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+# 0789 COSI Extension Images
+
+- Date: 2026-09-08
+- RFC PR: [microsoft/trident#789](https://github.com/microsoft/trident/pull/789)
+
+## Summary
+
+Carry sysext and confext images as compressed members of a COSI tar. An optional
+`extensions` section lists each [`ImageFile`](../../Reference/Composable-OS-Image.md#imagefile-object)
+and, when placement is prescribed, its destination `path`. Trident can deploy
+extensions with the OS image instead of fetching them separately.
+
+## Motivation and Goals
+
+Host Configuration extensions need separate hosting, downloads and updates.
+Bundling gives OS-coupled extensions the COSI's integrity chain and the same
+A/B rollout, reboot and rollback as the OS. Host Configuration extensions remain
+available for content released independently.
+
+## Scope
+
+### Requirements
+
+- COSI revision 1.3 adds optional `extensions.sysexts` and
+  `extensions.confexts` arrays, each entry `{ id, image, path? }`.
+- Clean Install and A/B Update deploy the effective set from COSI and Host
+  Configuration; collisions fail.
+- An extension-only change can use Runtime Update without rewriting partitions
+  or rebooting.
+
+### Out of Scope
+
+Changing the Host Configuration API, DDIs, SELinux support or signing;
+partition-less COSI files; and bundled extensions under
+[disk streaming](../../Explanation/Disk-Streaming.md), which must be refused
+until streaming can enable the merge units.
+
+### Exit Criteria
+
+Publish the 1.3 schema and samples. Deploy sysexts and confexts on Clean Install
+and A/B Update with slot-correct rollback; apply a verified extension-only
+change by Runtime Update. Reject collisions and unsupported disk streaming.
+
+## Dependencies
+
+[Image Customizer](https://github.com/microsoft/azure-linux-image-tools), or
+another writer, must emit the section. Extension-only updates also need a
+COSI-to-COSI writer that copies region images verbatim and an explicit
+Runtime Update request (including its precedence over `forceAbUpdate`).
+
+Fix the existing `set_up_extensions` replacement bug first: with the same ID
+and destination but different content, it installs the new file then deletes
+that path as the old file. Runtime stage/finalize also needs durable payloads;
+finalize and rollback construct their context with `image: None`.
+
+## Implementation
+
+### Tar Layout
+
+Payloads are ZSTD-compressed DDIs under `images/extensions/`; this preserves
+`ImageFile.path`'s `^images/.+` constraint. The primary GPT image must
+immediately follow `metadata.json`. Write extension members after the
+disk-region images and include their window sizes in `compression.maxWindowLog`.
+Older readers ignore the additional tar members.
+
+### Metadata Schema
+
+The two arrays mirror Host Configuration and determine the kind; each entry
+contains an `id`, an `image` and optional `path`. This is the proposed addition to
+the [1.2 schema](../../Reference/Composable-OS-Image/cosi-metadata-v1.2.schema.json):
+
+```json
+{
+  "properties": {
+    "extensions": { "$ref": "#/$defs/Extensions" }
+  },
+  "$defs": {
+    "Extensions": {
+      "type": "object",
+      "properties": {
+        "sysexts": { "type": "array", "items": { "$ref": "#/$defs/ExtensionImage" } },
+        "confexts": { "type": "array", "items": { "$ref": "#/$defs/ExtensionImage" } }
+      }
+    },
+    "ExtensionImage": {
+      "type": "object",
+      "required": ["id", "image"],
+      "properties": {
+        "id": { "type": "string", "minLength": 1 },
+        "image": { "$ref": "#/$defs/ImageFile" },
+        "path": { "type": "string", "pattern": "^/.+\\.raw$" }
+      }
+    }
+  }
+}
+```
+
+Example (`extensions` fragment of `metadata.json`):
+
+```json
+{
+  "sysexts": [{
+    "id": "gpu-driver",
+    "image": {
+      "path": "images/extensions/gpu-driver.rawzst",
+      "compressedSize": 41943040,
+      "uncompressedSize": 134217728,
