m/p/efivarfs: refactor
This accomplishes three things:
First, split out the variable access layer from the rest of the code.
This cleans up the attribute handling, which is now done centrally as
well as making the high-level functions very short and clean. They now
also return better errors.
Second this introduces proper types for LoadOption, which can now also
be unmarshaled which was a requirement for A/B updates. This required
implementation of EFI's DevicePath structure.
While refactoring the higher-level functions for this, this also
fixes a bug where the variable index (the 4 hex nibbles at the end) were
improperly generated as lowercase hex.
Third, this adds new high-level functions for interacting with more
boot-related variables needed for the A/B effort.
Change-Id: I53490fa4898a5e7a5498ecc05a9078bd2d66c26e
Reviewed-on: https://review.monogon.dev/c/monogon/+/1855
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/cloud/agent/install.go b/cloud/agent/install.go
index e05a57c..656f258 100644
--- a/cloud/agent/install.go
+++ b/cloud/agent/install.go
@@ -120,7 +120,7 @@
if err != nil {
return err
}
- bootEntryIdx, err := efivarfs.CreateBootEntry(be)
+ bootEntryIdx, err := efivarfs.AddBootEntry(be)
if err != nil {
return fmt.Errorf("error creating EFI boot entry: %w", err)
}
diff --git a/metropolis/installer/main.go b/metropolis/installer/main.go
index 7dc1da9..2515f53 100644
--- a/metropolis/installer/main.go
+++ b/metropolis/installer/main.go
@@ -315,7 +315,7 @@
}
// Create an EFI boot entry for Metropolis.
- en, err := efivarfs.CreateBootEntry(be)
+ en, err := efivarfs.AddBootEntry(be)
if err != nil {
panicf("While creating a boot entry: %v", err)
}
diff --git a/metropolis/node/build/mkimage/osimage/osimage.go b/metropolis/node/build/mkimage/osimage/osimage.go
index d730e68..97fa7c0 100644
--- a/metropolis/node/build/mkimage/osimage/osimage.go
+++ b/metropolis/node/build/mkimage/osimage/osimage.go
@@ -302,7 +302,7 @@
// argument. In case a regular file already exists at params.OutputPath,
// the function will fail. It returns nil on success or an error, if one
// did occur.
-func Create(params *Params) (*efivarfs.BootEntry, error) {
+func Create(params *Params) (*efivarfs.LoadOption, error) {
// Validate each parameter before use.
if params.OutputPath == "" {
return nil, fmt.Errorf("image output path must be set")
@@ -377,13 +377,19 @@
if err != nil {
return nil, fmt.Errorf("couldn't parse the GPT GUID: %w", err)
}
- be := efivarfs.BootEntry{
- Description: "Metropolis",
- Path: EFIPayloadPath,
- PartitionGUID: guid,
- PartitionNumber: 1,
- PartitionStart: esp.Start,
- PartitionSize: esp.End - esp.Start + 1,
+ be := efivarfs.LoadOption{
+ Description: "Metropolis",
+ FilePath: efivarfs.DevicePath{
+ &efivarfs.HardDrivePath{
+ PartitionNumber: 1,
+ PartitionStartBlock: esp.Start,
+ PartitionSizeBlocks: esp.End - esp.Start + 1,
+ PartitionMatch: efivarfs.PartitionGPT{
+ PartitionUUID: guid,
+ },
+ },
+ efivarfs.FilePath(EFIPayloadPath),
+ },
}
// Close the image and return the EFI boot entry.
