[PATCH v3 1/3] tools/mount-flatpak: init

[Alyssa Ross <hi@alyssa.is>]

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>
---
v3: Take the user data partition root and path to the Flatpak
    repository within it as separate parameters, to avoid calling
    Root::open() on an untrusted path.  Suggested by Demi out of
    band.
v2: https://spectrum-os.org/lists/archives/spectrum-devel/20251124194846.16338-1-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        | 252 +++++++++++++++++
 tools/mount-flatpak/src/metadata.rs    |  19 ++
 10 files changed, 1117 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 cc60228..6829185 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..fd2f74f
--- /dev/null
+++ b/tools/mount-flatpak/src/main.rs
@@ -0,0 +1,252 @@
+// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
+// SPDX-License-Identifier: EUPL-1.2+
+
+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 installation app");
+    exit(1);
+}
+
+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 source_installation_dir = Root::open(&user_data_path)
+        .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,
+    )
+    .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();
+    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();
+
+    full_app_path.pop();
+    full_app_path.push(&commit);
+    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}"))?;
+
+    let target_app_dir = target_installation_dir
+        .mkdir_all(&full_app_path, &PermissionsExt::from_mode(0o700))
+        .map_err(|e| format!("creating target app directory: {e}"))?;
+    let target_runtime_dir = target_installation_dir
+        .mkdir_all(&full_runtime_path, &PermissionsExt::from_mode(0o700))
+        .map_err(|e| format!("creating target runtime directory: {e}"))?;
+
+    let source_app_tree = open_tree(
+        &source_app_dir,
+        "",
+        OpenTreeFlags::AT_EMPTY_PATH
+            | OpenTreeFlags::OPEN_TREE_CLONE
+            | OpenTreeFlags::OPEN_TREE_CLOEXEC
+            | OpenTreeFlags::AT_RECURSIVE,
+    )
+    .map_err(|e| format!("cloning source app tree: {e}"))?;
+    let source_runtime_tree = open_tree(
+        &source_runtime_dir,
+        "",
+        OpenTreeFlags::AT_EMPTY_PATH
+            | OpenTreeFlags::OPEN_TREE_CLONE
+            | OpenTreeFlags::OPEN_TREE_CLOEXEC
+            | OpenTreeFlags::AT_RECURSIVE,
+    )
+    .map_err(|e| format!("cloning source runtime tree: {e}"))?;
+
+    move_mount(
+        source_app_tree,
+        "",
+        target_app_dir,
+        "",
+        MoveMountFlags::MOVE_MOUNT_F_EMPTY_PATH | MoveMountFlags::MOVE_MOUNT_T_EMPTY_PATH,
+    )
+    .map_err(|e| format!("mounting app directory: {e}"))?;
+    move_mount(
+        source_runtime_tree,
+        "",
+        target_runtime_dir,
+        "",
+        MoveMountFlags::MOVE_MOUNT_F_EMPTY_PATH | MoveMountFlags::MOVE_MOUNT_T_EMPTY_PATH,
+    )
+    .map_err(|e| format!("mounting runtime directory: {e}"))?;
+
+    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: 0,
+        userns_fd: 0,
+    };
+    let empty = b"\0";
+    // SAFETY: we pass a valid FD, and a valid mutable pointer with the correct size.
+    unsafe {
+        let r = libc::syscall(
+            libc::SYS_mount_setattr,
+            target_installation_dir.as_fd(),
+            empty.as_ptr() as *const libc::c_char,
+            (libc::AT_EMPTY_PATH | libc::AT_RECURSIVE) as libc::c_uint,
+            &mut attr as *mut libc::mount_attr,
+            size_of::<libc::mount_attr>() as libc::size_t,
+        );
+        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())?;
+    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())
+}

base-commit: 4200cea7eb31f5b8d35bf4c00b879aa25cf1fb90
-- 
2.51.0