+      "sha384": "3a1f9c0d4e5b6a7c8d9e0f1a2b3c4d5e6f708192a3b4c5d6e7f8091a2b3c4d5e6f708192a3b4c5d6e7f8091a2b3c4d5e"
+    }
+  }],
+  "confexts": [{
+    "id": "fleet-config",
+    "image": {
+      "path": "images/extensions/fleet-config.rawzst",
+      "compressedSize": 262144,
+      "uncompressedSize": 1048576,
+      "sha384": "c3d4e5f60718293a4b5c6d7e8f90a1b2c3d4e5f60718293a4b5c6d7e8f90a1b2c3d4e5f60718293a4b5c6d7e8f90a1b2"
+    },
+    "path": "/var/lib/confexts/fleet-config.raw"
+  }]
+}
+```
+
+`id` is the extension's stable logical identity, not the COSI root `id`, the
+host OS's `ID=`, or the image filename. For a sysext it must equal `SYSEXT_ID=` inside
+`/usr/lib/extension-release.d/extension-release.{name}`; for a confext it
+must equal `CONFEXT_ID=` inside
+`/etc/extension-release.d/extension-release.{name}`. Trident already requires
+those fields in DDIs. `(kind, id)` identifies an extension across COSI versions;
+its compressed hash triggers replacement. IDs must be unique within each
+array. Metadata can now match old and new extensions before reading their
+payloads, but Trident must still verify the declared ID against the DDI.
+
+`ImageFile` supplies the tar path, compressed size for ranged reads,
+uncompressed size for output bounds, and SHA-384 of the compressed member.
+Trident checks both sizes and the hash against the bytes read and written.
+It does not reuse Host Configuration's `Extension.sha384`, which hashes the
+uncompressed DDI; a source switch must replace the image, not compare those
+two hashes. Recompression may cause an unnecessary replacement, never a missed
+one.
+
+`path` is deliberately optional, **not** a missing piece of metadata. Omission
+leaves placement to Trident and may be the usual case for fully
+Trident-managed extensions. Today Trident derives `{name}.raw` from
+`extension-release.{name}` and uses `/var/lib/extensions/` or
+`/var/lib/confexts/`. An explicit `path` must use a permitted directory and
+match that name. [RFC 0838](https://github.com/microsoft/trident/pull/838)
+proposes fully Trident-managed, slotted sysext storage without `path`; it
+handles `/etc` layers separately. How bundled extensions select such a policy
+is unresolved; omitting `path` alone must not opt in.
+
+There is no `url` (the tar is the source), `kind` (the array supplies it), or
+separate name, enable flag or compatibility fields (the DDI supplies them, or
+systemd merges all discovered images). The name determines the required
+`{name}.raw` filename and can affect systemd's merge precedence; it is not
+Trident's update identity.
+
+### Versioning
+
+COSI revision 1.3 adds the optional section; older COSIs behave unchanged.
+Existing Trident accepts any `1.x` metadata and ignores unknown fields, so it
+silently skips bundled extensions. That omission does not self-heal after a
+Trident upgrade because the image is already recorded as deployed. Require a
+1.3-aware reader, warn on unknown minor versions in new readers, and use a
+[health check](../../Reference/Host-Configuration/API-Reference/Health.md) for
+required extensions when reader versions cannot be controlled.
+
+### Trident-Side Consumption
+
+Compute an effective set from COSI and Host Configuration entries, retaining
+each entry's source. Match old and new entries by `(kind, id)`; replace when
+content changes, and reconcile changes to placement or filename. The extensions
+subsystem reads bundled members instead of downloading them;
+`update_host_configuration` writes resolved paths back only for Host
+Configuration entries. `osconfig` enables the sysext/confext merge
+units for the effective set. Move the SELinux check into validation that also
+runs for Runtime Updates; its current `configure` hook does not.
+
+[Disk streaming](../../Explanation/Disk-Streaming.md) must reject bundled
+extensions: `osconfig` skips stream-image configuration, leaving the merge
+units disabled.