if err := diskImg.File.Close(); err != nil {
diff --git a/metropolis/pkg/efivarfs/BUILD.bazel b/metropolis/pkg/efivarfs/BUILD.bazel
index 33b0685..3e5339c 100644
--- a/metropolis/pkg/efivarfs/BUILD.bazel
+++ b/metropolis/pkg/efivarfs/BUILD.bazel
@@ -1,17 +1,32 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
go_library(
name = "efivarfs",
srcs = [
"boot.go",
+ "devicepath.go",
"efivarfs.go",
- "format.go",
+ "variables.go",
],
importpath = "source.monogon.dev/metropolis/pkg/efivarfs",
visibility = ["//visibility:public"],
deps = [
+ "//metropolis/pkg/msguid",
"@com_github_google_uuid//:uuid",
"@org_golang_x_text//encoding/unicode",
- "@org_golang_x_text//transform",
+ ],
+)
+
+go_test(
+ name = "efivarfs_test",
+ srcs = [
+ "boot_test.go",
+ "devicepath_test.go",
+ ],
+ embed = [":efivarfs"],
+ gc_goopts = ["-d=libfuzzer"],
+ deps = [
+ "@com_github_google_go_cmp//cmp",
+ "@com_github_google_uuid//:uuid",
],
)
diff --git a/metropolis/pkg/efivarfs/boot.go b/metropolis/pkg/efivarfs/boot.go
index 4178c8d..92b8ac9 100644
--- a/metropolis/pkg/efivarfs/boot.go
+++ b/metropolis/pkg/efivarfs/boot.go
@@ -24,159 +24,121 @@
package efivarfs
import (
+ "bytes"
+ "encoding/binary"
+ "errors"
"fmt"
"math"
"strings"
-
- "github.com/google/uuid"
- "golang.org/x/text/transform"
)
-// Note on binary format of EFI variables:
-// This code follows Section 3 "Boot Manager" of version 2.6 of the UEFI Spec:
-// http://www.uefi.org/sites/default/files/resources/UEFI%20Spec%202_6.pdf
-// It uses the binary representation from the Linux "efivars" filesystem.
-// Specifically, all binary data that is marshaled and unmarshaled is preceded by
-// 4 bytes of "Variable Attributes".
-// All binary data must have exactly the correct length and may not be padded
-// with extra bytes while reading or writing.
+type LoadOptionCategory uint8
-// Note on EFI variable attributes:
-// This code ignores all EFI variable attributes when reading.
-// This code always writes variables with the following attributes:
-// - EFI_VARIABLE_NON_VOLATILE (0x00000001)
-// - EFI_VARIABLE_BOOTSERVICE_ACCESS (0x00000002)
-// - EFI_VARIABLE_RUNTIME_ACCESS (0x00000004)
-const defaultAttrsByte0 uint8 = 7
+const (
+ // Boot entries belonging to the Boot category are normal boot entries.
+ LoadOptionCategoryBoot LoadOptionCategory = 0x0
+ // Boot entries belonging to the App category are not booted as part of
+ // the normal boot order, but are only launched via menu or hotkey.
+ // This category is optional for bootloaders to support, before creating
+ // new boot entries of this category firmware support needs to be
+ // confirmed.
+ LoadOptionCategoryApp LoadOptionCategory = 0x1
+)
-// BootEntry represents a subset of the contents of a Boot#### EFI variable.
-type BootEntry struct {
- Description string // eg. "Linux Boot Manager"
- Path string // eg. `\EFI\systemd\systemd-bootx64.efi`
- PartitionGUID uuid.UUID
- PartitionNumber uint32 // Starts with 1
- PartitionStart uint64 // LBA
- PartitionSize uint64 // LBA
+// LoadOption contains information on a payload to be loaded by EFI.
+type LoadOption struct {
+ // Human-readable description of what this load option loads.
+ // This is what's being shown by the firmware when selecting a boot option.
+ Description string
+ // If set, firmware will skip this load option when it is in BootOrder.
+ // It is unspecificed whether this prevents the user from booting the entry
+ // manually.
+ Inactive bool
+ // If set, this load option will not be shown in any menu for load option
+ // selection. This does not affect other functionality.
+ Hidden bool
+ // Category contains the category of the load entry. The selected category
+ // affects various firmware behaviors, see the individual value
+ // descriptions for more information.
+ Category LoadOptionCategory
+ // Path to the UEFI PE executable to execute when this load option is being
+ // loaded.
+ FilePath DevicePath
+ // OptionalData gets passed as an argument to the executed PE executable.
