generator: Fix orphaned /etc/shadow and /etc/gshadow entries before sysusers

There's a bit of a trap in the movement from nss-altfiles to systemd-sysusers;
if users/groups migrate from the former to the latter, they may leave
orphaned entires in the shadow files.

systemd-sysusers then tries to create those users/groups at boot it
finds them already in the shadow files and fatally errors.

Add a generator which enables a unit detects this situation and cleans
up the shadow entries.

Now in practice: we probably should have made sure that nss-altfiles
users don't have shadow entries at all, but that ship has sailed.

Fixes: #1179

Assisted-by: OpenCode (Claude Sonnet 4.6)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/lib/src/cli.rs

@@ -630,6 +630,9 @@ pub(crate) enum InternalsOpts {
         late_dir: Option<Utf8PathBuf>,
     },
     FixupEtcFstab,
+    /// Remove orphaned and duplicate entries from /etc/shadow and /etc/gshadow
+    /// before systemd-sysusers runs.
+    SysusersSync,
     /// Should only be used by `make update-generated`
     PrintJsonSchema {
         #[clap(long)]
@@ -2094,6 +2097,7 @@ async fn run_from_opt(opt: Opt) -> Result<()> {
                 Ok(())
             }
             InternalsOpts::FixupEtcFstab => crate::deploy::fixup_etc_fstab(&root),
+            InternalsOpts::SysusersSync => crate::sysusers_cleanup::run(&root),
             InternalsOpts::PrintJsonSchema { of } => {
                 let schema = match of {
                     SchemaType::Host => schema_for!(crate::spec::Host),

crates/lib/src/generator.rs

@@ -20,6 +20,7 @@ const FSTAB_ANACONDA_STAMP: &str = "Created by anaconda";
 pub(crate) const BOOTC_EDITED_STAMP: &str = "Updated by bootc-fstab-edit.service";
 const TRANSIENT_RELABEL_UNIT: &str = "bootc-early-overlay-relabel.service";
 const SYSINIT_TARGET: &str = "sysinit.target";
+const SHADOW_SYNC_UNIT: &str = "bootc-sysusers-shadow-sync.service";
 
 /// Called when the root is read-only composefs to reconcile /etc/fstab
 #[context("bootc generator")]
@@ -146,6 +147,11 @@ pub(crate) fn generator(root: &Dir, unit_dir: &Dir) -> Result<()> {
 
     unit_enablement_impl(sysroot, unit_dir)?;
 
+    // The shadow sync runs on all ostree boots; shadow_sync_generator_impl
+    // has its own guard that checks whether /etc/shadow exists.
+    let updated = shadow_sync_generator_impl(root, unit_dir)?;
+    tracing::trace!("Enabled shadow sync: {updated}");
+
     // Only run for overlayfs roots (composefs mounts an overlay, regular or transient).
     let st = rustix::fs::fstatfs(root.as_fd())?;
     if st.f_type != libc::OVERLAYFS_SUPER_MAGIC {
@@ -166,6 +172,31 @@ pub(crate) fn generator(root: &Dir, unit_dir: &Dir) -> Result<()> {
     Ok(())
 }
 
+/// Enable the statically-installed shadow sync unit by symlinking it into
+/// `sysinit.target.wants/` in the generator output directory.
+///
+/// The unit file itself lives at `/usr/lib/systemd/system/bootc-sysusers-shadow-sync.service`
+/// and is shipped with bootc; the generator only performs conditional enablement.
+///
+/// We check existence of `/etc/shadow` rather than writability because systemd
+/// sandboxes generators in a private read-only mount namespace (see
+/// `systemd.generator(7)`), so any writability check would always fail even
+/// though `/etc` will be on its own writable mount by the time the service
+/// actually runs.  The caller already gates on the ostree-booted marker,
+/// which guarantees we are on a bootc system where `/etc` is writable at
+/// service-run time.
+#[context("shadow sync generator")]
+pub(crate) fn shadow_sync_generator_impl(root: &Dir, unit_dir: &Dir) -> Result<bool> {
+    if !root.try_exists("etc/shadow")? {
+        tracing::trace!("/etc/shadow not found, skipping shadow sync");
+        return Ok(false);
+    }
+
+    tracing::debug!("/etc/shadow found, enabling {SHADOW_SYNC_UNIT}");
+    enable_unit(unit_dir, SHADOW_SYNC_UNIT, "sysinit.target")?;
+    Ok(true)
+}
+
 /// Parse /etc/fstab and check if the root mount is out of sync with the composefs
 /// state, and if so, fix it.
 fn generate_fstab_editor(unit_dir: &Dir) -> Result<()> {
@@ -417,5 +448,51 @@ UUID=341c4712-54e8-4839-8020-d94073b1dc8b /boot                   xfs     defaul
 
             Ok(())
         }
+
+        #[test]
+        fn test_shadow_sync_no_shadow() -> Result<()> {
+            // No /etc/shadow => should not enable (boot detection is caller's job)
+            let tempdir = fixture()?;
+            let unit_dir = &tempdir.open_dir("run/systemd/system")?;
+            let generated = shadow_sync_generator_impl(&tempdir, unit_dir)?;
+            assert!(!generated);
+            assert_eq!(unit_dir.entries()?.count(), 0);
+            Ok(())
+        }
+
+        #[test]
+        fn test_shadow_sync_enables_when_shadow_present() -> Result<()> {
+            // /etc/shadow present => enables the static unit
+            let tempdir = fixture()?;
+            tempdir.atomic_write("etc/shadow", "root:*:18912:0:99999:7:::\n")?;
+            let unit_dir = &tempdir.open_dir("run/systemd/system")?;
+            let generated = shadow_sync_generator_impl(&tempdir, unit_dir)?;
+            assert!(generated);
+            // The generator creates a symlink in sysinit.target.wants/; check the
+            // directory entry exists (symlink_contents creates an absolute-path symlink
+            // that cap-std won't follow in a tempdir, so we check metadata directly).
+            let wants = unit_dir.open_dir("sysinit.target.wants")?;
+            let meta = wants.symlink_metadata(SHADOW_SYNC_UNIT)?;
+            assert!(meta.is_symlink(), "expected symlink for {SHADOW_SYNC_UNIT}");
+            Ok(())
+        }
+
+        /// Verify that generator() enables the shadow sync unit on traditional
+        /// ostree boots (tmpfs root, OSTREE_BOOTED marker present).
+        #[test]
+        fn test_generator_shadow_sync_on_non_composefs() -> Result<()> {
+            let tempdir = fixture()?;
+            tempdir.atomic_write(OSTREE_BOOTED, "")?;
+            tempdir.atomic_write("etc/shadow", "root:*:18912:0:99999:7:::\n")?;
+            let unit_dir = &tempdir.open_dir("run/systemd/system")?;
+            generator(&tempdir, unit_dir)?;
+            let wants = unit_dir.open_dir("sysinit.target.wants")?;
+            let meta = wants.symlink_metadata(SHADOW_SYNC_UNIT)?;
+            assert!(
+                meta.is_symlink(),
+                "shadow sync unit must be enabled on non-composefs ostree systems"
+            );
+            Ok(())
+        }
     }
 }

crates/lib/src/lib.rs

@@ -94,6 +94,7 @@ mod reboot;
 pub mod spec;
 mod status;
 mod store;
+mod sysusers_cleanup;
 mod task;
 mod ukify;
 mod utils;