Merge pull request #2885 from cgwalters/doc-composefs

composefs: Add some basic docs

docs/composefs.md

@@ -0,0 +1,50 @@
+---
+nav_order: 10
+---
+
+# Using composefs with OSTree
+{: .no_toc }
+
+1. TOC
+{:toc}
+
+## composefs
+
+The [composefs](github.com/containers/composefs) project is a new
+hybrid Linux stacking filesystem that provides many benefits when
+used for bootable host systems, such as a strong story for integrity.
+
+At the current time, integration of composefs and ostree is experimental.
+[This issue](https://github.com/ostreedev/ostree/issues/2867) tracks the latest status.
+
+### Enabling composefs (unsigned)
+
+When building a disk image *or* to transition an existing system, run:
+
+```
+ostree config --repo=/ostree/repo set ex-integrity.composefs yes
+```
+
+This will ensure that any future deployments (e.g. created by `ostree admin upgrade`)
+have a `.ostree.cfs` file in the deployment directory which is a mountable
+composefs metadata file, with a "backing store" directory also shared with the current `/ostree/repo/objects`.
+
+**IMPORTANT** The integration with composefs is experimental and subject to change.  Please
+try it and report issues but do not deploy to production systems yet.
+
+## Comparison with other approaches
+
+There is also support for using [IMA](ima.md) with ostree.  In short, composefs
+provides much stronger and more efficient integrity:
+
+- composefs validates an entire filesystem tree, not just individual files
+- composefs makes files actually read-only, whereas IMA does not by default
+- composefs uses fs-verity which does on-demand verification
+
+## Further references
+
+- https://github.com/containers/composefs
+- https://www.kernel.org/doc/html/next/filesystems/fsverity.html
+
+<!-- SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later) -->
+

man/ostree.repo-config.xml

@@ -425,7 +425,16 @@ License along with this library. If not, see <https://www.gnu.org/licenses/>.
       </varlistentry>
 
     </variablelist>
+  </refsect1>
 
+  <refsect1>
+    <title>[ex-integrity] Section Options</title>
+
+    <para>
+      The "ex-" prefix here signifies experimental options.  The <literal>ex-integrity</literal> section
+      contains options related to system integrity.  Information about experimental
+      options is canonically found in upstream tracking issues.
+    </para>
   </refsect1>
 
   <refsect1>