+ // If zero-length a NULL value is passed to the executable.
+ OptionalData []byte
}
-// Marshal generates the binary representation of a BootEntry (EFI_LOAD_OPTION).
-// Description, DiskGUID and Path must be set.
-// Attributes of the boot entry (EFI_LOAD_OPTION.Attributes, not the same
-// as attributes of an EFI variable) are always set to LOAD_OPTION_ACTIVE.
-func (t *BootEntry) Marshal() ([]byte, error) {
- if t.Description == "" ||
- t.PartitionGUID.String() == "00000000-0000-0000-0000-000000000000" ||
- t.Path == "" ||
- t.PartitionNumber == 0 ||
- t.PartitionStart == 0 ||
- t.PartitionSize == 0 {
- return nil, fmt.Errorf("missing field, all are required: %+v", *t)
+// Marshal encodes a LoadOption into a binary EFI_LOAD_OPTION.
+func (e *LoadOption) Marshal() ([]byte, error) {
+ var data []byte
+ var attrs uint32
+ attrs |= (uint32(e.Category) & 0x1f) << 8
+ if e.Hidden {
+ attrs |= 0x08
}
-
- // EFI_LOAD_OPTION.FilePathList
- var dp []byte
-
- // EFI_LOAD_OPTION.FilePathList[0]
- dp = append(dp,
- 0x04, // Type ("Media Device Path")
- 0x01, // Sub-Type ("Hard Drive")
- 0x2a, 0x00, // Length (always 42 bytes for this type)
- )
- dp = append32(dp, t.PartitionNumber)
- dp = append64(dp, t.PartitionStart)
- dp = append64(dp, t.PartitionSize)
- // Append the partition GUID in the EFI format.
- dp = append(dp, MarshalEFIGUID(t.PartitionGUID)...)
-
- dp = append(dp,
- 0x02, // Partition Format ("GUID Partition Table")
- 0x02, // Signature Type ("GUID signature")
- )
-
- // EFI_LOAD_OPTION.FilePathList[1]
- enc := Encoding.NewEncoder()
- bsp := strings.ReplaceAll(t.Path, "/", "\\")
- path, _, e := transform.Bytes(enc, []byte(bsp))
- if e != nil {
- return nil, fmt.Errorf("while encoding Path: %v", e)
+ if !e.Inactive {
+ attrs |= 0x01
}
- path = append16(path, 0) // null terminate string
- filePathLen := len(path) + 4
- dp = append(dp,
- 0x04, // Type ("Media Device Path")
- 0x04, // Sub-Type ("File Path")
- )
- dp = append16(dp, uint16(filePathLen))
- dp = append(dp, path...)
-
- // EFI_LOAD_OPTION.FilePathList[2] ("Device Path End Structure")
- dp = append(dp,
- 0x7F, // Type ("End of Hardware Device Path")
- 0xFF, // Sub-Type ("End Entire Device Path")
- 0x04, 0x00, // Length (always 4 bytes for this type)
- )
-
- out := []byte{
- // EFI variable attributes
- defaultAttrsByte0, 0x00, 0x00, 0x00,
-
- // EFI_LOAD_OPTION.Attributes (only LOAD_OPTION_ACTIVE)
- 0x01, 0x00, 0x00, 0x00,
+ data = append32(data, attrs)
+ filePathRaw, err := e.FilePath.Marshal()
+ if err != nil {
+ return nil, fmt.Errorf("failed marshalling FilePath: %w", err)
}
-
- // EFI_LOAD_OPTION.FilePathListLength
- if len(dp) > math.MaxUint16 {
- // No need to also check for overflows for Path length field explicitly,
- // since if that overflows, this field will definitely overflow as well.
- // There is no explicit length field for Description, so no special
- // handling is required.
