Refuse a build output directory that is a link out of the module (issue 1589)

modules/mesh-build-go/cmd/mesh-build-go/archive.go

@@ -300,3 +300,46 @@ func within(base, path string) bool {
 	rel, err := filepath.Rel(base, path)
 	return err == nil && rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator))
 }
+
+// resolveDeepest follows the links of the deepest part of p that exists and appends the rest, so a path not yet made
+// is judged by where it would be made.
+func resolveDeepest(p string) (string, error) {
+	rest := ""
+	for cur := filepath.Clean(p); ; {
+		if real, err := filepath.EvalSymlinks(cur); err == nil {
+			return filepath.Join(real, rest), nil
+		} else if !os.IsNotExist(err) {
+			return "", err
+		}
+		parent := filepath.Dir(cur)
+		if parent == cur {
+			return "", fmt.Errorf("no part of %s exists", p)
+		}
+		rest = filepath.Join(filepath.Base(cur), rest)
+		cur = parent
+	}
+}
+
+// insideModule refuses a path whose real place is not inside the module's directory, which is itself inside the
+// workspace's tree: a link committed in the repository must not lead a build's writes or its pack out.
+func (m *Module) insideModule(workspace, dir, path string) error {
+	realTree, err := filepath.EvalSymlinks(filepath.Join(m.Work, workspace, "tree"))
+	if err != nil {
+		return err
+	}
+	realDir, err := filepath.EvalSymlinks(dir)
+	if err != nil {
+		return err
+	}
+	if !within(realTree, realDir) {
+		return fmt.Errorf("the module directory resolves to %s, which lies outside the workspace's tree %s", realDir, realTree)
+	}
+	real, err := resolveDeepest(path)
+	if err != nil {
+		return err
+	}
+	if !within(realDir, real) {
+		return fmt.Errorf("%s resolves to %s, which lies outside the module %s", path, real, realDir)
+	}
+	return nil
+}

modules/mesh-build-go/cmd/mesh-build-go/build.go

@@ -117,6 +117,9 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 
 	// Its own output directory, made empty: a build of the same tree twice is one build.
 	out := filepath.Join(dir, outDir, art.Name)
+	if err := m.insideModule(workspace, dir, out); err != nil {
+		return nil, fmt.Errorf("the output directory of %s: %w", art.Name, err)
+	}
 	if err := os.RemoveAll(out); err != nil {
 		return nil, err
 	}
@@ -133,6 +136,9 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 	if r := m.Run(ctx, m.buildEnv(dir), "go", cmd...); r.Status != 0 || r.Err != "" {
 		return nil, fmt.Errorf("go build of %s: %s", art.Name, causeLine(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
 	}
+	if err := m.insideModule(workspace, dir, out); err != nil {
+		return nil, fmt.Errorf("the output directory of %s: %w", art.Name, err)
+	}
 	res, err := m.packInto(workspace, module, art.Name, out)
 	if err != nil {
 		return nil, err

modules/mesh-build-go/cmd/mesh-build-go/build_test.go

@@ -215,3 +215,27 @@ func TestAModuleDirectoryOutsideTheWorkspaceIsRefused(t *testing.T) {
 		t.Errorf("the step ended %s after %d commands", st.State, len(*calls))
 	}
 }
+
+// Review of issue 1589: the output directory lies in the module; a committed `.mesh-build` that is a link out of it
+// must not take the build's writes, removals or the pack outside.
+func TestABuildOutputDirectoryLinkedOutsideTheModuleIsRefused(t *testing.T) {
+	m, states, calls := buildModule(t, manifestWith(goBundle), "the program")
+	outside := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(outside, "tools"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	kept := filepath.Join(outside, "tools", "precious")
+	if err := os.WriteFile(kept, []byte("mine"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(m.Work, "ws1", "tree", "modules", "demo", ".mesh-build")); err != nil {
+		t.Fatal(err)
+	}
+	_, st := runBuild(t, m, states, "tools")
+	if st.State != delivery.StepFailed || !strings.Contains(st.Reason, "outside") || !strings.Contains(st.Reason, outside) || len(*calls) != 0 {
+		t.Errorf("the step ended %s: %q after %d commands", st.State, st.Reason, len(*calls))
+	}
+	if body, err := os.ReadFile(kept); err != nil || string(body) != "mine" {
+		t.Errorf("a file outside the module was touched: %q, %v", body, err)
+	}
+}