) the power, direct
> +    or indirect, to cause the direction or management of such entity,
> +    whether by contract or otherwise, or (b) ownership of more than
> +    fifty percent (50%) of the outstanding shares or beneficial
> +    ownership of such entity.
> +
> +2. License Grants and Conditions
> +--------------------------------
> +
> +2.1. Grants
> +
> +Each Contributor hereby grants You a world-wide, royalty-free,
> +non-exclusive license:
> +
> +(a) under intellectual property rights (other than patent or trademark)
> +    Licensable by such Contributor to use, reproduce, make available,
> +    modify, display, perform, distribute, and otherwise exploit its
> +    Contributions, either on an unmodified basis, with Modifications, or
> +    as part of a Larger Work; and
> +
> +(b) under Patent Claims of such Contributor to make, use, sell, offer
> +    for sale, have made, import, and otherwise transfer either its
> +    Contributions or its Contributor Version.
> +
> +2.2. Effective Date
> +
> +The licenses granted in Section 2.1 with respect to any Contribution
> +become effective for each Contribution on the date the Contributor first
> +distributes such Contribution.
> +
> +2.3. Limitations on Grant Scope
> +
> +The licenses granted in this Section 2 are the only rights granted under
> +this License. No additional rights or licenses will be implied from the
> +distribution or licensing of Covered Software under this License.
> +Notwithstanding Section 2.1(b) above, no patent license is granted by a
> +Contributor:
> +
> +(a) for any code that a Contributor has removed from Covered Software;
> +    or
> +
> +(b) for infringements caused by: (i) Your and any other third party's
> +    modifications of Covered Software, or (ii) the combination of its
> +    Contributions with other software (except as part of its Contributor
> +    Version); or
> +
> +(c) under Patent Claims infringed by Covered Software in the absence of
> +    its Contributions.
> +
> +This License does not grant any rights in the trademarks, service marks,
> +or logos of any Contributor (except as may be necessary to comply with
> +the notice requirements in Section 3.4).
> +
> +2.4. Subsequent Licenses
> +
> +No Contributor makes additional grants as a result of Your choice to
> +distribute the Covered Software under a subsequent version of this
> +License (see Section 10.2) or under the terms of a Secondary License (if
> +permitted under the terms of Section 3.3).
> +
> +2.5. Representation
> +
> +Each Contributor represents that the Contributor believes its
> +Contributions are its original creation(s) or it has sufficient rights
> +to grant the rights to its Contributions conveyed by this License.
> +
> +2.6. Fair Use
> +
> +This License is not intended to limit any rights You have under
> +applicable copyright doctrines of fair use, fair dealing, or other
> +equivalents.
> +
> +2.7. Conditions
> +
> +Sections 3.1, 3.2, 3.3, and 3.4 are conditions of the licenses granted
> +in Section 2.1.
> +
> +3. Responsibilities
> +-------------------
> +
> +3.1. Distribution of Source Form
> +
> +All distribution of Covered Software in Source Code Form, including any
> +Modifications that You create or to which You contribute, must be under
> +the terms of this License. You must inform recipients that the Source
> +Code Form of the Covered Software is governed by the terms of this
> +License, and how they can obtain a copy of this License. You may not
> +attempt to alter or restrict the recipients' rights in the Source Code
> +Form.
> +
> +3.2. Distribution of Executable Form
> +
> +If You distribute Covered Software in Executable Form then:
> +
> +(a) such Covered Software must also be made available in Source Code
> +    Form, as described in Section 3.1, and You must inform recipients of
> +    the Executable Form how they can obtain a copy of such Source Code
> +    Form by reasonable means in a timely manner, at a charge no more
> +    than the cost of distribution to the recipient; and
> +
> +(b) You may distribute such Executable Form under the terms of this
> +    License, or sublicense it under different terms, provided that the
> +    license for the Executable Form does not attempt to limit or alter
> +    the recipients' rights in the Source Code Form under this License.
> +
> +3.3. Distribution of a Larger Work
> +
> +You may create and distribute a Larger Work under terms of Your choice,
> +provided that You also comply with the requirements of this License for
> +the Covered Software. If the Larger Work is a combination of Covered
> +Software with a work governed by one or more Secondary Licenses, and the
> +Covered Software is not Incompatible With Secondary Licenses, this
> +License permits You to additionally distribute such Covered Software
> +under the terms of such Secondary License(s), so that the recipient of
> +the Larger Work may, at their option, further distribute the Covered
> +Software under the terms of either this License or such Secondary
> +License(s).
> +
> +3.4. Notices
> +
> +You may not remove or alter the substance of any license notices
> +(including copyright notices, patent notices, disclaimers of warranty,
> +or limitations of liability) contained within the Source Code Form of
> +the Covered Software, except that You may alter any license notices to
> +the extent required to remedy known factual inaccuracies.
> +
> +3.5. Application of Additional Terms
> +
> +You may choose to offer, and to charge a fee for, warranty, support,
> +indemnity or liability obligations to one or more recipients of Covered
> +Software. However, You may do so only on Your own behalf, and not on
> +behalf of any Contributor. You must make it absolutely clear that any
> +such warranty, support, indemnity, or liability obligation is offered by
> +You alone, and You hereby agree to indemnify every Contributor for any
> +liability incurred by such Contributor as a result of warranty, support,
> +indemnity or liability terms You offer. You may include additional
> +disclaimers of warranty and limitations of liability specific to any
> +jurisdiction.
