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

[Alyssa Ross <hi@alyssa.is>]

Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
 LICENSES/MPL-2.0.txt                | 373 ++++++++++++++++++++++++++++
 tools/default.nix                   |   7 +-
 tools/meson.build                   |   1 +
 tools/mount-flatpak/config          |   7 +
 tools/mount-flatpak/keyfile.rs      | 189 ++++++++++++++
 tools/mount-flatpak/meson.build     |  18 ++
 tools/mount-flatpak/metadata.h      |   4 +
 tools/mount-flatpak/metadata.rs     |  47 ++++
 tools/mount-flatpak/mount-flatpak.c | 294 ++++++++++++++++++++++
 9 files changed, 938 insertions(+), 2 deletions(-)
 create mode 100644 LICENSES/MPL-2.0.txt
 create mode 100644 tools/mount-flatpak/config
 create mode 100644 tools/mount-flatpak/keyfile.rs
 create mode 100644 tools/mount-flatpak/meson.build
 create mode 100644 tools/mount-flatpak/metadata.h
 create mode 100644 tools/mount-flatpak/metadata.rs
 create mode 100644 tools/mount-flatpak/mount-flatpak.c

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/tools/default.nix b/tools/default.nix
index 27d4b82..27741b3 100644
--- a/tools/default.nix
+++ b/tools/default.nix
@@ -74,6 +74,7 @@ stdenv.mkDerivation (finalAttrs: {
       ./xdg-desktop-portal-spectrum
     ] ++ lib.optionals hostSupport [
       ./lsvm
+      ./mount-flatpak
       ./sd-notify-adapter.c
       ./start-vmm
       ./subprojects
@@ -88,8 +89,10 @@ stdenv.mkDerivation (finalAttrs: {
     ++ lib.optionals (appSupport || driverSupport) [ pkg-config ]
     ++ lib.optionals hostSupport [ rustc ]
     ++ lib.optionals driverSupport [ llvmPackages.clang-unwrapped ];
-  buildInputs = lib.optionals appSupport [ dbus ]
-    ++ lib.optionals driverSupport [ libbpf linuxHeaders ];
+  buildInputs = with stdenv.hostPlatform; lib.optionals appSupport [ dbus ]
+    ++ lib.optionals driverSupport [ libbpf linuxHeaders ]
+    # Workaround for <https://github.com/mesonbuild/meson/pull/15216>.
+    ++ lib.optionals (hostSupport && isMusl && isStatic) [ llvmPackages.libunwind ];
 
   postPatch = lib.optionals hostSupport (lib.concatMapStringsSep "\n" (crate: ''
     mkdir -p subprojects/packagecache
diff --git a/tools/meson.build b/tools/meson.build
index d465e99..c024ca6 100644
--- a/tools/meson.build
+++ b/tools/meson.build
@@ -30,6 +30,7 @@ if get_option('host')
     install: true)
 
