docs: keep the master's registry credentials private when running as nobody (#25)

CLAUDE.md

@@ -105,6 +105,12 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
   agent's own jobs always pass it explicitly.
 - **Override entries are never pruned.** A service deleted from the stack file keeps its
   `image:` line in `compose.override.yaml`.
+- **The override file gets the owner of the existing override, else of the stack's first
+  compose file.** The exception is a root-owned existing override (written before this
+  fix): then the compose file's owner is used, so it stops being root-owned. The agent runs
+  as root, so `WriteOverride` chowns the open temp file before the rename (never the path
+  afterwards: a symlink could be swapped in). A failure is a `warning:` line in the job
+  output, never a failed job.
 - **There is no `STACKS_ROOT`.** Stack directories must be mounted read-write at the host
   path (`/path:/path`), and `SELF_ID` must resolve to the agent's own container. On Docker
   Desktop/OrbStack bind sources are translated, so Compose jobs fail there.
@@ -116,6 +122,8 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
 - **Swarm updates need manager-node API access** for the agent doing the update.
 - **The image's default entrypoint is `updater-agent`.** The master overrides it with
   `updater-master`.
+- **The example stack runs the master as nobody (`user: "65534:65534"`).** Never add `USER`
+  to the Dockerfile: the agent uses the same image and needs root for the Docker socket.
 - **The webhook waits via a broker signal plus a store re-read, with a fallback poll**
   (the broker drops signals for slow subscribers). Marking and starting jobs runs on
   `context.WithoutCancel`, so a client that disconnects cannot leave a queued job no agent

README.md

@@ -28,14 +28,16 @@ a time per service, and one job at a time per agent.
 Use [`deploy/master.stack.yml`](deploy/master.stack.yml) as a starting point (Swarm stack;
 needs Postgres and an OIDC provider). The master listens for the UI on `PORT` (default
 `8080`, put it behind your reverse proxy) and for agents on `AGENT_PORT` (default `8090`,
-pinned TLS, reached directly on the internal network).
+pinned TLS, reached directly on the internal network). The example runs the master as
+`nobody` (`65534`); it needs no root. The agent runs as root because access to the Docker
+socket is root on the host anyway.
 
 | Variable | Required | Default | Purpose |
 |---|---|---|---|
 | `UPDATER_API_KEY` | no | — | Bearer token for `POST /update`. Without it the webhook answers `503` and the master logs `webhook_disabled`. |
 | `UPDATE_TIMEOUT` | no | `5m` | How long one `POST /update` waits for its auto jobs (Go duration). |
 | `POLL_INTERVAL` | no | `6h` | Time between registry polling cycles (Go duration, must be positive). The first cycle runs one minute after start. |
-| `DOCKER_CONFIG` | no | `~/.docker` | Directory with a `config.json` holding registry credentials for polling. Only inline `auths` entries work; see [Registry polling](#registry-polling). |
+| `DOCKER_CONFIG` | no | `~/.docker` | Directory with a `config.json` holding registry credentials for polling, readable by the master's user. Only inline `auths` entries work; see [Registry polling](#registry-polling). |
 
 The master's other settings (`DATABASE_URL`, `PUBLIC_URL`, `AGENT_URL`, `OIDC_*`,
 `ALLOWED_GROUP`, `AGENT_IMAGE`, `PORT`, `AGENT_PORT`, `LOG_LEVEL`) are shown in
@@ -146,11 +148,14 @@ multi-arch image the digest is the index digest, the same one `docker inspect` s
   tags continue. A later success clears it.
 - **Credentials.** The master reads `$DOCKER_CONFIG/config.json` (a file written by
   `docker login`), mounted read-only into the container; without it registries are accessed
-  anonymously. Only inline `auths` entries are supported. A `credsStore` or `credHelpers`
+  anonymously. The directory and file must be readable by the master's user (`65534` in the
+  example stack) without being world-readable: `chown -R 65534:65534` the directory and keep
+  `0700`/`0600`. Only inline `auths` entries are supported. A `credsStore` or `credHelpers`
   entry makes every lookup fail with `error getting credentials ... executable file not found
   in $PATH`, because the container has no credential helper binaries. The master logs
-  `registry_credentials` at startup with the directory and whether `config.json` exists,
-  never its contents.
+  `registry_credentials` at startup with the directory, `config_json_found` and
+  `config_json_readable`, never the contents. If the file exists but cannot be opened it also
+  logs a `registry_credentials_unreadable` warning with the error.
 - **Requests go wherever the image name points.** The master sends the lookup to the
   registry host in the image name, including a `GET /v2/` ping that follows redirects.
   Anyone who can start containers on a managed host can therefore make the master send