> +
> +4. Inability to Comply Due to Statute or Regulation
> +---------------------------------------------------
> +
> +If it is impossible for You to comply with any of the terms of this
> +License with respect to some or all of the Covered Software due to
> +statute, judicial order, or regulation then You must: (a) comply with
> +the terms of this License to the maximum extent possible; and (b)
> +describe the limitations and the code they affect. Such description must
> +be placed in a text file included with all distributions of the Covered
> +Software under this License. Except to the extent prohibited by statute
> +or regulation, such description must be sufficiently detailed for a
> +recipient of ordinary skill to be able to understand it.
> +
> +5. Termination
> +--------------
> +
> +5.1. The rights granted under this License will terminate automatically
> +if You fail to comply with any of its terms. However, if You become
> +compliant, then the rights granted under this License from a particular
> +Contributor are reinstated (a) provisionally, unless and until such
> +Contributor explicitly and finally terminates Your grants, and (b) on an
> +ongoing basis, if such Contributor fails to notify You of the
> +non-compliance by some reasonable means prior to 60 days after You have
> +come back into compliance. Moreover, Your grants from a particular
> +Contributor are reinstated on an ongoing basis if such Contributor
> +notifies You of the non-compliance by some reasonable means, this is the
> +first time You have received notice of non-compliance with this License
> +from such Contributor, and You become compliant prior to 30 days after
> +Your receipt of the notice.
> +
> +5.2. If You initiate litigation against any entity by asserting a patent
> +infringement claim (excluding declaratory judgment actions,
> +counter-claims, and cross-claims) alleging that a Contributor Version
> +directly or indirectly infringes any patent, then the rights granted to
> +You by any and all Contributors for the Covered Software under Section
> +2.1 of this License shall terminate.
> +
> +5.3. In the event of termination under Sections 5.1 or 5.2 above, all
> +end user license agreements (excluding distributors and resellers) which
> +have been validly granted by You or Your distributors under this License
> +prior to termination shall survive termination.
> +
> +************************************************************************
> +*                                                                      *
> +*  6. Disclaimer of Warranty                                           *
> +*  -------------------------                                           *
> +*                                                                      *
> +*  Covered Software is provided under this License on an "as is"       *
> +*  basis, without warranty of any kind, either expressed, implied, or  *
> +*  statutory, including, without limitation, warranties that the       *
> +*  Covered Software is free of defects, merchantable, fit for a        *
> +*  particular purpose or non-infringing. The entire risk as to the     *
> +*  quality and performance of the Covered Software is with You.        *
> +*  Should any Covered Software prove defective in any respect, You     *
> +*  (not any Contributor) assume the cost of any necessary servicing,   *
> +*  repair, or correction. This disclaimer of warranty constitutes an   *
> +*  essential part of this License. No use of any Covered Software is   *
> +*  authorized under this License except under this disclaimer.         *
> +*                                                                      *
> +************************************************************************
> +
> +************************************************************************
> +*                                                                      *
> +*  7. Limitation of Liability                                          *
> +*  --------------------------                                          *
> +*                                                                      *
> +*  Under no circumstances and under no legal theory, whether tort      *
> +*  (including negligence), contract, or otherwise, shall any           *
> +*  Contributor, or anyone who distributes Covered Software as          *
> +*  permitted above, be liable to You for any direct, indirect,         *
> +*  special, incidental, or consequential damages of any character      *
> +*  including, without limitation, damages for lost profits, loss of    *
> +*  goodwill, work stoppage, computer failure or malfunction, or any    *
> +*  and all other commercial damages or losses, even if such party      *
> +*  shall have been informed of the possibility of such damages. This   *
> +*  limitation of liability shall not apply to liability for death or   *
> +*  personal injury resulting from such party's negligence to the       *
> +*  extent applicable law prohibits such limitation. Some               *
> +*  jurisdictions do not allow the exclusion or limitation of           *
> +*  incidental or consequential damages, so this exclusion and          *
> +*  limitation may not apply to You.                                    *
> +*                                                                      *
> +************************************************************************
> +
> +8. Litigation
> +-------------
> +
> +Any litigation relating to this License may be brought only in the
> +courts of a jurisdiction where the defendant maintains its principal
> +place of business and such litigation shall be governed by laws of that
> +jurisdiction, without reference to its conflict-of-law provisions.
> +Nothing in this Section shall prevent a party's ability to bring
> +cross-claims or counter-claims.
> +
> +9. Miscellaneous
> +----------------
> +
> +This License represents the complete agreement concerning the subject
> +matter hereof. If any provision of this License is held to be
> +unenforceable, such provision shall be reformed only to the extent
> +necessary to make it enforceable. Any law or regulation which provides
> +that the language of a contract shall be construed against the drafter
> +shall not be used to construe this License against a Contributor.
> +
> +10. Versions of the License
> +---------------------------
> +
> +10.1. New Versions
> +
> +Mozilla Foundation is the license steward. Except as provided in Section
> +10.3, no one other than the license steward has the right to modify or
> +publish new versions of this License. Each version will be given a
> +distinguishing version number.