- return nil, fmt.Errorf("variable too large, use shorter strings")
+ if len(filePathRaw) > math.MaxUint16 {
+ return nil, fmt.Errorf("failed marshalling FilePath: value too big (%d)", len(filePathRaw))
}
- out = append16(out, uint16(len(dp)))
-
- // EFI_LOAD_OPTION.Description
- desc, _, e := transform.Bytes(enc, []byte(t.Description))
- if e != nil {
- return nil, fmt.Errorf("while encoding Description: %v", e)
+ data = append16(data, uint16(len(filePathRaw)))
+ if strings.IndexByte(e.Description, 0x00) != -1 {
+ return nil, fmt.Errorf("failed to encode Description: contains invalid null bytes")
}
- desc = append16(desc, 0) // null terminate string
- out = append(out, desc...)
-
- // EFI_LOAD_OPTION.FilePathList
- out = append(out, dp...)
-
- // EFI_LOAD_OPTION.OptionalData is always empty
-
- return out, nil
+ encodedDesc, err := Encoding.NewEncoder().Bytes([]byte(e.Description))
+ if err != nil {
+ return nil, fmt.Errorf("failed to encode Description: %w", err)
+ }
+ data = append(data, encodedDesc...)
+ data = append(data, 0x00, 0x00) // Final UTF-16/UCS-2 null code
+ data = append(data, filePathRaw...)
+ data = append(data, e.OptionalData...)
+ return data, nil
}
-// UnmarshalBootEntry loads a BootEntry from its binary representation.
-// WARNING: UnmarshalBootEntry only loads the Description field.
-// Everything else is ignored (and not validated if possible)
-func UnmarshalBootEntry(d []byte) (*BootEntry, error) {
- descOffset := 4 /* EFI Var Attrs */ + 4 /* EFI_LOAD_OPTION.Attributes */ + 2 /*FilePathListLength*/
- if len(d) < descOffset {
- return nil, fmt.Errorf("too short: %v bytes", len(d))
+// UnmarshalLoadOption decodes a binary EFI_LOAD_OPTION into a LoadOption.
+func UnmarshalLoadOption(data []byte) (*LoadOption, error) {
+ if len(data) < 6 {
+ return nil, fmt.Errorf("invalid load option: minimum 6 bytes are required, got %d", len(data))
}
- descBytes := []byte{}
- var foundNull bool
- for i := descOffset; i+1 < len(d); i += 2 {
- a := d[i]
- b := d[i+1]
- if a == 0 && b == 0 {
- foundNull = true
- break
- }
- descBytes = append(descBytes, a, b)
+ var opt LoadOption
+ attrs := binary.LittleEndian.Uint32(data[:4])
+ opt.Category = LoadOptionCategory((attrs >> 8) & 0x1f)
+ opt.Hidden = attrs&0x08 != 0
+ opt.Inactive = attrs&0x01 == 0
+ lenPath := binary.LittleEndian.Uint16(data[4:6])
+ // Search for UTF-16 null code
+ nullIdx := bytes.Index(data[6:], []byte{0x00, 0x00})
+ if nullIdx == -1 {
+ return nil, errors.New("no null code point marking end of Description found")
}
- if !foundNull {
- return nil, fmt.Errorf("didn't find null terminator for Description")
+ descriptionEnd := 6 + nullIdx + 1
+ descriptionRaw := data[6:descriptionEnd]
+ description, err := Encoding.NewDecoder().Bytes(descriptionRaw)
+ if err != nil {
+ return nil, fmt.Errorf("error decoding UTF-16 in Description: %w", err)
}
- descDecoded, _, e := transform.Bytes(Encoding.NewDecoder(), descBytes)
- if e != nil {
- return nil, fmt.Errorf("while decoding Description: %v", e)
+ descriptionEnd += 2 // 2 null bytes terminating UTF-16 string
+ opt.Description = string(description)
+ if descriptionEnd+int(lenPath) > len(data) {
+ return nil, fmt.Errorf("declared length of FilePath (%d) overruns available data (%d)", lenPath, len(data)-descriptionEnd)
}
- return &BootEntry{Description: string(descDecoded)}, nil
+ filePathData := data[descriptionEnd : descriptionEnd+int(lenPath)]
+ opt.FilePath, err = UnmarshalDevicePath(filePathData)
+ if err != nil {
+ return nil, fmt.Errorf("failed unmarshaling FilePath: %w", err)
+ }
+ if descriptionEnd+int(lenPath) < len(data) {
+ opt.OptionalData = data[descriptionEnd+int(lenPath):]
+ }
+ return &opt, nil
}
// BootOrder represents the contents of the BootOrder EFI variable.