+
+#### Deployed Extension Inventory
+
+Host Configuration does not record bundled extensions. Persist an inventory
+(kind, ID, name, resolved placement, source and source hash) in Host Status so
+Trident can compare old/new sets and resume separate stage/finalize calls.
+Retain deployed and pending inventories and image summaries alongside
+`spec_old` and `spec`. Promote after successful Runtime Update, or after
+post-boot commit for A/B; discard pending data on rollback. Extend
+`ManualRollbackChainItem` to carry historical inventory and summary.
+
+Manual runtime rollback also needs the old DDI bytes. Either retain them while
+the rollback chain references them or refuse rollback across a bundled
+extension change; metadata alone cannot restore them.
+
+#### Destination Validation
+
+Existing A/B location validation reads only Host Configuration entries before
+loading COSI. Validate the *resolved* effective set after reading the image:
+every destination must be writable and, when A/B is configured, slot-scoped.
+A/B volumes qualify; the root-verity `/etc` overlay qualifies during
+provisioning if its backing volume is A/B. Slotted storage from
+[RFC 0838](https://github.com/microsoft/trident/pull/838) qualifies only with
+its loader capability; `path` omission alone must not bypass the A/B check.
+
+#### Where Images Are Written
+
+Clean Install and A/B Update place files in the target root (the inactive slot
+for A/B). Runtime Update places them in the running root without provisioning
+partitions. Stage temporary files on the destination filesystem for atomic
+placement; the current fixed `/var/lib/extensions/.staging` falls back to a
+non-atomic cross-filesystem copy.
+
+#### Rollback
+
+An A/B rollback restores the previous extension set only when placement is
+slot-scoped. Writing a bundled image to a shared volume would change the running
+slot and defeat rollback; see [Destination Validation](#destination-validation).
+
+#### Extension-Only Updates
+
+Today any change to the COSI metadata hash forces A/B servicing, including an
+extension-only change. A partition-less COSI is not an option: `os.image`
+describes a complete OS. Compare the new COSI against a stored summary of the
+deployed OS instead.
+
+##### Deployed Image Summary
+
+Store each region's hash, uncompressed size and identity (partition number,
+mount point, `fsType`, `fsUuid`, `partType`, verity root hash), plus disk geometry,
+`osArch`, `osRelease` and `bootloader`. Exclude extensions, packages, compression
+settings and `id`: none changes the deployed OS. A structured summary identifies
+which region differs. If missing or of an unknown summary version, fall back to
+the existing metadata-hash comparison.
+
+Equal summaries and equal extension sets require no servicing; equal summaries
+with changed extensions permit Runtime Update; a changed summary requires A/B.
+Other subsystems can still require A/B. A no-op needs a metadata-only Host Status
+update, since `update` currently returns without recording the accepted image.
+
+##### Constructing an Extension-Only Update
+
+Copy an existing COSI's region images and metadata entries verbatim; replace
+only the extension members and metadata (and `compression.maxWindowLog` or `id`
+if needed). Rebuilding filesystems changes bytes and hashes, even for the same
+files, and safely selects A/B instead of skipping an OS change.
+
+##### Requested and Verified, Never Inferred
+
+Runtime Update requires an explicit request, matching OS summaries, a
+committed/active source slot, and no A/B requirement from any other subsystem.
+If any condition fails, report an error; do not partially apply changes or
+silently switch to A/B.
+
+##### Rollback on This Path
+
+Runtime Update changes the running slot. Retain the old DDI until finalize and
+auto-rollback complete; both can run with `image: None`. Historical manual
+rollback needs longer retention or must be refused.
+
+#### Interaction With Host Configuration `sysexts` and `confexts`
+
+Merge both sources. Reject duplicate extension IDs per kind or destination
+collisions after resolving placement, including omitted `path`. Silent
+precedence could run an unvalidated combination; identical entries still
+collide. COSI ID duplicates can be rejected from metadata; Host Configuration
+IDs still require reading their DDIs. Enforce the Host Configuration's
+documented ID-uniqueness rule too.