> +
> +10.2. Effect of New Versions
> +
> +You may distribute the Covered Software under the terms of the version
> +of the License under which You originally received the Covered Software,
> +or under the terms of any subsequent version published by the license
> +steward.
> +
> +10.3. Modified Versions
> +
> +If you create software not governed by this License, and you want to
> +create a new license for such software, you may create and use a
> +modified version of this License if you rename the license and remove
> +any references to the name of the license steward (except to note that
> +such modified license differs from this License).
> +
> +10.4. Distributing Source Code Form that is Incompatible With Secondary
> +Licenses
> +
> +If You choose to distribute Source Code Form that is Incompatible With
> +Secondary Licenses under the terms of this version of the License, the
> +notice described in Exhibit B of this License must be attached.
> +
> +Exhibit A - Source Code Form License Notice
> +-------------------------------------------
> +
> +  This Source Code Form is subject to the terms of the Mozilla Public
> +  License, v. 2.0. If a copy of the MPL was not distributed with this
> +  file, You can obtain one at https://mozilla.org/MPL/2.0/.
> +
> +If it is not possible or desirable to put the notice in a particular
> +file, then You may include the notice in a location (such as a LICENSE
> +file in a relevant directory) where a recipient would be likely to look
> +for such a notice.
> +
> +You may add additional accurate notices of copyright ownership.
> +
> +Exhibit B - "Incompatible With Secondary Licenses" Notice
> +---------------------------------------------------------
> +
> +  This Source Code Form is "Incompatible With Secondary Licenses", as
> +  defined by the Mozilla Public License, v. 2.0.
> diff --git a/pkgs/default.nix b/pkgs/default.nix
> index bc25420..29fefb4 100644
> --- a/pkgs/default.nix
> +++ b/pkgs/default.nix
> @@ -36,6 +36,7 @@ let
>        path: (import path { inherit (self) callPackage; }).override;
>  
>      rootfs = self.callSpectrumPackage ../host/rootfs {};
> +    mount-flatpak = self.callSpectrumPackage ../tools/mount-flatpak {};
>      spectrum-build-tools = self.callSpectrumPackage ../tools {
>        appSupport = false;
>        buildSupport = true;
> diff --git a/tools/mount-flatpak/Cargo.lock b/tools/mount-flatpak/Cargo.lock
> new file mode 100644
> index 0000000..8cb5663
> --- /dev/null
> +++ b/tools/mount-flatpak/Cargo.lock
> @@ -0,0 +1,238 @@
> +# This file is automatically @generated by Cargo.
> +# It is not intended for manual editing.
> +version = 4
> +
> +[[package]]
> +name = "bitflags"
> +version = "2.10.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "812e12b5285cc515a9c72a5c1d3b6d46a19dac5acfef5265968c166106e31dd3"
> +
> +[[package]]
> +name = "cfg-if"
> +version = "1.0.4"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
> +
> +[[package]]
> +name = "either"
> +version = "1.15.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
> +
> +[[package]]
> +name = "errno"
> +version = "0.3.14"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
> +dependencies = [
> + "libc",
> + "windows-sys",
> +]
> +
> +[[package]]
> +name = "fastrand"
> +version = "2.3.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
> +
> +[[package]]
> +name = "getrandom"
> +version = "0.3.4"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "899def5c37c4fd7b2664648c28120ecec138e4d395b459e5ca34f9cce2dd77fd"
> +dependencies = [
> + "cfg-if",
> + "libc",
> + "r-efi",
> + "wasip2",
> +]
> +
> +[[package]]
> +name = "itertools"
> +version = "0.14.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "2b192c782037fadd9cfa75548310488aabdbf3d2da73885b31bd0abd03351285"
> +dependencies = [
> + "either",
> +]
> +
> +[[package]]
> +name = "libc"
> +version = "0.2.177"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "2874a2af47a2325c2001a6e6fad9b16a53b802102b528163885171cf92b15976"
> +
> +[[package]]
> +name = "linux-raw-sys"
> +version = "0.11.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "df1d3c3b53da64cf5760482273a98e575c651a67eec7f77df96b5b642de8f039"
> +
> +[[package]]
> +name = "memchr"
> +version = "2.7.6"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "f52b00d39961fc5b2736ea853c9cc86238e165017a493d1d5c8eac6bdc4cc273"
> +
> +[[package]]
> +name = "mount-flatpak"
> +version = "0.0.0"
> +dependencies = [
> + "libc",
> + "pathrs",
> + "rustix",
> +]
> +
> +[[package]]
> +name = "once_cell"
> +version = "1.21.3"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
> +
> +[[package]]
> +name = "pathrs"
> +version = "0.2.1"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "c63f58f2463b3eecc7d285eace144d8c4820631d018d927049f8569be4b3b1a4"
> +dependencies = [
> + "bitflags",
> + "itertools",
> + "libc",
> + "memchr",
> + "once_cell",
> + "rustix",
> + "rustversion",
> + "static_assertions",
> + "tempfile",
> + "thiserror",
> +]
> +
> +[[package]]
> +name = "proc-macro2"
> +version = "1.0.103"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "5ee95bc4ef87b8d5ba32e8b7714ccc834865276eab0aed5c9958d00ec45f49e8"
> +dependencies = [
> + "unicode-ident",
> +]
> +
> +[[package]]
> +name = "quote"