cmd/master/main.go

@@ -48,19 +48,29 @@ const pollFirstDelay = time.Minute
 const pollWorkers = 4
 
 // registryCredentialsDir returns the directory the registry client reads
-// credentials from (DOCKER_CONFIG, else ~/.docker) and whether a config.json
-// is in it. Only the path is logged, never the file.
-func registryCredentialsDir() (dir string, found bool) {
+// credentials from (DOCKER_CONFIG, else ~/.docker), whether a config.json is
+// in it, and the error from opening it when it exists but cannot be read (for
+// example because the master runs as another user than the file owner). The
+// file is opened and closed, never read; only the path is logged.
+func registryCredentialsDir() (dir string, found bool, readErr error) {
 	dir = os.Getenv("DOCKER_CONFIG")
 	if dir == "" {
 		home, err := os.UserHomeDir()
 		if err != nil {
-			return "", false
+			return "", false, nil
 		}
 		dir = filepath.Join(home, ".docker")
 	}
-	_, err := os.Stat(filepath.Join(dir, "config.json"))
-	return dir, err == nil
+	path := filepath.Join(dir, "config.json")
+	if _, err := os.Stat(path); err != nil {
+		return dir, false, nil
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return dir, true, err
+	}
+	_ = f.Close()
+	return dir, true, nil
 }
 
 func run() error {
@@ -154,8 +164,12 @@ func run() error {
 		BaseContext: baseContext,
 	}
 
-	credDir, credFound := registryCredentialsDir()
-	logger.Info("registry_credentials", "docker_config", credDir, "config_json_found", credFound)
+	credDir, credFound, credReadErr := registryCredentialsDir()
+	logger.Info("registry_credentials", "docker_config", credDir, "config_json_found", credFound,
+		"config_json_readable", credFound && credReadErr == nil)
+	if credReadErr != nil {
+		logger.Warn("registry_credentials_unreadable", "docker_config", credDir, "error", credReadErr)
+	}
 	pollCtx, stopPolling := context.WithCancel(ctx)
 	defer stopPolling()
 	registryPoller := poller.New(st, jobService, registry.New("package-updater-master/"+version, logger), broker, poller.Config{

cmd/master/main_test.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRegistryCredentialsDir(t *testing.T) {
+	t.Run("missing file", func(t *testing.T) {
+		dir := t.TempDir()
+		t.Setenv("DOCKER_CONFIG", dir)
+		got, found, readErr := registryCredentialsDir()
+		assert.Equal(t, dir, got)
+		assert.False(t, found)
+		assert.NoError(t, readErr)
+	})
+
+	t.Run("readable file", func(t *testing.T) {
+		dir := t.TempDir()
+		require.NoError(t, os.WriteFile(filepath.Join(dir, "config.json"), []byte("{}"), 0o600))
+		t.Setenv("DOCKER_CONFIG", dir)
+		_, found, readErr := registryCredentialsDir()
+		assert.True(t, found)
+		assert.NoError(t, readErr)
+	})
+
+	t.Run("unreadable file", func(t *testing.T) {
+		if os.Geteuid() == 0 {
+			t.Skip("root can read any file")
+		}
+		dir := t.TempDir()
+		require.NoError(t, os.WriteFile(filepath.Join(dir, "config.json"), []byte("{}"), 0o000))
+		t.Setenv("DOCKER_CONFIG", dir)
+		_, found, readErr := registryCredentialsDir()
+		assert.True(t, found)
+		assert.Error(t, readErr)
+	})
+}

