fs: don't log the contents of objects without a String method

The logging functions take an object which is rendered into the log
line. Rendering it with %+v dumps all its fields, which for an object
holding backend config would include decrypted credentials. Every
object currently logged is a string or has a String method, so render
anything else as just its type to keep credentials out of the logs.

See: https://github.com/rclone/rclone/security/code-scanning/183

fs/log.go

@@ -146,8 +146,17 @@ func logSlog(level LogLevel, text string, attrs []any) {
 
 func logSlogWithObject(level LogLevel, o any, text string, attrs []any) {
 	if o != nil {
+		var object string
+		switch o.(type) {
+		case fmt.Stringer, string:
+			object = fmt.Sprint(o)
+		default:
+			// Don't render the fields of arbitrary objects as they
+			// may contain sensitive data such as credentials.
+			object = fmt.Sprintf("%T", o)
+		}
 		attrs = slices.Concat(attrs, []any{
-			"object", fmt.Sprintf("%+v", o),
+			"object", object,
 			"objectType", fmt.Sprintf("%T", o),
 		})
 	}

fs/log_test.go

@@ -1,8 +1,10 @@
 package fs
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
+	"log/slog"
 	"strconv"
 	"testing"
 
@@ -32,6 +34,30 @@ func TestLogValue(t *testing.T) {
 	assert.Equal(t, "", x.String())
 }
 
+func TestLogSlogWithObject(t *testing.T) {
+	var buf bytes.Buffer
+	oldLogger := logger
+	defer func() { logger = oldLogger }()
+	SetLogger(slog.NewTextHandler(&buf, nil))
+
+	// Objects with a String method are rendered with it
+	logSlogWithObject(LogLevelError, withString{}, "message", nil)
+	assert.Contains(t, buf.String(), "object=hello")
+
+	// Plain strings are rendered as themselves
+	buf.Reset()
+	logSlogWithObject(LogLevelError, "potato", "message", nil)
+	assert.Contains(t, buf.String(), "object=potato")
+
+	// Anything else shows only its type as it may contain
+	// sensitive data such as credentials
+	buf.Reset()
+	type secrets struct{ Password string }
+	logSlogWithObject(LogLevelError, &secrets{Password: "SECRET"}, "message", nil)
+	assert.Contains(t, buf.String(), "object=*fs.secrets")
+	assert.NotContains(t, buf.String(), "SECRET")
+}
+
 func TestLogLevelString(t *testing.T) {
 	for _, test := range []struct {
 		in   LogLevel