> +version = "1.0.42"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "a338cc41d27e6cc6dce6cefc13a0729dfbb81c262b1f519331575dd80ef3067f"
> +dependencies = [
> + "proc-macro2",
> +]
> +
> +[[package]]
> +name = "r-efi"
> +version = "5.3.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
> +
> +[[package]]
> +name = "rustix"
> +version = "1.1.2"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "cd15f8a2c5551a84d56efdc1cd049089e409ac19a3072d5037a17fd70719ff3e"
> +dependencies = [
> + "bitflags",
> + "errno",
> + "libc",
> + "linux-raw-sys",
> + "windows-sys",
> +]
> +
> +[[package]]
> +name = "rustversion"
> +version = "1.0.22"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "b39cdef0fa800fc44525c84ccb54a029961a8215f9619753635a9c0d2538d46d"
> +
> +[[package]]
> +name = "static_assertions"
> +version = "1.1.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
> +
> +[[package]]
> +name = "syn"
> +version = "2.0.111"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "390cc9a294ab71bdb1aa2e99d13be9c753cd2d7bd6560c77118597410c4d2e87"
> +dependencies = [
> + "proc-macro2",
> + "quote",
> + "unicode-ident",
> +]
> +
> +[[package]]
> +name = "tempfile"
> +version = "3.23.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "2d31c77bdf42a745371d260a26ca7163f1e0924b64afa0b688e61b5a9fa02f16"
> +dependencies = [
> + "fastrand",
> + "getrandom",
> + "once_cell",
> + "rustix",
> + "windows-sys",
> +]
> +
> +[[package]]
> +name = "thiserror"
> +version = "2.0.17"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "f63587ca0f12b72a0600bcba1d40081f830876000bb46dd2337a3051618f4fc8"
> +dependencies = [
> + "thiserror-impl",
> +]
> +
> +[[package]]
> +name = "thiserror-impl"
> +version = "2.0.17"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "3ff15c8ecd7de3849db632e14d18d2571fa09dfc5ed93479bc4485c7a517c913"
> +dependencies = [
> + "proc-macro2",
> + "quote",
> + "syn",
> +]
> +
> +[[package]]
> +name = "unicode-ident"
> +version = "1.0.22"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "9312f7c4f6ff9069b165498234ce8be658059c6728633667c526e27dc2cf1df5"
> +
> +[[package]]
> +name = "wasip2"
> +version = "1.0.1+wasi-0.2.4"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "0562428422c63773dad2c345a1882263bbf4d65cf3f42e90921f787ef5ad58e7"
> +dependencies = [
> + "wit-bindgen",
> +]
> +
> +[[package]]
> +name = "windows-link"
> +version = "0.2.1"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
> +
> +[[package]]
> +name = "windows-sys"
> +version = "0.61.2"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "ae137229bcbd6cdf0f7b80a31df61766145077ddf49416a728b02cb3921ff3fc"
> +dependencies = [
> + "windows-link",
> +]
> +
> +[[package]]
> +name = "wit-bindgen"
> +version = "0.46.0"
> +source = "registry+https://github.com/rust-lang/crates.io-index"
> +checksum = "f17a85883d4e6d00e8a97c586de764dabcc06133f7f1d55dce5cdc070ad7fe59"
> diff --git a/tools/mount-flatpak/Cargo.lock.license b/tools/mount-flatpak/Cargo.lock.license
> new file mode 100644
> index 0000000..d705e97
> --- /dev/null
> +++ b/tools/mount-flatpak/Cargo.lock.license
> @@ -0,0 +1,2 @@
> +SPDX-License-Identifier: CC0-1.0
> +SPDX-FileCopyrightText: 2024 Alyssa Ross <hi@alyssa.is>
> diff --git a/tools/mount-flatpak/Cargo.toml b/tools/mount-flatpak/Cargo.toml
> new file mode 100644
> index 0000000..670be2f
> --- /dev/null
> +++ b/tools/mount-flatpak/Cargo.toml
> @@ -0,0 +1,11 @@
> +# SPDX-License-Identifier: CC0-1.0
> +# SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +
> +[package]
> +name = "mount-flatpak"
> +edition = "2024"
> +
> +[dependencies]
> +libc = "0.2.177"
> +pathrs = "0.2.1"
> +rustix = "1.1.2"
> diff --git a/tools/mount-flatpak/config b/tools/mount-flatpak/config
> new file mode 100644
> index 0000000..b3c2bee
> --- /dev/null
> +++ b/tools/mount-flatpak/config
> @@ -0,0 +1,7 @@
> +# SPDX-License-Identifier: CC0-1.0
> +# SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +
> +[core]
> +repo_version=1
> +mode=bare-user-only
> +min-free-space-size=500MB
> diff --git a/tools/mount-flatpak/default.nix b/tools/mount-flatpak/default.nix
> new file mode 100644
> index 0000000..f364fd7
> --- /dev/null
> +++ b/tools/mount-flatpak/default.nix
> @@ -0,0 +1,25 @@
> +# SPDX-FileCopyrightText: 2024-2025 Alyssa Ross <hi@alyssa.is>
> +# SPDX-License-Identifier: MIT
> +
> +import ../../lib/call-package.nix (
> +{ src, lib, rustPlatform }:
> +
> +rustPlatform.buildRustPackage {
> +  name = "mount-flatpak";
> +
> +  src = lib.fileset.toSource {
> +    root = ../..;
> +    fileset = lib.fileset.intersection src ./.;
> +  };
> +  sourceRoot = "source/tools/mount-flatpak";
> +
> +  cargoLock.lockFile = ./Cargo.lock;
> +
> +  env = {
> +    MOUNT_FLATPAK_CONFIG_PATH = "${placeholder "out"}/share/spectrum/flatpak-config";
> +  };
> +
> +  postInstall = ''
> +    install -Dm 0755 config $MOUNT_FLATPAK_CONFIG_PATH
> +  '';
> +}) (_: {})
> diff --git a/tools/mount-flatpak/src/keyfile.rs b/tools/mount-flatpak/src/keyfile.rs
> new file mode 100644
> index 0000000..fa746b3
> --- /dev/null
> +++ b/tools/mount-flatpak/src/keyfile.rs
> @@ -0,0 +1,189 @@
> +// Copyright 2021 System76 <info@system76.com>
> +// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +// SPDX-License-Identifier: MPL-2.0
> +
> +// Adapted from freedesktop-desktop-entry 0.7.19.