deploy/master.stack.yml

@@ -12,12 +12,17 @@
 #      The reverse proxy must allow a read timeout longer than UPDATE_TIMEOUT (default 5m).
 #   5. Optional, for registry polling of private images: put a config.json written by
 #      `docker login` (inline "auths" only; no credsStore) in /path/to/docker-config and
-#      uncomment DOCKER_CONFIG and the volumes entry below. The directory must exist on every
-#      manager node (the master is placed on a manager), or Swarm rejects the task. Without
-#      it registries are accessed anonymously.
+#      uncomment DOCKER_CONFIG and the volumes entry below. The master runs as 65534:65534, so
+#      the directory and config.json must be readable by that user while staying private:
+#      `chown -R 65534:65534 /path/to/docker-config` (keep 0700/0600), or set `user:` below
+#      to the owner's uid:gid. Do not make config.json world-readable; it holds registry
+#      passwords. The directory must exist on every manager node (the master is placed on a
+#      manager), or Swarm rejects the task. Without it registries are accessed anonymously.
 services:
   master:
     image: gitea.shcizo.se/shcizo/package-updater:latest
+    # Runs as nobody: the master needs no root (the agent does, for the Docker socket).
+    user: "65534:65534"
     entrypoint: ["/bin/sh", "-c", "DATABASE_URL=$$(cat /run/secrets/pu_database_url) OIDC_CLIENT_SECRET=$$(cat /run/secrets/pu_oidc_client_secret) UPDATER_API_KEY=$$(cat /run/secrets/pu_updater_api_key) exec /usr/local/bin/updater-master"]
     environment:
       PUBLIC_URL: https://updater.example.com

internal/agent/updater/compose.go

@@ -152,9 +152,15 @@ func (e *ComposeExecutor) Execute(ctx context.Context, job protocol.JobStart, ou
 	}
 	fmt.Fprintln(out, resolveNote(job.Ref, pinned))
 
-	if _, err := WriteOverride(dir, service, pinned); err != nil {
+	// Giving the override the stack's owner is best effort: a failure is only
+	// a warning, the update itself is fine.
+	_, ownerErr, err := WriteOverride(dir, service, pinned, files[0])
+	if err != nil {
 		return "", err
 	}
+	if ownerErr != nil {
+		fmt.Fprintf(out, "warning: could not give the override the stack's owner: %v\n", ownerErr)
+	}
 
 	base := composeBase(project, workingDir, files, envs)
 	for _, step := range [][]string{{"pull", service}, {"up", "-d", service}} {

internal/agent/updater/compose_test.go

@@ -69,6 +69,7 @@ func TestComposeExecutor(t *testing.T) {
 	root, _ := filepath.EvalSymlinks(t.TempDir())
 	stack := filepath.Join(root, "slusk")
 	require.NoError(t, os.MkdirAll(stack, 0o755))
+	composeFile := stackFile(t, stack)
 	job := protocol.JobStart{Kind: protocol.KindCompose, ComposeProject: "slusk", ComposeService: "web", Ref: "gitea.shcizo.se/shcizo/slusk:1.4"}
 	pinned := job.Ref + "@" + d
 
@@ -107,6 +108,19 @@ func TestComposeExecutor(t *testing.T) {
 		_, doc := readOverride(t, stack)
 		assert.Equal(t, pinned, serviceImage(doc, "web"))
 		assert.Contains(t, out.String(), "resolved "+pinned)
+		assert.NotContains(t, out.String(), "warning")
+	})
+	t.Run("an unreadable owner is a warning and the job still succeeds", func(t *testing.T) {
+		t.Cleanup(cleanOverride)
+		require.NoError(t, os.Rename(composeFile, composeFile+".bak"))
+		t.Cleanup(func() { os.Rename(composeFile+".bak", composeFile) })
+		var out strings.Builder
+		got, err := setup(fakeCompose{rw, []container.Summary{ct}}, &fakeDist{digest: d}, &recorder{}).Execute(context.Background(), job, &out)
+		require.NoError(t, err)
+		assert.Equal(t, pinned, got)
+		assert.Contains(t, out.String(), "warning: could not give the override the stack's owner: read owner: ")
+		_, doc := readOverride(t, stack)
+		assert.Equal(t, pinned, serviceImage(doc, "web"))
 	})
 	t.Run("a pinned ref is written verbatim and not resolved", func(t *testing.T) {
 		t.Cleanup(cleanOverride)

