composefs-backend: Implement bootc usr-overlay

Similar to ostree, mount a transient overlayfs on /usr

Signed-off-by: Pragyan Poudyal <[email protected]>

Author: Johan-Liebert1

crates/initramfs/src/lib.rs

@@ -179,7 +179,9 @@ fn overlay_state(base: impl AsFd, state: impl AsFd, source: &str) -> Result<()>
     mount_at_wrapper(fs, base, ".").context("Moving mount")
 }
 
-fn overlay_transient(base: impl AsFd) -> Result<()> {
+/// Mounts a transient overlayfs with passed in fd as the lowerdir
+#[context("Mounting transient overlayfs")]
+pub fn overlay_transient(base: impl AsFd) -> Result<()> {
     overlay_state(base, prepare_mount(mount_tmpfs()?)?, "transient")
 }
 

crates/lib/src/bootc_composefs/state.rs

@@ -2,10 +2,13 @@ use std::os::unix::fs::symlink;
 use std::{fs::create_dir_all, process::Command};
 
 use anyhow::{Context, Result};
+use bootc_initramfs_setup::overlay_transient;
 use bootc_kernel_cmdline::utf8::Cmdline;
 use bootc_mount::tempmount::TempMount;
 use bootc_utils::CommandRunExt;
 use camino::Utf8PathBuf;
+use cap_std_ext::cap_std::ambient_authority;
+use cap_std_ext::cap_std::fs::Dir;
 use cap_std_ext::{cap_std, dirext::CapStdExtDirExt};
 use composefs::fsverity::{FsVerityHashValue, Sha256HashValue};
 use fn_error_context::context;
@@ -163,3 +166,25 @@ pub(crate) fn write_composefs_state(
 
     Ok(())
 }
+
+pub(crate) fn composefs_usr_overlay() -> Result<()> {
+    let usr = Dir::open_ambient_dir("/usr", ambient_authority()).context("Opening /usr")?;
+    let is_usr_mounted = usr
+        .is_mountpoint(".")
+        .context("Failed to get mount details for /usr")?;
+
+    let is_usr_mounted =
+        is_usr_mounted.ok_or_else(|| anyhow::anyhow!("Falied to get mountinfo"))?;
+
+    if is_usr_mounted {
+        println!("A writeable overlayfs is already mounted on /usr");
+        return Ok(());
+    }
+
+    overlay_transient(usr)?;
+
+    println!("A writeable overlayfs is now mounted on /usr");
+    println!("All changes there will be discarded on reboot.");
+
+    Ok(())
+}

crates/lib/src/cli.rs

@@ -33,6 +33,7 @@ use serde::{Deserialize, Serialize};
 use crate::bootc_composefs::{
     finalize::{composefs_native_finalize, get_etc_diff},
     rollback::composefs_rollback,
+    state::composefs_usr_overlay,
     status::composefs_booted,
     switch::switch_composefs,
     update::upgrade_composefs,
@@ -1289,7 +1290,17 @@ async fn run_from_opt(opt: Opt) -> Result<()> {
             Ok(())
         }
         Opt::Edit(opts) => edit(opts).await,
-        Opt::UsrOverlay => usroverlay().await,
+        Opt::UsrOverlay => {
+            #[cfg(feature = "composefs-backend")]
+            if composefs_booted()?.is_some() {
+                composefs_usr_overlay()
+            } else {
+                usroverlay().await
+            }
+
+            #[cfg(not(feature = "composefs-backend"))]
+            usroverlay().await
+        }
         Opt::Container(opts) => match opts {
             ContainerOpts::Lint {
                 rootfs,