> +
> +use std::collections::BTreeMap;
> +use std::fmt::{self, Display, Formatter};
> +
> +type Group = BTreeMap<String, String>;
> +
> +#[derive(Debug)]
> +pub enum DecodeError {
> +    KeyValueWithoutAGroup,
> +    InvalidKey,
> +    InvalidValue,
> +}
> +
> +impl Display for DecodeError {
> +    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
> +        match self {
> +            Self::KeyValueWithoutAGroup => write!(f, "key/value without a group"),
> +            Self::InvalidKey => write!(f, "invalid key"),
> +            Self::InvalidValue => write!(f, "invalid value"),
> +        }
> +    }
> +}
> +
> +pub fn parse(input: &str) -> Result<BTreeMap<String, Group>, DecodeError> {
> +    let mut groups = BTreeMap::default();
> +    let mut active_group: Option<ActiveGroup> = None;
> +    let mut active_keys: Option<ActiveKeys> = None;
> +
> +    for line in input.lines() {
> +        process_line(line, &mut groups, &mut active_group, &mut active_keys)?;
> +    }
> +
> +    if let Some(active_keys) = active_keys.take() {
> +        match &mut active_group {
> +            Some(active_group) => {
> +                active_group
> +                    .group
> +                    .insert(active_keys.key_name, active_keys.value);
> +            }
> +            None => return Err(DecodeError::KeyValueWithoutAGroup),
> +        }
> +    }
> +
> +    if let Some(mut group) = active_group.take() {
> +        groups
> +            .entry(group.group_name)
> +            .or_default()
> +            .append(&mut group.group);
> +    }
> +
> +    Ok(groups)
> +}
> +
> +struct ActiveGroup {
> +    group_name: String,
> +    group: Group,
> +}
> +
> +struct ActiveKeys {
> +    key_name: String,
> +    value: String,
> +}
> +
> +#[inline(never)]
> +fn process_line(
> +    line: &str,
> +    groups: &mut BTreeMap<String, Group>,
> +    active_group: &mut Option<ActiveGroup>,
> +    active_keys: &mut Option<ActiveKeys>,
> +) -> Result<(), DecodeError> {
> +    if line.trim().is_empty() || line.starts_with('#') {
> +        return Ok(());
> +    }
> +
> +    let line_bytes = line.as_bytes();
> +
> +    // if group
> +    if line_bytes[0] == b'[' {
> +        if let Some(end) = line_bytes[1..].iter().rposition(|&b| b == b']') {
> +            let group_name = &line[1..end + 1];
> +
> +            if let Some(active_keys) = active_keys.take() {
> +                match active_group {
> +                    Some(active_group) => {
> +                        active_group
> +                            .group
> +                            .insert(active_keys.key_name, active_keys.value);
> +                    }
> +                    None => return Err(DecodeError::KeyValueWithoutAGroup),
> +                }
> +            }
> +
> +            if let Some(mut group) = active_group.take() {
> +                groups
> +                    .entry(group.group_name)
> +                    .or_default()
> +                    .append(&mut group.group);
> +            }
> +
> +            active_group.replace(ActiveGroup {
> +                group_name: group_name.to_string(),
> +                group: Group::default(),
> +            });
> +        }
> +    }
> +    // else, if value
> +    else if let Some(delimiter) = line_bytes.iter().position(|&b| b == b'=') {
> +        let key = &line[..delimiter];
> +        let value = format_value(&line[delimiter + 1..])?;
> +
> +        if key.is_empty() {
> +            return Err(DecodeError::InvalidKey);
> +        }
> +
> +        if let Some(active_keys) = active_keys.take() {
> +            match active_group {
> +                Some(active_group) => {
> +                    active_group
> +                        .group
> +                        .insert(active_keys.key_name, active_keys.value);
> +                }
> +                None => return Err(DecodeError::KeyValueWithoutAGroup),
> +            }
> +        }
> +        active_keys.replace(ActiveKeys {
> +            key_name: key.trim().to_string(),
> +            value,
> +        });
> +    }
> +    Ok(())
> +}
> +
> +// https://specifications.freedesktop.org/desktop-entry-spec/latest/value-types.html
> +#[inline]