internal/agent/updater/override.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/api/types/mount"
@@ -34,15 +35,68 @@ var overrideNames = []string{
 // keeping every other entry in the file, and returns the file's path. It
 // refuses when the file exists without the header, or when another
 // auto-loaded override file exists in dir. The write is atomic.
-func WriteOverride(dir, service, ref string) (string, error) {
+//
+// composeFile is the stack's first compose file. The new file gets the owner
+// of the existing override, or of composeFile when there is none or the
+// existing one is owned by root (see pickOwner). The agent runs as root and the
+// rename creates a new file, so without this the stack's owner would lose
+// control of the file. ownerErr reports that the owner could not be read or
+// set; the override is still written. err means it was not written.
+func WriteOverride(dir, service, ref, composeFile string) (path string, ownerErr error, err error) {
+	overridePath := filepath.Join(dir, OverrideName)
+	return writeOverride(dir, service, ref, func() (fileOwner, error) {
+		return resolveOwner(overridePath, composeFile)
+	})
+}
+
+// fileOwner is the uid and gid of a file.
+type fileOwner struct{ uid, gid int }
+
+// readOwner returns the owner of the file at path.
+func readOwner(path string) (fileOwner, error) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return fileOwner{}, fmt.Errorf("read owner: %w", err)
+	}
+	st, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		return fileOwner{}, fmt.Errorf("read owner of %s: not available on this platform", path)
+	}
+	return fileOwner{int(st.Uid), int(st.Gid)}, nil
+}
+
+// pickOwner chooses the owner of a new override: the existing override's
+// (nil when there is none), unless that is root, then the compose file's.
+// Overrides written before the agent set owners are root-owned and would
+// otherwise stay so. compose is only called when needed.
+func pickOwner(override *fileOwner, compose func() (fileOwner, error)) (fileOwner, error) {
+	if override != nil && override.uid != 0 {
+		return *override, nil
+	}
+	return compose()
+}
+
+// resolveOwner applies pickOwner to the files on disk.
+func resolveOwner(overridePath, composeFile string) (fileOwner, error) {
+	var existing *fileOwner
+	if o, err := readOwner(overridePath); err == nil {
+		existing = &o
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return fileOwner{}, err
+	}
+	return pickOwner(existing, func() (fileOwner, error) { return readOwner(composeFile) })
+}
+
+// writeOverride is WriteOverride with the owner lookup injected, for tests.
+func writeOverride(dir, service, ref string, owner func() (fileOwner, error)) (string, error, error) {
 	for _, name := range overrideNames {
 		if name == OverrideName {
 			continue
 		}
 		if _, err := os.Lstat(filepath.Join(dir, name)); err == nil {
-			return "", fmt.Errorf("refused: %s exists; the agent only manages %s and never edits operator files", name, OverrideName)
+			return "", nil, fmt.Errorf("refused: %s exists; the agent only manages %s and never edits operator files", name, OverrideName)
 		} else if !errors.Is(err, os.ErrNotExist) {
-			return "", fmt.Errorf("check %s: %w", name, err)
+			return "", nil, fmt.Errorf("check %s: %w", name, err)
 		}
 	}
 	path := filepath.Join(dir, OverrideName)
@@ -51,12 +105,12 @@ func WriteOverride(dir, service, ref string) (string, error) {
 	switch {
 	case errors.Is(err, os.ErrNotExist):
 	case err != nil:
-		return "", fmt.Errorf("read override: %w", err)
+		return "", nil, fmt.Errorf("read override: %w", err)
 	case !bytes.HasPrefix(raw, []byte(overrideHeader)):
-		return "", fmt.Errorf("refused: %s exists and was not written by package-updater", path)
+		return "", nil, fmt.Errorf("refused: %s exists and was not written by package-updater", path)
 	default:
 		if err := yaml.Unmarshal(raw, &doc); err != nil {
-			return "", fmt.Errorf("parse override: %w", err)
+			return "", nil, fmt.Errorf("parse override: %w", err)
 		}
 		if doc == nil {
 			doc = map[string]any{}
@@ -75,30 +129,38 @@ func WriteOverride(dir, service, ref string) (string, error) {
 	doc["services"] = services
 	body, err := yaml.Marshal(doc)
 	if err != nil {
-		return "", fmt.Errorf("encode override: %w", err)
+		return "", nil, fmt.Errorf("encode override: %w", err)
 	}
 	// Temp file in the same directory + rename: a crash never leaves a
 	// half-written override that compose would then load.
 	tmp, err := os.CreateTemp(dir, "."+OverrideName+".*")
 	if err != nil {
-		return "", fmt.Errorf("write override: %w", err)
+		return "", nil, fmt.Errorf("write override: %w", err)
 	}
 	defer os.Remove(tmp.Name()) // no-op after a successful rename
 	if _, err := tmp.WriteString(overrideHeader + string(body)); err != nil {
 		tmp.Close()
-		return "", fmt.Errorf("write override: %w", err)
+		return "", nil, fmt.Errorf("write override: %w", err)
 	}
 	if err := tmp.Chmod(0o644); err != nil {
 		tmp.Close()
-		return "", fmt.Errorf("write override: %w", err)
+		return "", nil, fmt.Errorf("write override: %w", err)
+	}
+	// Chown the open file, not the path after the rename: a path could have
+	// been swapped for a symlink by someone with write access to the stack.
+	var ownerErr error
+	if o, err := owner(); err != nil {
+		ownerErr = err
+	} else if err := tmp.Chown(o.uid, o.gid); err != nil {
+		ownerErr = fmt.Errorf("set owner %d:%d: %w", o.uid, o.gid, err)
 	}
 	if err := tmp.Close(); err != nil {
-		return "", fmt.Errorf("write override: %w", err)
+		return "", nil, fmt.Errorf("write override: %w", err)
 	}
 	if err := os.Rename(tmp.Name(), path); err != nil {
-		return "", fmt.Errorf("write override: %w", err)
+		return "", nil, fmt.Errorf("write override: %w", err)
 	}
-	return path, nil
+	return path, ownerErr, nil
 }
 
