src/pkg/cosboot/grubconfig.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/exec"
	"regexp"
	"strconv"
	"strings"

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

var (
	defaultARegex  = regexp.MustCompile(`defaultA=(.*)`)
	defaultBRegex  = regexp.MustCompile(`defaultB=(.*)`)
	menuEntryRegex = regexp.MustCompile(`(?m)^menuentry\s+.*\{[\s\S]*?^\}`)
)

func readGPTOnCOS() (partutil.SFDiskTable, error) {
	var stdout bytes.Buffer
	var stderr bytes.Buffer
	cmd := exec.Command("rootdev", "-s", "-d")
	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 partutil.SFDiskTable{}, fmt.Errorf("cannot determine boot disk: %v", err)
	}
	disk := strings.TrimSpace(string(stdout.Bytes()))
	return partutil.ReadPartitionTableJSON(disk)
}

func isBootA(disk string) (bool, error) {
	var gpt partutil.SFDiskTable
	if disk == "" {
		var err error
		gpt, err = readGPTOnCOS()
		if err != nil {
			return false, err
		}
	} else {
		var err error
		gpt, err = partutil.ReadPartitionTableJSON(disk)
		if err != nil {
			return false, err
		}
	}
	part2Attrs := gpt.PartitionTable.Partitions[1].Attrs
	part2Prio, err := partutil.GPTPriority(part2Attrs)
	if err != nil {
		return false, fmt.Errorf("could not get partition 2 GPTPriority: %v", err)
	}
	part4Attrs := gpt.PartitionTable.Partitions[3].Attrs
	part4Prio, err := partutil.GPTPriority(part4Attrs)
	if err != nil {
		return false, fmt.Errorf("could not get partition 4 GPTPriority: %v", err)
	}
	return part2Prio >= part4Prio, nil
}

func NextGRUBCmdLine(disk string) (string, error) {
	grubConfig, err := ReadEFIFile(disk, partutil.GRUBCfgEFIPath)
	if err != nil {
		return "", err
	}
	grubConfigStr := string(grubConfig)
	matches := defaultARegex.FindStringSubmatch(grubConfigStr)
	if len(matches) < 2 {
		return "", fmt.Errorf("could not find defaultA in grub config: %v", grubConfigStr)
	}
	defaultA, err := strconv.Atoi(matches[1])
	if err != nil {
		return "", err
	}
	matches = defaultBRegex.FindStringSubmatch(grubConfigStr)
	if len(matches) < 2 {
		return "", fmt.Errorf("could not find defaultB in grub config: %v", grubConfigStr)
	}
	defaultB, err := strconv.Atoi(matches[1])
	if err != nil {
		return "", err
	}
	useBootA, err := isBootA(disk)
	if err != nil {
		return "", fmt.Errorf("Failed to determine if next boot is boot A on disk %q: %v", disk, err)
	}
	menuEntries := menuEntryRegex.FindAllString(grubConfigStr, -1)
	var menuEntry string
	if useBootA {
		menuEntry = strings.TrimSpace(menuEntries[defaultA])
	} else {
		menuEntry = strings.TrimSpace(menuEntries[defaultB])
	}
	lines := strings.Split(menuEntry, "\n")
	if len(lines) < 3 {
		return "", fmt.Errorf("Menuentry is malformed: %v", menuEntry)
	}
	cmdline := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(lines[1]), "linux"))
	return cmdline, nil
}