Prevent rootfs copy from following symlinks

2 files changed, 46 insertions, 2 deletions

diff --git a/src/rootfs.rs b/src/rootfs.rs
index 9246aa8..ea7d87b 100644
--- a/src/rootfs.rs
+++ b/src/rootfs.rs
@@ -173,8 +173,7 @@ fn copy_entry(
         if let Some(parent) = output.parent() {
             fs::create_dir_all(parent)?;
         }
-        fs::copy(source, &output)
-            .with_context(|| format!("copy {} to {}", source.display(), output.display()))?;
+        replace_with_file(source, &output)?;
         fs::set_permissions(&output, metadata.permissions())?;
         manifest.record(destination);
     } else {
@@ -213,6 +212,19 @@ fn enable_service(rootfs: &Path, service: &ServiceName, manifest: &mut MutationM
     Ok(())
 }
 
+fn replace_with_file(source: &Path, output: &Path) -> Result<()> {
+    match fs::symlink_metadata(output) {
+        Ok(metadata) if metadata.file_type().is_dir() => {
+            bail!("cannot replace directory with file: {}", output.display());
+        }
+        Ok(_) => fs::remove_file(output).with_context(|| format!("replace {}", output.display()))?,
+        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
+        Err(error) => return Err(error).with_context(|| format!("inspect {}", output.display())),
+    }
+    fs::copy(source, output).with_context(|| format!("copy {} to {}", source.display(), output.display()))?;
+    Ok(())
+}
+
 fn replace_with_symlink(target: &Path, output: &Path) -> Result<()> {
     match fs::symlink_metadata(output) {
         Ok(metadata) if metadata.file_type().is_dir() => {
diff --git a/tests/rootfs_finalization.rs b/tests/rootfs_finalization.rs
index 3a98290..a450b7e 100644
--- a/tests/rootfs_finalization.rs
+++ b/tests/rootfs_finalization.rs
@@ -115,3 +115,35 @@ fn finalization_is_idempotent_for_matching_symlinks() {
         .iter()
         .any(|path| path == "etc/controller/controller-link"));
 }
+
+#[test]
+fn finalization_replaces_a_destination_symlink_instead_of_following_it() {
+    let fixture = tempdir().expect("fixture directory");
+    let source = fixture.path().join("overlay");
+    fs::create_dir_all(&source).expect("create source tree");
+    fs::write(source.join("controller.conf"), "new controller configuration\n")
+        .expect("write source file");
+    let rootfs = tempdir().expect("rootfs directory");
+    let outside = fixture.path().join("outside.conf");
+    fs::write(&outside, "must not change\n").expect("write outside file");
+    fs::create_dir_all(rootfs.path().join("etc/controller")).expect("create destination directory");
+    symlink(&outside, rootfs.path().join("etc/controller/controller.conf"))
+        .expect("create destination symlink");
+    let finalization = RootfsFinalization::new(
+        vec![CopyTree::new(&source, "etc/controller").expect("valid copy destination")],
+        InitrdOem::new(std::iter::empty::<&str>(), std::iter::empty::<&str>()),
+        vec![],
+    );
+
+    finalization.apply(rootfs.path()).expect("finalization replaces symlink safely");
+
+    assert_eq!(fs::read_to_string(&outside).expect("read outside file"), "must not change\n");
+    assert!(fs::symlink_metadata(rootfs.path().join("etc/controller/controller.conf"))
+        .expect("inspect copied file")
+        .file_type()
+        .is_file());
+    assert_eq!(
+        fs::read_to_string(rootfs.path().join("etc/controller/controller.conf")).expect("read copied file"),
+        "new controller configuration\n"
+    );
+}