@@ -184,7 +146,7 @@
// Marshal generates the binary representation of a BootOrder.
func (t *BootOrder) Marshal() []byte {
- out := []byte{defaultAttrsByte0, 0x00, 0x00, 0x00}
+ var out []byte
for _, v := range *t {
out = append16(out, v)
}
@@ -193,10 +155,10 @@
// UnmarshalBootOrder loads a BootOrder from its binary representation.
func UnmarshalBootOrder(d []byte) (*BootOrder, error) {
- if len(d) < 4 || len(d)%2 != 0 {
+ if len(d)%2 != 0 {
return nil, fmt.Errorf("invalid length: %v bytes", len(d))
}
- l := (len(d) - 4) / 2
+ l := len(d) / 2
out := make(BootOrder, l)
for i := 0; i < l; i++ {
out[i] = uint16(d[4+2*i]) | uint16(d[4+2*i+1])<<8
@@ -219,16 +181,3 @@
byte(v>>24&0xFF),
)
}
-
-func append64(d []byte, v uint64) []byte {
- return append(d,
- byte(v&0xFF),
- byte(v>>8&0xFF),
- byte(v>>16&0xFF),
- byte(v>>24&0xFF),
- byte(v>>32&0xFF),
- byte(v>>40&0xFF),
- byte(v>>48&0xFF),
- byte(v>>56&0xFF),
- )
-}
diff --git a/metropolis/pkg/efivarfs/boot_test.go b/metropolis/pkg/efivarfs/boot_test.go
new file mode 100644
index 0000000..9abd8f9
--- /dev/null
+++ b/metropolis/pkg/efivarfs/boot_test.go
@@ -0,0 +1,58 @@
+package efivarfs
+
+import (
+ "bytes"
+ "encoding/hex"
+ "testing"
+
+ "github.com/google/go-cmp/cmp"
+ "github.com/google/uuid"
+)
+
+// Generated with old working marshaler and manually double-checked
+var ref, _ = hex.DecodeString(
+ "010000004a004500780061006d0070006c006500000004012a00010000000" +
+ "500000000000000080000000000000014b8a76bad9dd11180b400c04fd430" +
+ "c8020204041c005c0074006500730074005c0061002e00650066006900000" +
+ "07fff0400",
+)
+
+func TestEncoding(t *testing.T) {
+ opt := LoadOption{
+ Description: "Example",
+ FilePath: DevicePath{
+ &HardDrivePath{
+ PartitionNumber: 1,
+ PartitionStartBlock: 5,
+ PartitionSizeBlocks: 8,
+ PartitionMatch: PartitionGPT{
+ PartitionUUID: uuid.NameSpaceX500,
+ },
+ },
+ FilePath("/test/a.efi"),
+ },
+ }
+ got, err := opt.Marshal()
+ if err != nil {
+ t.Fatal(err)
+ }
+ if !bytes.Equal(ref, got) {
+ t.Fatalf("expected %x, got %x", ref, got)
+ }
+ got2, err := UnmarshalLoadOption(got)
+ if err != nil {
+ t.Fatalf("failed to unmarshal marshaled LoadOption: %v", err)
+ }
+ diff := cmp.Diff(&opt, got2)
+ if diff != "" {
+ t.Errorf("marshal/unmarshal wasn't transparent: %v", diff)
+ }
+}
+
+func FuzzDecode(f *testing.F) {
+ f.Add(ref)
+ f.Fuzz(func(t *testing.T, a []byte) {
+ // Just try to see if it crashes
+ _, _ = UnmarshalLoadOption(a)
+ })
+}
diff --git a/metropolis/pkg/efivarfs/devicepath.go b/metropolis/pkg/efivarfs/devicepath.go
new file mode 100644
index 0000000..1606fd6
--- /dev/null
+++ b/metropolis/pkg/efivarfs/devicepath.go
@@ -0,0 +1,323 @@
+package efivarfs
+
+import (
+ "bytes"
+ "encoding/binary"
+ "errors"
+ "fmt"
+ "math"
+ "strings"
+
+ "github.com/google/uuid"
+
+ "source.monogon.dev/metropolis/pkg/msguid"
+)
+
+// DevicePath represents a path consisting of one or more elements to an
+// entity implementing an EFI protocol. It's very broadly used inside EFI
+// for representing all sorts of abstract paths. In the context of this
+// package it is used to represent paths to EFI loaders.
