src/pkg/cosboot/efi.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

import (
	"bytes"
	"fmt"
	"log"
	"os"
	"os/exec"
	"path/filepath"

	"cos.googlesource.com/cos/tools.git/src/pkg/tools/partutil"
)

func readEFIFileOnCOS(path string) ([]byte, error) {
	efiPath, err := partutil.MountEFIPartition()
	if err != nil {
		return nil, err
	}
	data, err := os.ReadFile(filepath.Join(efiPath, path))
	if err != nil {
		unmountErr := partutil.UnmountEFIPartition()
		if unmountErr != nil {
			log.Printf("ERROR: failed to unmount EFI partition: %v", unmountErr)
		}
		return nil, fmt.Errorf("failed to read EFI file %q: %v", path, err)
	}
	if err := partutil.UnmountEFIPartition(); err != nil {
		return nil, fmt.Errorf("failed to unmount EFI partition: %v", err)
	}
	return data, nil
}

func readEFIFileOffline(disk, path string) ([]byte, error) {
	sfdiskTable, err := partutil.ReadPartitionTableJSON(disk)
	if err != nil {
		return nil, fmt.Errorf("failed to get GPT from %q: %v", disk, err)
	}
	partTable := sfdiskTable.PartitionTable
	var efiPart *partutil.Partition
	for i := range partTable.Partitions {
		if partTable.Partitions[i].Name == "EFI-SYSTEM" {
			efiPart = &partTable.Partitions[i]
			break
		}
	}
	if efiPart == nil {
		return nil, fmt.Errorf("failed to find EFI partition on %q", disk)
	}
	tmpDir, err := os.MkdirTemp("", "cos-efi")
	if err != nil {
		return nil, err
	}
	defer os.RemoveAll(tmpDir)
	start := efiPart.Start * partTable.SectorSize
	size := efiPart.Size * partTable.SectorSize
	buf := make([]byte, size)
	in, err := os.Open(disk)
	if err != nil {
		return nil, err
	}
	defer in.Close()
	if _, err := in.ReadAt(buf, int64(start)); err != nil {
		return nil, err
	}
	efiImage := filepath.Join(tmpDir, "efi.bin")
	if err := os.WriteFile(efiImage, buf, 0644); err != nil {
		return nil, err
	}
	var stdout bytes.Buffer
	var stderr bytes.Buffer
	cmd := exec.Command("mcopy", "-i", efiImage, "::"+path, "-")
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		log.Printf("Command %q failed with stderr: %v", cmd.String(), string(stderr.Bytes()))
		return nil, fmt.Errorf("cannot mcopy from EFI image sourced from %q: %v", disk, err)
	}
	return stdout.Bytes(), nil
}

func ReadEFIFile(disk, path string) ([]byte, error) {
	if disk == "" {
		return readEFIFileOnCOS(path)
	} else {
		return readEFIFileOffline(disk, path)
	}
}