 // SelfInspector is the subset of the Docker SDK MountRoots needs.

internal/agent/updater/override_test.go

@@ -29,11 +29,96 @@ func serviceImage(doc map[string]any, name string) any {
 	return entry["image"]
 }
 
+// stackFile creates dir/compose.yaml and returns its path.
+func stackFile(t *testing.T, dir string) string {
+	t.Helper()
+	p := filepath.Join(dir, "compose.yaml")
+	require.NoError(t, os.WriteFile(p, []byte("services: {}\n"), 0o644))
+	return p
+}
+
+func TestPickOwner(t *testing.T) {
+	compose := fileOwner{1000, 1000}
+	fromCompose := func() (fileOwner, error) { return compose, nil }
+	failing := func() (fileOwner, error) { return fileOwner{}, errors.New("boom") }
+	tests := []struct {
+		name     string
+		override *fileOwner
+		compose  func() (fileOwner, error)
+		want     fileOwner
+		wantErr  bool
+	}{
+		{"no override uses the compose file", nil, fromCompose, compose, false},
+		{"override owned by a user wins", &fileOwner{1001, 1002}, fromCompose, fileOwner{1001, 1002}, false},
+		{"root-owned override uses the compose file", &fileOwner{0, 0}, fromCompose, compose, false},
+		{"compose file is not read when the override decides", &fileOwner{1001, 1002}, failing, fileOwner{1001, 1002}, false},
+		{"unreadable compose file is an error", nil, failing, fileOwner{}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := pickOwner(tt.override, tt.compose)
+			assert.Equal(t, tt.wantErr, err != nil)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}
+
+func TestResolveOwner(t *testing.T) {
+	me := fileOwner{os.Getuid(), os.Getgid()}
+	t.Run("no override uses the compose file", func(t *testing.T) {
+		dir := t.TempDir()
+		got, err := resolveOwner(filepath.Join(dir, OverrideName), stackFile(t, dir))
+		require.NoError(t, err)
+		assert.Equal(t, me, got)
+	})
+	t.Run("existing override is used without the compose file", func(t *testing.T) {
+		dir := t.TempDir()
+		require.NoError(t, os.WriteFile(filepath.Join(dir, OverrideName), nil, 0o644))
+		got, err := resolveOwner(filepath.Join(dir, OverrideName), filepath.Join(dir, "missing.yaml"))
+		require.NoError(t, err)
+		assert.Equal(t, me, got)
+	})
+	t.Run("neither file is an error", func(t *testing.T) {
+		dir := t.TempDir()
+		_, err := resolveOwner(filepath.Join(dir, OverrideName), filepath.Join(dir, "missing.yaml"))
+		assert.Error(t, err)
+	})
+}
+
+func TestWriteOverride_Owner(t *testing.T) {
+	t.Run("success sets no ownerErr", func(t *testing.T) {
+		dir := t.TempDir()
+		_, ownerErr, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
+		require.NoError(t, err)
+		assert.NoError(t, ownerErr)
+	})
+	t.Run("unreadable owner is reported and the file is still written", func(t *testing.T) {
+		dir := t.TempDir()
+		_, ownerErr, err := WriteOverride(dir, "web", "a:1@sha256:aa", filepath.Join(dir, "missing.yaml"))
+		require.NoError(t, err)
+		assert.ErrorContains(t, ownerErr, "read owner")
+		_, doc := readOverride(t, dir)
+		assert.Equal(t, "a:1@sha256:aa", serviceImage(doc, "web"))
+	})
+	t.Run("chown failure is reported and the file is still written", func(t *testing.T) {
+		if os.Getuid() == 0 {
+			t.Skip("root can chown to anyone")
+		}
+		dir := t.TempDir()
+		_, ownerErr, err := writeOverride(dir, "web", "a:1@sha256:aa", func() (fileOwner, error) { return fileOwner{0, 0}, nil })