+// See https://uefi.org/specs/UEFI/2.10/10_Protocols_Device_Path_Protocol.html
+// for more information.
+type DevicePath []DevicePathElem
+
+// DevicePathElem is a common interface for all UEFI device path elements.
+type DevicePathElem interface {
+ typ() uint8
+ subType() uint8
+ data() ([]byte, error)
+}
+
+type pathElemUnmarshalFunc func([]byte) (DevicePathElem, error)
+
+// PartitionMBR matches a drive or partition formatted with legacy MBR
+// (Master Boot Record).
+type PartitionMBR struct {
+ // DiskSignature contains a 4-byte signature identifying the drive, located
+ // just after the 440 bytes of boot sector loading code.
+ // Note that since MBR does not have per-partition signatures, this is
+ // combined with PartitionNumber to select a partition.
+ DiskSignature [4]byte
+}
+
+func (p PartitionMBR) partitionSignature() (sig [16]byte) {
+ copy(sig[:4], p.DiskSignature[:])
+ return
+}
+
+func (p PartitionMBR) partitionFormat() uint8 {
+ return 0x01
+}
+
+func (p PartitionMBR) signatureType() uint8 {
+ return 0x01
+}
+
+// PartitionGPT matches a partition on a drive formatted with GPT.
+type PartitionGPT struct {
+ // UUID of the partition to be matched. Conversion into mixed-endian format
+ // is taken care of, a standard big-endian UUID can be put in here.
+ PartitionUUID uuid.UUID
+}
+
+func (p PartitionGPT) partitionSignature() [16]byte {
+ return msguid.From(p.PartitionUUID)
+}
+
+func (p PartitionGPT) partitionFormat() uint8 {
+ return 0x02
+}
+
+func (p PartitionGPT) signatureType() uint8 {
+ return 0x02
+}
+
+// PartitionUnknown is being used to represent unknown partitioning schemas or
+// combinations of PartitionFormat/SignatureType. It contains raw uninterpreted
+// data.
+type PartitionUnknown struct {
+ PartitionSignature [16]byte
+ PartitionFormat uint8
+ SignatureType uint8
+}
+
+func (p PartitionUnknown) partitionSignature() [16]byte {
+ return p.PartitionSignature
+}
+
+func (p PartitionUnknown) partitionFormat() uint8 {
+ return p.PartitionFormat
+}
+
+func (p PartitionUnknown) signatureType() uint8 {
+ return p.SignatureType
+}
+
+type PartitionMatch interface {
+ partitionSignature() [16]byte
+ partitionFormat() uint8
+ signatureType() uint8
+}
+
+// HardDrivePath matches whole drives or partitions on GPT/MBR formatted
+// drives.
+type HardDrivePath struct {
+ // Partition number, starting at 1. If zero or unset, the whole drive is
+ // selected.
+ PartitionNumber uint32
+ // Block address at which the partition starts. Not used for matching
+ // partitions in EDK2.
+ PartitionStartBlock uint64
+ // Number of blocks occupied by the partition starting from the
+ // PartitionStartBlock. Not used for matching partitions in EDK2.
+ PartitionSizeBlocks uint64
+ // PartitionMatch is used to match drive or partition signatures.
+ // Use PartitionMBR and PartitionGPT types here.
