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env.go
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env.go
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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2017 Canonical Ltd
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 3 as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package ubootenv
import (
"bufio"
"bytes"
"encoding/binary"
"errors"
"fmt"
"hash/crc32"
"io"
"os"
"path/filepath"
"sort"
"strings"
)
// Env contains the data of the uboot environment
type Env struct {
fname string
size int
headerFlagByte bool
data map[string]string
}
// little endian helpers
func readUint32(data []byte) uint32 {
var ret uint32
buf := bytes.NewBuffer(data)
binary.Read(buf, binary.LittleEndian, &ret)
return ret
}
func writeUint32(u uint32) []byte {
buf := bytes.NewBuffer(nil)
binary.Write(buf, binary.LittleEndian, &u)
return buf.Bytes()
}
const sizeOfUint32 = 4
func calcHeaderSize(headerFlagByte bool) int {
if headerFlagByte {
// If uboot uses a header flag byte, header is 4 byte crc + flag byte
return sizeOfUint32 + 1
}
// otherwise, just a 4 byte crc
return sizeOfUint32
}
type CreateOptions struct {
HeaderFlagByte bool
}
// Create a new empty uboot env file with the given size
func Create(fname string, size int, opts CreateOptions) (*Env, error) {
f, err := os.Create(fname)
if err != nil {
return nil, err
}
defer f.Close()
env := &Env{
fname: fname,
size: size,
headerFlagByte: opts.HeaderFlagByte,
data: make(map[string]string),
}
return env, nil
}
// OpenFlags instructs open how to alter its behavior.
type OpenFlags int
const (
// OpenBestEffort instructs OpenWithFlags to skip malformed data without returning an error.
OpenBestEffort OpenFlags = 1 << iota
)
// Open opens a existing uboot env file
func Open(fname string) (*Env, error) {
return OpenWithFlags(fname, OpenFlags(0))
}
// OpenWithFlags opens a existing uboot env file, passing additional flags.
func OpenWithFlags(fname string, flags OpenFlags) (*Env, error) {
f, err := os.Open(fname)
if err != nil {
return nil, err
}
defer f.Close()
contentWithHeader, err := io.ReadAll(f)
if err != nil {
return nil, err
}
// Most systems have SYS_REDUNDAND_ENVIRONMENT=y, so try that first
tryHeaderFlagByte := true
env, err := readEnv(contentWithHeader, flags, tryHeaderFlagByte)
// if there is a bad CRC, maybe we just assumed the wrong header size
if errors.Is(err, errBadCrc) {
tryHeaderFlagByte := false
env, err = readEnv(contentWithHeader, flags, tryHeaderFlagByte)
}
// if error was not one of the ones that might indicate we assumed the wrong
// header size, or there is still an error after checking both header sizes
// something is actually wrong
if err != nil {
return nil, fmt.Errorf("cannot open %q: %w", fname, err)
}
env.fname = fname
return env, nil
}
var errBadCrc = errors.New("bad CRC")
func readEnv(contentWithHeader []byte, flags OpenFlags, headerFlagByte bool) (*Env, error) {
// The minimum valid env is 6 bytes (4 byte CRC + 2 null bytes for EOF)
// The maximum header length is 5 bytes (4 byte CRC, + )
// If we always make sure our env is 6 bytes long, we'll never run into
// trouble doing some sort of OOB slicing below, but also we will
// accept all legal envs
if len(contentWithHeader) < 6 {
return nil, errors.New("smaller than expected environment block")
}
headerSize := calcHeaderSize(headerFlagByte)
crc := readUint32(contentWithHeader)
payload := contentWithHeader[headerSize:]
actualCRC := crc32.ChecksumIEEE(payload)
if crc != actualCRC {
return nil, fmt.Errorf("%w %v != %v", errBadCrc, crc, actualCRC)
}
if eof := bytes.Index(payload, []byte{0, 0}); eof >= 0 {
payload = payload[:eof]
}
data, err := parseData(payload, flags)
if err != nil {
return nil, err
}
env := &Env{
size: len(contentWithHeader),
headerFlagByte: headerFlagByte,
data: data,
}
return env, nil
}
func parseData(data []byte, flags OpenFlags) (map[string]string, error) {