> +fn format_value(input: &str) -> Result<String, DecodeError> {
> +    let input = if let Some(input) = input.strip_prefix(" ") {
> +        input
> +    } else {
> +        input
> +    };
> +
> +    let mut res = String::with_capacity(input.len());
> +
> +    let mut last: usize = 0;
> +
> +    for (i, v) in input.as_bytes().iter().enumerate() {
> +        if *v != b'\\' {
> +            continue;
> +        }
> +
> +        // edge case for //
> +        if last > i {
> +            continue;
> +        }
> +
> +        // when there is an \ at the end
> +        if input.len() <= i + 1 {
> +            return Err(DecodeError::InvalidValue);
> +        }
> +
> +        if last < i {
> +            res.push_str(&input[last..i]);
> +        }
> +
> +        last = i + 2;
> +
> +        match input.as_bytes()[i + 1] {
> +            b's' => res.push(' '),
> +            b'n' => res.push('\n'),
> +            b't' => res.push('\t'),
> +            b'r' => res.push('\r'),
> +            b'\\' => res.push('\\'),
> +            _ => {
> +                return Err(DecodeError::InvalidValue);
> +            }
> +        }
> +    }
> +
> +    if last < input.len() {
> +        res.push_str(&input[last..input.len()]);
> +    }
> +
> +    Ok(res)
> +}
> diff --git a/tools/mount-flatpak/src/main.rs b/tools/mount-flatpak/src/main.rs
> new file mode 100644
> index 0000000..6fc0556
> --- /dev/null
> +++ b/tools/mount-flatpak/src/main.rs
> @@ -0,0 +1,288 @@
> +// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +// SPDX-License-Identifier: EUPL-1.2+
> +
> +//! Flatpak installations look like this:
> +//!
> +//! ```text
> +//! flatpak/
> +//! ├── app/
> +//! │   └── org.gnome.TextEditor/
> +//! │       ├── current -> x86_64/stable
> +//! │       └── x86_64/
> +//! │           └── stable/
> +//! │               ├── 0029140121b39f5b7cf4d44fd46b0708eee67f395b5e1291628612a0358fb909/
> +//! │               │   └── …
> +//! │               └── active -> 0029140121b39f5b7cf4d44fd46b0708eee67f395b5e1291628612a0358fb909
> +//! ├── db/
> +//! ├── exports/
> +//! │   └── …
> +//! ├── repo
> +//! │   ├── config
> +//! │   ├── objects
> +//! │   ├── tmp
> +//! │   │   └── cache
> +//! │   │       └── …
> +//! │   └── …
> +//! └── runtime
> +//!     ├── org.gnome.Platform
> +//!     │   └── x86_64
> +//!     │       └── 49
> +//!     │           ├── active -> bf6aa432cb310726f4ac0ec08cc88558619e1d4bd4b964e27e95187ecaad5400
> +//!     │           └── bf6aa432cb310726f4ac0ec08cc88558619e1d4bd4b964e27e95187ecaad5400
> +//!     │               └── …
> +//!     └── …
> +//! ```
> +//!
> +//! The purpose of this program is to use bind mounts to construct a
> +//! Flatpak installation containing only a single application and
> +//! runtime, which can be passed through to a VM without exposing
> +//! other installed applications.
> +
> +mod keyfile;
> +mod metadata;
> +
> +use std::borrow::Cow;
> +use std::env::{ArgsOs, args_os};
> +use std::ffi::OsStr;
> +use std::io;
> +use std::os::unix::prelude::*;
> +use std::path::{Path, PathBuf};
> +use std::process::exit;
> +
> +use pathrs::Root;
> +use pathrs::flags::{OpenFlags, ResolverFlags};
> +use rustix::fs::{CWD, FileType, fstat};
> +use rustix::mount::{MoveMountFlags, OpenTreeFlags, move_mount, open_tree};
> +
> +use metadata::extract_runtime;
> +
> +fn ex_usage() -> ! {
> +    eprintln!("Usage: mount-flatpak userdata installation app");
> +    exit(1);
> +}
> +
> +fn mount_commit(
> +    source_commit: &dyn AsFd,
> +    target_installation: &Root,
> +    path: &Path,
> +) -> Result<(), String> {
> +    let source_commit_tree = open_tree(
> +        source_commit,
> +        "",
> +        OpenTreeFlags::AT_EMPTY_PATH
> +            | OpenTreeFlags::OPEN_TREE_CLONE
> +            | OpenTreeFlags::OPEN_TREE_CLOEXEC
> +            | OpenTreeFlags::AT_RECURSIVE,
> +    )
> +    .map_err(|e| format!("cloning source commit tree: {e}"))?;
> +    let target_commit_dir = target_installation
> +        .mkdir_all(path, &PermissionsExt::from_mode(0o700))
> +        .map_err(|e| format!("creating target commit directory: {e}"))?;

Does this check for path being absolute?  I don't know how Rust
handles joining a relative path to an absolute one.