   subdir('lsvm')
+  subdir('mount-flatpak')
   subdir('start-vmm')
 endif
 
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/keyfile.rs b/tools/mount-flatpak/keyfile.rs
new file mode 100644
index 0000000..fa746b3
--- /dev/null
+++ b/tools/mount-flatpak/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/meson.build b/tools/mount-flatpak/meson.build
new file mode 100644
index 0000000..e0c0a85
--- /dev/null
+++ b/tools/mount-flatpak/meson.build
@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
+
+rust_lib = static_library('metadata', 'metadata.rs', rust_abi : 'c')
+
+conf_data = configuration_data()
+conf_data.set_quoted('CONFIG_PATH',
+  get_option('prefix') / get_option('datadir') / 'spectrum/flatpak-config')
+configure_file(output : 'config.h', configuration : conf_data)
+
+executable('mount-flatpak', 'mount-flatpak.c',
+  c_args : '-D_GNU_SOURCE',
+  link_with : rust_lib,
+  install : true)
+
+install_data('config',
+  rename : 'flatpak-config',
+  install_dir : get_option('datadir') / 'spectrum')
diff --git a/tools/mount-flatpak/metadata.h b/tools/mount-flatpak/metadata.h
new file mode 100644
index 0000000..d6441b7
--- /dev/null
+++ b/tools/mount-flatpak/metadata.h
@@ -0,0 +1,4 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
+
+void extract_runtime(int metadata, char *runtime);
diff --git a/tools/mount-flatpak/metadata.rs b/tools/mount-flatpak/metadata.rs
new file mode 100644
index 0000000..0398416
--- /dev/null
+++ b/tools/mount-flatpak/metadata.rs
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
+
+mod keyfile;
+
+use std::env::args_os;
+use std::ffi::OsString;
+use std::fs::File;
+use std::io::{read_to_string, stderr, Write};
+use std::os::fd::OwnedFd;
+use std::os::unix::prelude::*;
+use std::process::exit;
+
+use keyfile::parse;
+
+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())
+}
+
+// SAFETY: we do not expect "extract_runtime" to collide with another symbol.
+#[unsafe(export_name = "extract_runtime")]
+pub extern "C" fn extract_runtime_c(metadata: OwnedFd, out: &mut [u8; 256]) {
+    let error = match extract_runtime(metadata.into()) {
+        Err(e) => e,
+        Ok(runtime) if runtime.len() >= out.len() => "runtime name too long".to_string(),
+        Ok(runtime) => {
+            out[..runtime.len()].copy_from_slice(runtime.as_bytes());
+            out[runtime.len()] = 0;
+            return;
+        }
+    };
+
+    let prog_name = args_os()
+        .next()
+        .unwrap_or_else(|| OsString::from("mount-flatpak"));
+    stderr().write_all(prog_name.as_bytes()).unwrap();
+    eprintln!(": {error}");
+    exit(1);
+}
diff --git a/tools/mount-flatpak/mount-flatpak.c b/tools/mount-flatpak/mount-flatpak.c
new file mode 100644
index 0000000..8e09d1d
--- /dev/null
+++ b/tools/mount-flatpak/mount-flatpak.c
@@ -0,0 +1,294 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Alyssa Ross <hi@alyssa.is>
+
+#include "config.h"
+#include "metadata.h"
+
+#include <err.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/stat.h>
+#include <sys/syscall.h>
+
+#include <linux/mount.h>
+#include <linux/openat2.h>
+
+static void bind_mount(int source_fd, const char *source,
+                       int target_fd, const char *target)
+{
+	int source_tree = syscall(SYS_open_tree, source_fd, source,
+	                          AT_EMPTY_PATH | OPEN_TREE_CLOEXEC |
+	                          OPEN_TREE_CLONE | AT_RECURSIVE);
+
+	if (source_tree == -1)
+		err(EXIT_FAILURE, "open_tree %s", source);
+	if (syscall(SYS_move_mount, source_tree, "", target_fd, target,
+	            MOVE_MOUNT_F_EMPTY_PATH | MOVE_MOUNT_T_EMPTY_PATH) == -1)
+		err(EXIT_FAILURE, "move_mount");
+}
+
+static int mkdir_openat(int dirfd, const char *path, mode_t mode)
+{
+	int fd;
+	if (mkdirat(dirfd, path, mode) == -1)
+		err(EXIT_FAILURE, "mkdirat %s", path);
+	if ((fd = openat(dirfd, path, O_PATH | O_DIRECTORY | O_CLOEXEC)) == -1)
+		err(EXIT_FAILURE, "openat %s", path);
+	return fd;
+}
+
+// By failing on EEXIST when creating each directory,
+// we can be sure we don't end up following .. components.