+		require.NoError(t, err)
+		assert.ErrorContains(t, ownerErr, "set owner 0:0")
+		_, doc := readOverride(t, dir)
+		assert.Equal(t, "a:1@sha256:aa", serviceImage(doc, "web"))
+	})
+}
+
 func TestWriteOverride(t *testing.T) {
 	t.Run("new file", func(t *testing.T) {
 		dir := t.TempDir()
-		path, err := WriteOverride(dir, "web", "a:1@sha256:aa")
+		path, ownerErr, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
 		require.NoError(t, err)
+		assert.NoError(t, ownerErr)
 		assert.Equal(t, filepath.Join(dir, OverrideName), path)
 		raw, doc := readOverride(t, dir)
 		assert.True(t, len(raw) > len(overrideHeader) && raw[:len(overrideHeader)] == overrideHeader)
@@ -42,7 +127,7 @@ func TestWriteOverride(t *testing.T) {
 	t.Run("keeps other services", func(t *testing.T) {
 		dir := t.TempDir()
 		require.NoError(t, os.WriteFile(filepath.Join(dir, OverrideName), []byte(overrideHeader+"services:\n  db:\n    image: db:1\n"), 0o644))
-		_, err := WriteOverride(dir, "web", "a:1@sha256:aa")
+		_, _, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
 		require.NoError(t, err)
 		_, doc := readOverride(t, dir)
 		assert.Equal(t, "db:1", serviceImage(doc, "db"))
@@ -51,7 +136,7 @@ func TestWriteOverride(t *testing.T) {
 	t.Run("replaces own entry", func(t *testing.T) {
 		dir := t.TempDir()
 		require.NoError(t, os.WriteFile(filepath.Join(dir, OverrideName), []byte(overrideHeader+"services:\n  web:\n    image: old\n"), 0o644))
-		_, err := WriteOverride(dir, "web", "a:2@sha256:bb")
+		_, _, err := WriteOverride(dir, "web", "a:2@sha256:bb", stackFile(t, dir))
 		require.NoError(t, err)
 		_, doc := readOverride(t, dir)
 		assert.Equal(t, "a:2@sha256:bb", serviceImage(doc, "web"))
@@ -60,7 +145,7 @@ func TestWriteOverride(t *testing.T) {
 		dir := t.TempDir()
 		const body = "services:\n  web:\n    environment: [A=1]\n"
 		require.NoError(t, os.WriteFile(filepath.Join(dir, OverrideName), []byte(body), 0o644))
-		_, err := WriteOverride(dir, "web", "a:1@sha256:aa")
+		_, _, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
 		assert.ErrorContains(t, err, "refused")
 		got, _ := os.ReadFile(filepath.Join(dir, OverrideName))
 		assert.Equal(t, body, string(got))
@@ -69,7 +154,7 @@ func TestWriteOverride(t *testing.T) {
 		t.Run("refuses next to "+other, func(t *testing.T) {
 			dir := t.TempDir()
 			require.NoError(t, os.WriteFile(filepath.Join(dir, other), []byte("services: {}\n"), 0o644))
-			_, err := WriteOverride(dir, "web", "a:1@sha256:aa")
+			_, _, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
 			assert.ErrorContains(t, err, other)
 			_, statErr := os.Stat(filepath.Join(dir, OverrideName))
 			assert.True(t, errors.Is(statErr, os.ErrNotExist))
@@ -77,12 +162,13 @@ func TestWriteOverride(t *testing.T) {
 	}
 	t.Run("leaves no temp files", func(t *testing.T) {
 		dir := t.TempDir()
-		_, err := WriteOverride(dir, "web", "a:1@sha256:aa")
+		_, _, err := WriteOverride(dir, "web", "a:1@sha256:aa", stackFile(t, dir))
 		require.NoError(t, err)
 		entries, err := os.ReadDir(dir)
 		require.NoError(t, err)
-		require.Len(t, entries, 1)
-		assert.Equal(t, OverrideName, entries[0].Name())
+		require.Len(t, entries, 2)
+		assert.Equal(t, "compose.override.yaml", entries[0].Name())
+		assert.Equal(t, "compose.yaml", entries[1].Name())
 	})
 }