> +    move_mount(
> +        source_commit_tree,
> +        "",
> +        target_commit_dir,
> +        "",
> +        MoveMountFlags::MOVE_MOUNT_F_EMPTY_PATH | MoveMountFlags::MOVE_MOUNT_T_EMPTY_PATH,
> +    )
> +    .map_err(|e| format!("mounting commit: {e}"))
> +}
> +
> +fn run(mut args: ArgsOs) -> Result<(), String> {
> +    let Some(user_data_path) = args.next().map(PathBuf::from) else {
> +        ex_usage();
> +    };
> +    let Some(installation_path) = args.next().map(PathBuf::from) else {
> +        ex_usage();
> +    };
> +    let Some(app) = args.next() else {
> +        ex_usage();
> +    };
> +    if args.next().is_some() {
> +        ex_usage();
> +    }
> +
> +    let mut user_data =
> +        Root::open(&user_data_path).map_err(|e| format!("opening user data partition: {e}"))?;
> +    user_data.set_resolver_flags(ResolverFlags::NO_SYMLINKS);
> +
> +    let mut source_installation_dir = user_data
> +        .open_subpath(&installation_path, OpenFlags::O_PATH)
> +        .map(Root::from_fd)
> +        .map_err(|e| format!("opening source flatpak installation: {e}"))?;
> +    source_installation_dir.set_resolver_flags(ResolverFlags::NO_SYMLINKS);
> +
> +    std::fs::create_dir("flatpak")
> +        .map_err(|e| format!("creating target flatpak installation: {e}"))?;
> +
> +    let target_installation_dir = open_tree(
> +        CWD,
> +        "flatpak",
> +        OpenTreeFlags::OPEN_TREE_CLONE
> +            | OpenTreeFlags::OPEN_TREE_CLOEXEC
> +            | OpenTreeFlags::AT_RECURSIVE
> +            | OpenTreeFlags::AT_SYMLINK_NOFOLLOW,
> +    )
> +    .map_err(|e| format!("opening target flatpak installation: {e}"))?;
> +    let mut target_installation_dir = Root::from_fd(target_installation_dir);
> +    target_installation_dir.set_resolver_flags(ResolverFlags::NO_SYMLINKS);
> +
> +    let mut full_app_path = installation_path.join("app");
> +    full_app_path.push(&app);
> +    full_app_path.push("current");
> +    let arch_and_branch = source_installation_dir
> +        .readlink(&full_app_path)
> +        .map_err(|e| format!("reading current app arch and branch: {e}"))?;
> +    let mut components = arch_and_branch.components();
> +    let arch = components.next().unwrap().as_os_str();

Still panics if there is no next component.

> +    let branch = components.as_path().as_os_str();
> +    if branch.is_empty() {
> +        return Err("can't infer branch from \"current\" link".to_string());
> +    }
> +
> +    full_app_path.pop();
> +    full_app_path.push(&arch_and_branch);
> +    full_app_path.push("active");
> +    let commit = source_installation_dir
> +        .readlink(&full_app_path)
> +        .map_err(|e| format!("reading active app commit: {e}"))?
> +        .into_os_string();

This can (and should) be moved into mount_commit().

> +    full_app_path.pop();
> +    full_app_path.push(&commit);

I strongly recommend validating the commit here.  This avoids Rust’s
stdlib path functions potentially mishandling absolute paths, ".",
or ".." (by popping off the last components).  I don’t know if
they *do* mishandle it, but validation is obviously correct, and not
validating isn’t.

> +    let source_app_dir = source_installation_dir
> +        .resolve(&full_app_path)
> +        .map_err(|e| format!("opening source app directory: {e}"))?;
> +
> +    let metadata = source_installation_dir
> +        .resolve(full_app_path.join("metadata"))
> +        .map_err(|e| format!("resolving app metadata: {e}"))?;
> +
> +    let metadata_stat =
> +        fstat(&metadata).map_err(|e| format!("checking app metadata is a regular file: {e}"))?;
> +    let metadata_type = FileType::from_raw_mode(metadata_stat.st_mode);
> +    if !metadata_type.is_file() {
> +        let e = format!("type of app metadata is {metadata_type:?}, not RegularFile");
> +        return Err(e);
> +    }
> +    let metadata = metadata
> +        .reopen(OpenFlags::O_RDONLY)
> +        .map_err(|e| format!("opening app metadata: {e}"))?;
> +
> +    let runtime =
> +        extract_runtime(metadata).map_err(|e| format!("reading runtime from metadata: {e}"))?;
> +
> +    let mut full_runtime_path = installation_path.join("runtime");
> +    full_runtime_path.push(runtime);
> +    full_runtime_path.push("active");
> +    let runtime_commit = source_installation_dir
> +        .readlink(&full_runtime_path)
> +        .map_err(|e| format!("reading active runtime commit: {e}"))?
> +        .into_os_string();
> +
> +    full_runtime_path.pop();
> +    full_runtime_path.push(&runtime_commit);
> +    let source_runtime_dir = source_installation_dir
> +        .resolve(&full_runtime_path)
> +        .map_err(|e| format!("opening source runtime directory: {e}"))?;

Same comments as above.  Also, does the error contain the path name?
I'm guessing it doesn’t, but it would be best to check that.