+static int mkdirs_beneath(int root, const char *target)
+{
+	int last, fd = root;
+	size_t len = strlen(target);
+	char *end, *path = malloc(len + 1), *dir = path;
+	if (!dir)
+		err(EXIT_FAILURE, "malloc %zu", len + 1);
+	memcpy(dir, target, len + 1);
+
+	do {
+		// Find next non-empty directory component
+		end = strchrnul(dir, '/');
+		while (*(end + 1) == '/')
+			end++;
+
+		// Replace path separator with string terminator.
+		*end = 0;
+
+		// Update fd to a new child, and close the previous one
+		// unless it was the one provided by the caller.
+		last = fd;
+		fd = mkdir_openat(fd, dir, 0755);
+		if (last != root)
+			close(last);
+
+		dir = end + 1;
+	} while (end != &path[len]);
+
+	free(path);
+	return fd;
+}
+
+static int openat_beneath(int dirfd, const char *path, int flags)
+{
+	struct open_how how = {
+		.flags = flags,
+		.resolve = RESOLVE_BENEATH | RESOLVE_NO_MAGICLINKS,
+	};
+	int r = syscall(SYS_openat2, dirfd, path, &how, sizeof how);
+	if (r == -1)
+		err(EXIT_FAILURE, "openat2 %s", path);
+	return r;
+}
+
+static int resolve_link(int dirfd, const char *path, char **target)
+{
+	int r;
+	struct stat sb;
+	if (fstatat(dirfd, path, &sb, AT_SYMLINK_NOFOLLOW) == -1)
+		err(EXIT_FAILURE, "fstatat %s", path);
+	if (!(*target = malloc(sb.st_size + 1)))
+		err(EXIT_FAILURE, "malloc %zu", sb.st_size + 1);
+	if ((r = readlinkat(dirfd, path, *target, sb.st_size + 1)) <= -1)
+		err(EXIT_FAILURE, "readlinkat %s", path);
+	if (r == sb.st_size + 1)
+		errx(EXIT_FAILURE, "symlink target lengthened");
+	(*target)[r] = 0; // NOLINT(clang-analyzer-security.ArrayBound)
+	return openat_beneath(dirfd, *target, O_PATH | O_CLOEXEC | O_DIRECTORY);
+}
+
+static void write_to(int dirfd, const char *path, const char *text)
+{
+	FILE *file;
+	size_t len = strlen(text);
+	int f = openat(dirfd, path,
+	               O_WRONLY | O_CLOEXEC | O_NOFOLLOW | O_CREAT, 0644);
+
+	if (f == -1)
+		err(EXIT_FAILURE, "openat %s", path);
+	if (!(file = fdopen(f, "w")))
+		err(EXIT_FAILURE, "fdopen %s", path);
+
+	if (fwrite(text, 1, len, file) != len)
+		err(EXIT_FAILURE, "fwrite");
+	if (fclose(file) == EOF)
+		err(EXIT_FAILURE, "fclose");
+}
+
+static void set_up_app_dir(int source_commit_dir, int installation_dir,
+                           const char *id, const char *arch,
+                           const char *branch, const char *commit)
+{
+	int app_dir, id_dir, arch_dir, branch_dir;
+
+	app_dir = mkdir_openat(installation_dir, "app", 0755);
+
+	id_dir = mkdir_openat(app_dir, id, 0755);
+	close(app_dir);
+
+	arch_dir = mkdir_openat(id_dir, arch, 0755);
+	close(id_dir);
+
+	branch_dir = mkdir_openat(arch_dir, branch, 0755);
+	close(arch_dir);
+
+	if (mkdirat(branch_dir, commit, 0755) == -1)
+		err(EXIT_FAILURE, "mkdirat %s", commit);
+	bind_mount(source_commit_dir, "", branch_dir, commit);
+	close(branch_dir);
+}
+
+static int mount_app(int source_installation_dir, int target_installation_dir,
+                     int params_dir, const char *id)
+{
+	char *arch_and_branch, *arch_end, *commit;
+	int app_dir, id_dir, branch_dir, commit_dir;
+
+	app_dir = openat_beneath(source_installation_dir, "app",
+	                         O_PATH | O_CLOEXEC | O_DIRECTORY);
+	id_dir = openat_beneath(app_dir, id, O_PATH | O_CLOEXEC | O_DIRECTORY);
+	close(app_dir);
+
+	branch_dir = resolve_link(id_dir, "current", &arch_and_branch);
+	close(id_dir);
+
+	commit_dir = resolve_link(branch_dir, "active", &commit);
+	close(branch_dir);
+
+	if (!(arch_end = strchr(arch_and_branch, '/')))
+		errx(EXIT_FAILURE, "unexpected current format");
+	*arch_end = 0;
+
+	set_up_app_dir(commit_dir, target_installation_dir, id,
+	               arch_and_branch, arch_end + 1, commit);
+	write_to(params_dir, "commit", commit);
+	write_to(params_dir, "arch", arch_and_branch);
+	write_to(params_dir, "branch", arch_end + 1);
+
+	free(arch_and_branch);
+	free(commit);
+	return commit_dir;
+}
+
+static void set_up_runtime_dir(int source_commit_dir, int installation_dir,
+                               const char *ref, const char *commit)
