fix(agentcompat): secure legacy file manager boundaries

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

integration/agentcompat/internal/scenario/legacy_fm.go

@@ -101,12 +101,14 @@ func (LegacyFM) Run(ctx context.Context, input LegacyFMInput) (result Result, ru
 	payload := bytes.Repeat(payloadPattern, (1<<20+257)/len(payloadPattern)+1)
 	payload = payload[:1<<20+257]
 	sentinel := []byte("outside-fm-root-sentinel")
-	sentinelPaths := []string{
-		filepath.Join(agentInstance.WorkspaceRoot(), "outside-fm-root-a.txt"),
-		filepath.Join(agentInstance.WorkspaceRoot(), "outside-fm-root-b.txt"),
+	workspaceRoot, err := os.OpenRoot(agentInstance.WorkspaceRoot())
+	if err != nil {
+		return finishLegacyFM(assertions, err)
 	}
-	for _, path := range sentinelPaths {
-		if err := os.WriteFile(path, sentinel, 0o600); err != nil {
+	defer workspaceRoot.Close()
+	sentinelNames := []string{"outside-fm-root-a.txt", "outside-fm-root-b.txt"}
+	for _, name := range sentinelNames {
+		if err := workspaceRoot.WriteFile(name, sentinel, 0o600); err != nil {
 			return finishLegacyFM(assertions, err)
 		}
 	}
@@ -252,7 +254,7 @@ func (LegacyFM) Run(ctx context.Context, input LegacyFMInput) (result Result, ru
 		return finishLegacyFM(assertions, err)
 	}
 
-	sentinelErr := verifyLegacyFMSentinels(sentinelPaths, sentinel)
+	sentinelErr := verifyLegacyFMSentinels(workspaceRoot, sentinelNames, sentinel)
 	assertions.Record("outside-root sentinels remain unchanged", sentinelErr == nil, errorText(sentinelErr))
 	if sentinelErr != nil {
 		return finishLegacyFM(assertions, sentinelErr)

integration/agentcompat/internal/scenario/legacy_fm_support.go

@@ -21,6 +21,9 @@ type legacyFMUserForm struct {
 	Password string `json:"password"`
 }
 
+const legacyFMForeignUsername = "agentcompat-fm-foreign"
+const legacyFMForeignPassword = "agentcompat-fm-password" // #nosec G101 -- Ephemeral localhost integration fixture, not a production credential.
+
 type legacyFMFrameWriter interface {
 	WriteFrame(context.Context, client.Frame) error
 }
@@ -116,13 +119,11 @@ func findLegacyFMServerID(ctx context.Context, dashboardInstance *dashboard.Dash
 }
 
 func createForeignLegacyFMClient(ctx context.Context, dashboardInstance *dashboard.Dashboard) (*client.Client, func() error, error) {
-	const username = "agentcompat-fm-foreign"
-	const password = "agentcompat-fm-password"
 	admin := dashboardInstance.Clients().REST
 	userID, err := client.DoREST[legacyFMUserForm, uint64](ctx, admin, client.RESTRequest[legacyFMUserForm]{
 		Method: http.MethodPost,
 		Path:   "/api/v1/user",
-		Body:   &legacyFMUserForm{Role: 1, Username: username, Password: password},
+		Body:   &legacyFMUserForm{Role: 1, Username: legacyFMForeignUsername, Password: legacyFMForeignPassword},
 	})
 	if err != nil {
 		return nil, func() error { return nil }, err
@@ -137,7 +138,7 @@ func createForeignLegacyFMClient(ctx context.Context, dashboardInstance *dashboa
 	if err != nil {
 		return nil, func() error { return nil }, errors.Join(err, cleanup())
 	}
-	if _, err := loginClient.Login(ctx, client.LoginRequest{Username: username, Password: password}); err != nil {
+	if _, err := loginClient.Login(ctx, client.LoginRequest{Username: legacyFMForeignUsername, Password: legacyFMForeignPassword}); err != nil {
 		return nil, func() error { return nil }, errors.Join(err, cleanup())
 	}
 	pat, err := client.DoREST[patRequest, patResponse](ctx, loginClient, client.RESTRequest[patRequest]{

integration/agentcompat/internal/scenario/legacy_fm_verification.go

@@ -133,9 +133,9 @@ func newLegacyFMRunID() (string, error) {
 	return hex.EncodeToString(bytes), nil
 }
 
-func verifyLegacyFMSentinels(sentinelPaths []string, sentinel []byte) error {
-	for _, path := range sentinelPaths {
-		content, err := os.ReadFile(path)
+func verifyLegacyFMSentinels(root *os.Root, sentinelNames []string, sentinel []byte) error {
+	for _, name := range sentinelNames {
+		content, err := root.ReadFile(name)
 		if err != nil {
 			return err
 		}