> +    mount_commit(&source_app_dir, &target_installation_dir, &full_app_path)?;
> +    mount_commit(
> +        &source_runtime_dir,
> +        &target_installation_dir,
> +        &full_runtime_path,
> +    )?;
> +
> +    target_installation_dir
> +        .mkdir_all("repo/objects", &PermissionsExt::from_mode(0o700))
> +        .map_err(|e| format!("creating repo/objects: {e}"))?;
> +    target_installation_dir
> +        .mkdir_all("repo/tmp/cache", &PermissionsExt::from_mode(0o700))
> +        .map_err(|e| format!("creating repo/tmp/cache: {e}"))?;
> +    let config_target = target_installation_dir
> +        .create_file(
> +            "repo/config",
> +            OpenFlags::O_WRONLY | OpenFlags::O_CLOEXEC,
> +            &PermissionsExt::from_mode(0o700),
> +        )
> +        .map_err(|e| format!("creating repo/config: {e}"))?;
> +    let config_source_path = env!("MOUNT_FLATPAK_CONFIG_PATH");
> +    let config_source = open_tree(
> +        CWD,
> +        config_source_path,
> +        OpenTreeFlags::OPEN_TREE_CLONE | OpenTreeFlags::OPEN_TREE_CLOEXEC,
> +    )
> +    .map_err(|e| format!("opening {config_source_path}: {e}"))?;
> +    move_mount(
> +        config_source,
> +        "",
> +        config_target,
> +        "",
> +        MoveMountFlags::MOVE_MOUNT_F_EMPTY_PATH | MoveMountFlags::MOVE_MOUNT_T_EMPTY_PATH,
> +    )
> +    .map_err(|e| format!("mounting config: {e}"))?;
> +
> +    let mut attr = libc::mount_attr {
> +        attr_clr: libc::MOUNT_ATTR_NOSYMFOLLOW,
> +        attr_set: libc::MOUNT_ATTR_RDONLY | libc::MOUNT_ATTR_NODEV,
> +        propagation: libc::MS_SLAVE,
> +        userns_fd: 0,
> +    };
> +    let empty = b"\0";
> +    // SAFETY: we pass a valid FD, valid C string, and a valid mutable pointer with the
> +    // correct size.
> +    unsafe {
> +        let r = libc::syscall(
> +            libc::SYS_mount_setattr,
> +            target_installation_dir.as_fd().as_raw_fd() as libc::c_long,
> +            empty.as_ptr() as *const libc::c_char,
> +            (libc::AT_EMPTY_PATH | libc::AT_RECURSIVE) as libc::c_long,
> +            &mut attr as *mut libc::mount_attr,
> +            size_of::<libc::mount_attr>() as libc::c_long,
> +        );
> +        if r == -1 {
> +            return Err(format!(
> +                "setting target mount attributes: {}",
> +                io::Error::last_os_error()
> +            ));
> +        }
> +    }
> +    move_mount(
> +        target_installation_dir,
> +        "",
> +        CWD,
> +        "flatpak",
> +        MoveMountFlags::MOVE_MOUNT_F_EMPTY_PATH,
> +    )
> +    .map_err(|e| format!("mounting target installation dir: {e}"))?;
> +
> +    std::fs::create_dir("params").map_err(|e| format!("creating params directory: {e}"))?;
> +    std::fs::write("params/id", app.as_bytes()).map_err(|e| format!("writing params/id: {e}"))?;
> +    std::fs::write("params/commit", commit.as_bytes())
> +        .map_err(|e| format!("writing params/commit: {e}"))?;
> +    std::fs::write("params/arch", arch.as_bytes())
> +        .map_err(|e| format!("writing params/arch: {e}"))?;
> +    std::fs::write("params/branch", branch.as_bytes())
> +        .map_err(|e| format!("writing params/branch: {e}"))?;
> +    std::fs::write("params/runtime-commit", runtime_commit.as_bytes())
> +        .map_err(|e| format!("writing params/runtime-commit: {e}"))?;
> +
> +    Ok(())
> +}
> +
> +fn main() {
> +    let mut args = args_os();
> +
> +    let prog_name = args
> +        .next()
> +        .as_ref()
> +        .map(Path::new)
> +        .and_then(Path::file_name)
> +        .map_or(Cow::Borrowed("mount-flatpak"), OsStr::to_string_lossy)
> +        .into_owned();
> +
> +    if let Err(e) = run(args) {
> +        eprintln!("{prog_name}: {e}");
> +        exit(1);
> +    }
> +}
> diff --git a/tools/mount-flatpak/src/metadata.rs b/tools/mount-flatpak/src/metadata.rs
> new file mode 100644
> index 0000000..dfc05b1
> --- /dev/null
> +++ b/tools/mount-flatpak/src/metadata.rs
> @@ -0,0 +1,19 @@
> +// SPDX-License-Identifier: EUPL-1.2+
> +// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
> +
> +use std::fs::File;
> +use std::io::read_to_string;
> +
> +use crate::keyfile::parse;
> +
> +pub fn extract_runtime(mut metadata: File) -> Result<String, String> {
> +    let metadata = read_to_string(&mut metadata).map_err(|e| e.to_string())?;

Is this going to be in the VM's cgroup?

> +    let group = parse(&metadata).map_err(|e| e.to_string())?;
> +    let application = group
> +        .get("Application")
> +        .ok_or_else(|| "Application group missing".to_string())?;
> +    Ok(application
> +        .get("runtime")
> +        .ok_or_else(|| "runtime property missing".to_string())?
> +        .clone())
> +}
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)