+{
+	int runtime_dir, branch_dir;
+
+	runtime_dir = mkdir_openat(installation_dir, "runtime", 0755);
+
+	branch_dir = mkdirs_beneath(runtime_dir, ref);
+	close(runtime_dir);
+
+	if (mkdirat(branch_dir, commit, 0755) == -1)
+		err(EXIT_FAILURE, "mkdirat %s", commit);
+	bind_mount(source_commit_dir, "", branch_dir, commit);
+	close(branch_dir);
+}
+
+static void mount_runtime(int source_installation_dir,
+                          int target_installation_dir,
+                          int params_dir, int app_commit_dir)
+{
+	char runtime[256], *commit;
+	int runtime_dir, branch_dir, commit_dir;
+	int metadata = openat_beneath(app_commit_dir, "metadata",
+	                              O_RDONLY | O_CLOEXEC);
+
+	extract_runtime(metadata, runtime);
+
+	runtime_dir = openat_beneath(source_installation_dir, "runtime",
+	                             O_PATH | O_CLOEXEC | O_DIRECTORY);
+
+	branch_dir = openat_beneath(runtime_dir, runtime,
+	                            O_PATH | O_CLOEXEC | O_DIRECTORY);
+	close(runtime_dir);
+
+	commit_dir = resolve_link(branch_dir, "active", &commit);
+	close(branch_dir);
+
+	set_up_runtime_dir(commit_dir, target_installation_dir,
+	                   runtime, commit);
+	write_to(params_dir, "runtime-commit", commit);
+
+	free(commit);
+	close(commit_dir);
+}
+
+static void set_up_repo(int target_installation_dir)
+{
+	int config;
+
+	if (mkdirat(target_installation_dir, "repo", 0755) == -1)
+		err(EXIT_FAILURE, "mkdir repo");
+	if (mkdirat(target_installation_dir, "repo/objects", 0755) == -1)
+		err(EXIT_FAILURE, "mkdir repo/objects");
+	if (mkdirat(target_installation_dir, "repo/tmp", 0775) == -1)
+		err(EXIT_FAILURE, "mkdir repo/tmp");
+	if (mkdirat(target_installation_dir, "repo/tmp/cache", 0775) == -1)
+		err(EXIT_FAILURE, "mkdir repo/tmp/cache");
+
+	if ((config = openat(target_installation_dir, "repo/config",
+	                     O_WRONLY | O_CLOEXEC | O_NOFOLLOW | O_CREAT,
+	                     0644)) == -1)
+		err(EXIT_FAILURE, "openat repo/config");
+
+	bind_mount(AT_FDCWD, CONFIG_PATH, config, "");
+
+	close(config);
+}
+
+int main(int, char **argv)
+{
+	char *installation_path, *id;
+	int params_dir, source_installation_dir, target_installation_dir,
+	    app_commit_dir;
+	struct mount_attr attr = {
+		.attr_clr = MOUNT_ATTR_NOSYMFOLLOW,
+		.attr_set = MOUNT_ATTR_RDONLY | MOUNT_ATTR_NODEV,
+	};
+
+	if (!(installation_path = *++argv))
+		errx(EXIT_FAILURE, "missing installation path");
+	if (!(id = *++argv))
+		errx(EXIT_FAILURE, "missing app ID");
+
+	if ((source_installation_dir = open(installation_path,
+	                                    O_PATH | O_CLOEXEC | O_DIRECTORY)) == -1)
+		err(EXIT_FAILURE, "open %s", installation_path);
+
+	params_dir = mkdir_openat(AT_FDCWD, "params", 0755);
+	write_to(params_dir, "id", id);
+
+	if (mkdir("flatpak", 0755) == -1)
+		err(EXIT_FAILURE, "mkdir flatpak");
+	if ((target_installation_dir = syscall(SYS_open_tree, AT_FDCWD,
+	                                       "flatpak",
+	                                       AT_EMPTY_PATH | OPEN_TREE_CLONE |
+	                                       OPEN_TREE_CLOEXEC |
+	                                       AT_RECURSIVE)) == -1)
+		err(EXIT_FAILURE, "open_tree flatpak");
+
+	app_commit_dir = mount_app(source_installation_dir,
+	                           target_installation_dir, params_dir, id);
+	mount_runtime(source_installation_dir, target_installation_dir,
+	              params_dir, app_commit_dir);
+	close(source_installation_dir);
+	close(params_dir);
+
+	set_up_repo(target_installation_dir);
+
+	if (syscall(SYS_mount_setattr, target_installation_dir, "",
+	            AT_EMPTY_PATH | AT_RECURSIVE, &attr, sizeof attr) == -1)
+		err(EXIT_FAILURE, "mount_setattr flatpak");
+	if (syscall(SYS_move_mount, target_installation_dir, "",
+	            AT_FDCWD, "flatpak", MOVE_MOUNT_F_EMPTY_PATH) == -1)
+		err(EXIT_FAILURE, "move_mount flatpak flatpak");
+	close(target_installation_dir);
+}

base-commit: 4b89596e112d26c4bd60bc46bb45c5d9a0875314
-- 
2.51.0