Confine subprocess diagnostics and retain redacted tails

internal/fileops/confined.go

@@ -184,15 +184,21 @@ func syncOpenedDirectory(parent *os.Root) error {
 }
 
 // OpenFileConfined opens a file after verifying its parent hierarchy without
-// following symbolic links. Existing symbolic-link leaves are rejected.
+// following symbolic links. Existing symbolic-link and non-regular leaves are
+// rejected.
 func OpenFileConfined(path string, flags int, mode os.FileMode) (*os.File, error) {
 	parent, name, err := openConfinedParent(path, false, 0)
 	if err != nil {
 		return nil, err
 	}
 	defer func() { _ = parent.Close() }()
-	if info, err := parent.Lstat(name); err == nil && info.Mode()&os.ModeSymlink != 0 {
-		return nil, fmt.Errorf("destination file %q is a symbolic link", name)
+	if info, err := parent.Lstat(name); err == nil {
+		if info.Mode()&os.ModeSymlink != 0 {
+			return nil, fmt.Errorf("destination file %q is a symbolic link", name)
+		}
+		if !info.Mode().IsRegular() {
+			return nil, fmt.Errorf("destination file %q is not a regular file", name)
+		}
 	} else if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return nil, fmt.Errorf("inspect destination file %q: %w", name, err)
 	}
@@ -206,7 +212,7 @@ func OpenFileConfined(path string, flags int, mode os.FileMode) (*os.File, error
 		return nil, fmt.Errorf("inspect opened destination file %q: %w", name, err)
 	}
 	current, err := parent.Lstat(name)
-	if err != nil || current.Mode()&os.ModeSymlink != 0 || !os.SameFile(opened, current) {
+	if err != nil || !opened.Mode().IsRegular() || current.Mode()&os.ModeSymlink != 0 || !current.Mode().IsRegular() || !os.SameFile(opened, current) {
 		_ = file.Close()
 		if err != nil {
 			return nil, fmt.Errorf("reinspect destination file %q: %w", name, err)

internal/fileops/confined_open_test.go

@@ -0,0 +1,56 @@
+package fileops
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestOpenFileConfinedRejectsNonRegularLeaf(t *testing.T) {
+	leaf := filepath.Join(t.TempDir(), "diagnostic.log")
+	if err := os.Mkdir(leaf, 0o700); err != nil {
+		t.Fatalf("Mkdir() error = %v", err)
+	}
+	file, err := OpenFileConfined(leaf, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, WorkspaceFileMode)
+	if file != nil {
+		_ = file.Close()
+		t.Fatal("OpenFileConfined() returned a file for a directory")
+	}
+	if err == nil || !strings.Contains(err.Error(), "not a regular file") {
+		t.Fatalf("OpenFileConfined() error = %v, want regular-file rejection", err)
+	}
+}
+
+func TestOpenFileConfinedRejectsSymlinkWithoutTruncatingTarget(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("creating symlinks requires privileges that are not available on every Windows runner")
+	}
+	dir := t.TempDir()
+	target := filepath.Join(dir, "target.log")
+	const original = "outside content"
+	if err := os.WriteFile(target, []byte(original), 0o600); err != nil {
+		t.Fatalf("WriteFile(target) error = %v", err)
+	}
+	leaf := filepath.Join(dir, "diagnostic.log")
+	if err := os.Symlink(target, leaf); err != nil {
+		t.Fatalf("Symlink() error = %v", err)
+	}
+
+	file, err := OpenFileConfined(leaf, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, WorkspaceFileMode)
+	if file != nil {
+		_ = file.Close()
+		t.Fatal("OpenFileConfined() returned a file for a symbolic link")
+	}
+	if err == nil || !strings.Contains(err.Error(), "symbolic link") {
+		t.Fatalf("OpenFileConfined() error = %v, want symbolic-link rejection", err)
+	}
+	data, readErr := os.ReadFile(target)
+	if readErr != nil {
+		t.Fatalf("ReadFile(target) error = %v", readErr)
+	}
+	if string(data) != original {
+		t.Fatalf("target content = %q, want %q", data, original)
+	}
+}