+
+### Validation
+
+In `cosi/validation.rs`, add `V1_3` error kinds for invalid explicit paths
+(absolute, `.raw`, allowed directory), duplicate `(kind, id)` pairs, duplicate
+explicit destinations, reused tar members and collisions with disk-region
+images. Reuse
+`Extension::validate_sysext`/`validate_confext` path rules. A missing member is
+detected when read: `Cosi::new` only scans as far as `metadata.json`.
+
+After decompressing and mounting the DDI, check actual compressed and
+uncompressed sizes, the compressed SHA-384, exactly one `extension-release`
+whose `SYSEXT_ID` or `CONFEXT_ID` equals the metadata `id`, the `{name}.raw`
+destination suffix, resolved placement, and duplicate destinations/IDs across
+both sources. Omitted paths cannot be collision-checked from metadata alone.
+
+Warn on definite OS `ID` mismatch with COSI `osRelease`, not `VERSION_ID`:
+systemd may accept a version mismatch via `SYSEXT_LEVEL` or `CONFEXT_LEVEL`.
+Do not warn on `ID=_any`; it declares compatibility, not delivery method.
+
+### SELinux
+
+Still unsupported in enforcing mode on systemd 255. Check the effective set in
+validation for every servicing type, not only the Host Configuration lists in
+`configure`, which skips Runtime Updates.
+
+## Testing and Metrics
+
+- v1.3 schema and valid/invalid samples, including missing or empty IDs;
+  semantic path and duplicate-ID rules in Rust tests.
+- Clean Install and A/B Update with both kinds, merge-unit activation and
+  rollback (including absent `path` and slotted placement once supported).
+- Runtime Update with identical region images, distinct stage/finalize and
+  auto-rollback; same-ID/same-path replacement and historical rollback refusal.
+- Reject mixed-source collisions, invalid destinations, enforcing SELinux,
+  mismatched metadata/DDI IDs, changed OS summaries under requested Runtime
+  Update, and disk streaming.
+
+## Implementation Plan
+
+1. Fix [prerequisites](#dependencies); publish COSI 1.3 schema and samples.
+2. Read, validate and install bundled extensions; persist the effective set.
+3. Add the image summary, copy-based writer and verified Runtime Update path.
+
+Step 2 can ship before step 3, but the RFC is not complete without both.
+
+## Counter-Arguments
+
+### Drawbacks
+
+Bundling still ties distribution to a new COSI, increases its size and gives
+hosts no per-extension opt-out. Use Host Configuration for extensions that must
+move independently. Older readers silently omit bundled extensions.
+
+### Alternatives
+
+Embedding `.raw` files in a partition image hides them from COSI metadata,
+fails when the destination is a separately created filesystem, and changes
+verity hashes. A separate Host Configuration URL remains preferable for
+independent release cycles.
+
+Flatcar's sysext-bakery pairs pre-built DDIs with
+[`systemd-sysupdate`](https://www.freedesktop.org/software/systemd/man/latest/systemd-sysupdate.html),
+hosting artefacts over HTTP with a `SHA256SUMS` manifest. Like `os.sysexts`, it
+optimises for extensions that move independently of the OS, though without an
+OCI transport. Bundling optimises for extensions that must move with it.
+
+## Open Questions
+
+- **Slotted placement:** how does a COSI entry request the policy in
+  [RFC 0838](https://github.com/microsoft/trident/pull/838)? Omitting `path`
+  cannot silently change an existing image's placement or imply loader support.
+- **Runtime request:** where is the explicit request recorded across
+  stage/finalize, and how does it interact with `forceAbUpdate`?
+- **Manual runtime rollback:** keep historical DDIs as long as the rollback
+  chain needs them, or refuse rollback across bundled extension changes?
+- **Operator override:** retain the hard error on bundled/Host Configuration
+  collisions, or permit an explicit override when a hotfix is needed?