src/pkg/cosboot/cosboot.go

// Copyright 2026 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package cosboot exports utilities for interacting with the COS boot process.
package cosboot

import (
	"bytes"
	"debug/pe"
	"fmt"
	"strings"
)

type BootPath string

const (
	UnknownBoot = BootPath("UnknownBoot")
	GRUBBoot    = BootPath("GRUBBoot")
	UKI         = BootPath("UKI")
)

func isShim(exe []byte) (bool, error) {
	exeFile, err := pe.NewFile(bytes.NewReader(exe))
	if err != nil {
		// Invalid PE file - not shim
		return false, nil
	}
	defer exeFile.Close()
	section := exeFile.Section(".sbat")
	if section == nil {
		// No SBAT - not shim
		return false, nil
	}
	data, err := section.Data()
	if err != nil {
		return false, err
	}
	sbat := string(data)
	if strings.Contains(sbat, "https://github.com/rhboot/shim") {
		return true, nil
	}
	return false, nil
}

func DiskBootPath(disk string) (BootPath, error) {
	bootx64, err := ReadEFIFile(disk, "/efi/boot/bootx64.efi")
	if err != nil {
		return UnknownBoot, fmt.Errorf("could not read bootx64.efi: %v", err)
	}
	bootx64IsShim, err := isShim(bootx64)
	if err != nil {
		return UnknownBoot, fmt.Errorf("could not determine if bootx64.efi is shim: %v", err)
	}
	if bootx64IsShim {
		return GRUBBoot, nil
	} else {
		return UKI, nil
	}
}