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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2020 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 efi supports reading EFI variables.
package efi
import (
"bytes"
"encoding/binary"
"errors"
"fmt"
"io"
"os"
"path/filepath"
"unicode/utf16"
"github.com/snapcore/snapd/dirs"
"github.com/snapcore/snapd/osutil"
)
var ErrNoEFISystem = errors.New("not a supported EFI system")
type VariableAttr uint32
// see https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/efi.h?h=v5.4.32
const (
VariableNonVolatile VariableAttr = 0x00000001
VariableBootServiceAccess VariableAttr = 0x00000002
VariableRuntimeAccess VariableAttr = 0x00000004
)
var (
openEFIVar = openEFIVarImpl
)
const expectedEFIvarfsDir = "/sys/firmware/efi/efivars"
func openEFIVarImpl(name string) (r io.ReadCloser, attr VariableAttr, size int64, err error) {
mounts, err := osutil.LoadMountInfo()
if err != nil {
return nil, 0, 0, err
}
found := false
for _, mnt := range mounts {
if mnt.MountDir == expectedEFIvarfsDir {
if mnt.FsType == "efivarfs" {
found = true
break
}
}
}
if !found {
return nil, 0, 0, ErrNoEFISystem
}
varf, err := os.Open(filepath.Join(dirs.GlobalRootDir, expectedEFIvarfsDir, name))
if err != nil {
return nil, 0, 0, err
}
defer func() {
if err != nil {
varf.Close()
}
}()
fi, err := varf.Stat()
if err != nil {
return nil, 0, 0, err
}
sz := fi.Size()
if sz < 4 {
return nil, 0, 0, fmt.Errorf("unexpected size: %d", sz)
}
if err = binary.Read(varf, binary.LittleEndian, &attr); err != nil {
return nil, 0, 0, err
}
return varf, attr, sz - 4, nil
}
func cannotReadError(name string, err error) error {
return fmt.Errorf("cannot read EFI var %q: %v", name, err)
}
// ReadVarBytes will attempt to read the bytes of the value of the
// specified EFI variable, specified by its full name composed of the
// variable name and vendor ID. It also returns the attribute value
// attached to it. It expects to use the efivars filesystem at
// /sys/firmware/efi/efivars.
// https://www.kernel.org/doc/Documentation/filesystems/efivarfs.txt
// for more details.
func ReadVarBytes(name string) ([]byte, VariableAttr, error) {
varf, attr, _, err := openEFIVar(name)
if err != nil {
if err == ErrNoEFISystem {
return nil, 0, err
}
return nil, 0, cannotReadError(name, err)
}
defer varf.Close()
b, err := io.ReadAll(varf)
if err != nil {
return nil, 0, cannotReadError(name, err)
}
return b, attr, nil
}
// ReadVarString will attempt to read the string value of the
// specified EFI variable, specified by its full name composed of the
// variable name and vendor ID. The string value is expected to be
// encoded as UTF16. It also returns the attribute value attached to
// it. It expects to use the efivars filesystem at
// /sys/firmware/efi/efivars.
// https://www.kernel.org/doc/Documentation/filesystems/efivarfs.txt
// for more details.
func ReadVarString(name string) (string, VariableAttr, error) {
varf, attr, sz, err := openEFIVar(name)
if err != nil {
if err == ErrNoEFISystem {
return "", 0, err
}
return "", 0, cannotReadError(name, err)
}
defer varf.Close()
// TODO: consider using golang.org/x/text/encoding/unicode here
if sz%2 != 0 {
return "", 0, fmt.Errorf("EFI var %q is not a valid UTF16 string, it has an extra byte", name)
}
n := int(sz / 2)
if n == 0 {
return "", attr, nil
}
r16 := make([]uint16, n)
if err := binary.Read(varf, binary.LittleEndian, r16); err != nil {
return "", 0, cannotReadError(name, err)
}
if r16[n-1] == 0 {
n--
}
b := &bytes.Buffer{}
for _, r := range utf16.Decode(r16[:n]) {
b.WriteRune(r)
}
return b.String(), attr, nil
}
// MockVars mocks EFI variables as read by ReadVar*, only to be used
// from tests. Set vars to nil to mock a non-EFI system.
func MockVars(vars map[string][]byte, attrs map[string]VariableAttr) (restore func()) {
osutil.MustBeTestBinary("MockVars only to be used from tests")
old := openEFIVar
openEFIVar = func(name string) (io.ReadCloser, VariableAttr, int64, error) {
if vars == nil {
return nil, 0, 0, ErrNoEFISystem
}
if val, ok := vars[name]; ok {
attr, ok := attrs[name]
if !ok {
attr = VariableRuntimeAccess | VariableBootServiceAccess
}
return io.NopCloser(bytes.NewBuffer(val)), attr, int64(len(val)), nil
}
return nil, 0, 0, fmt.Errorf("EFI variable %s not mocked", name)
}
return func() {
openEFIVar = old
}
}
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