out := make(map[string]string)
for _, envStr := range bytes.Split(data, []byte{0}) {
if len(envStr) == 0 || envStr[0] == 0 || envStr[0] == 255 {
continue
}
l := strings.SplitN(string(envStr), "=", 2)
if len(l) != 2 || l[0] == "" {
if flags&OpenBestEffort == OpenBestEffort {
continue
}
return nil, fmt.Errorf("cannot parse line %q as key=value pair", envStr)
}
key := l[0]
value := l[1]
out[key] = value
}
return out, nil
}
func (env *Env) String() string {
out := ""
env.iterEnv(func(key, value string) {
out += fmt.Sprintf("%s=%s\n", key, value)
})
return out
}
func (env *Env) Size() int {
return env.size
}
func (env *Env) HeaderFlagByte() bool {
return env.headerFlagByte
}
// Get the value of the environment variable
func (env *Env) Get(name string) string {
return env.data[name]
}
// Set an environment name to the given value, if the value is empty
// the variable will be removed from the environment
func (env *Env) Set(name, value string) {
if name == "" {
panic(fmt.Sprintf("Set() can not be called with empty key for value: %q", value))
}
if value == "" {
delete(env.data, name)
return
}
env.data[name] = value
}
// iterEnv calls the passed function f with key, value for environment
// vars. The order is guaranteed (unlike just iterating over the map)
func (env *Env) iterEnv(f func(key, value string)) {
keys := make([]string, 0, len(env.data))
for k := range env.data {
keys = append(keys, k)
}
sort.Strings(keys)
for _, k := range keys {
if k == "" {
panic("iterEnv iterating over a empty key")
}
f(k, env.data[k])
}
}
// Save will write out the environment data
func (env *Env) Save() error {
headerSize := calcHeaderSize(env.headerFlagByte)
w := bytes.NewBuffer(nil)
// will panic if the buffer can't grow, all writes to
// the buffer will be ok because we sized it correctly
w.Grow(env.size - headerSize)
// write the payload
env.iterEnv(func(key, value string) {
w.Write([]byte(fmt.Sprintf("%s=%s", key, value)))
w.Write([]byte{0})
})
// write double \0 to mark the end of the env
w.Write([]byte{0})
// no keys, so no previous \0 was written so we write one here
if len(env.data) == 0 {
w.Write([]byte{0})
}
// write ff into the remaining parts
writtenSoFar := w.Len()
for i := 0; i < env.size-headerSize-writtenSoFar; i++ {
w.Write([]byte{0xff})
}
// checksum
crc := crc32.ChecksumIEEE(w.Bytes())
// ensure dir sync
dir, err := os.Open(filepath.Dir(env.fname))
if err != nil {
return err
}
defer dir.Close()
// Note that we overwrite the existing file and do not do
// the usual write-rename. The rationale is that we want to
// minimize the amount of writes happening on a potential
// FAT partition where the env is loaded from. The file will
// always be of a fixed size so we know the writes will not
// fail because of ENOSPC.
//
// The size of the env file never changes so we do not
// truncate it.
//
// We also do not O_TRUNC to avoid reallocations on the FS
// to minimize risk of fs corruption.
f, err := os.OpenFile(env.fname, os.O_WRONLY, 0666)
if err != nil {
return err
}
defer f.Close()
if _, err := f.Write(writeUint32(crc)); err != nil {
return err
}
// padding bytes (e.g. for redundant header)
pad := make([]byte, headerSize-binary.Size(crc))
if _, err := f.Write(pad); err != nil {
return err
}
if _, err := f.Write(w.Bytes()); err != nil {
return err
}
if err := f.Sync(); err != nil {
return err
}
return dir.Sync()
}
// Import is a helper that imports a given text file that contains
// "key=value" paris into the uboot env. Lines starting with ^# are
// ignored (like the input file on mkenvimage)
func (env *Env) Import(r io.Reader) error {
scanner := bufio.NewScanner(r)
for scanner.Scan() {
line := scanner.Text()
if strings.HasPrefix(line, "#") || len(line) == 0 {
continue
}
l := strings.SplitN(line, "=", 2)
if len(l) == 1 {
return fmt.Errorf("Invalid line: %q", line)
}
env.data[l[0]] = l[1]
}
return scanner.Err()
}