+ PartitionMatch PartitionMatch
+}
+
+func (h *HardDrivePath) typ() uint8 {
+ return 4
+}
+
+func (h *HardDrivePath) subType() uint8 {
+ return 1
+}
+
+func (h *HardDrivePath) data() ([]byte, error) {
+ out := make([]byte, 38)
+ le := binary.LittleEndian
+ le.PutUint32(out[0:4], h.PartitionNumber)
+ le.PutUint64(out[4:12], h.PartitionStartBlock)
+ le.PutUint64(out[12:20], h.PartitionSizeBlocks)
+ if h.PartitionMatch == nil {
+ return nil, errors.New("PartitionMatch needs to be set")
+ }
+ sig := h.PartitionMatch.partitionSignature()
+ copy(out[20:36], sig[:])
+ out[36] = h.PartitionMatch.partitionFormat()
+ out[37] = h.PartitionMatch.signatureType()
+ return out, nil
+}
+
+func unmarshalHardDrivePath(data []byte) (DevicePathElem, error) {
+ var h HardDrivePath
+ if len(data) != 38 {
+ return nil, fmt.Errorf("invalid HardDrivePath element, expected 38 bytes, got %d", len(data))
+ }
+ le := binary.LittleEndian
+ h.PartitionNumber = le.Uint32(data[0:4])
+ h.PartitionStartBlock = le.Uint64(data[4:12])
+ h.PartitionSizeBlocks = le.Uint64(data[12:20])
+ partitionFormat := data[36]
+ signatureType := data[37]
+ var rawSig [16]byte
+ copy(rawSig[:], data[20:36])
+ switch {
+ case partitionFormat == 1 && signatureType == 1:
+ // MBR
+ var mbr PartitionMBR
+ copy(mbr.DiskSignature[:], rawSig[:4])
+ h.PartitionMatch = mbr
+ case partitionFormat == 2 && signatureType == 2:
+ // GPT
+ h.PartitionMatch = PartitionGPT{
+ PartitionUUID: msguid.To(rawSig),
+ }
+ default:
+ // Unknown
+ h.PartitionMatch = PartitionUnknown{
+ PartitionSignature: rawSig,
+ PartitionFormat: partitionFormat,
+ SignatureType: signatureType,
+ }
+ }
+ return &h, nil
+}
+
+// FilePath contains a backslash-separated path or part of a path to a file on
+// a filesystem.
+type FilePath string
+
+func (f FilePath) typ() uint8 {
+ return 4
+}
+
+func (f FilePath) subType() uint8 {
+ return 4
+}
+
+func (f FilePath) data() ([]byte, error) {
+ if strings.IndexByte(string(f), 0x00) != -1 {
+ return nil, fmt.Errorf("contains invalid null bytes")
+ }
+ withBackslashes := bytes.ReplaceAll([]byte(f), []byte(`/`), []byte(`\`))
+ out, err := Encoding.NewEncoder().Bytes(withBackslashes)
+ if err != nil {
+ return nil, fmt.Errorf("failed to encode FilePath to UTF-16: %w", err)
+ }
+ return append(out, 0x00, 0x00), nil
+}
+
+func unmarshalFilePath(data []byte) (DevicePathElem, error) {
+ if len(data) < 2 {
+ return nil, fmt.Errorf("FilePath must be at least 2 bytes because of UTF-16 null terminator")
+ }
+ out, err := Encoding.NewDecoder().Bytes(data)
+ if err != nil {
+ return nil, fmt.Errorf("error decoding FilePath UTF-16 string: %w", err)
+ }
+ nullIdx := bytes.IndexByte(out, 0x00)
+ if nullIdx != len(out)-1 {
+ return nil, fmt.Errorf("FilePath not properly null-terminated")
+ }
+ withoutBackslashes := strings.Replace(string(out[:len(out)-1]), `\`, `/`, -1)
+ return FilePath(withoutBackslashes), nil
+}
+
+// Map key contains type and subtype
+var pathElementUnmarshalMap = map[[2]byte]pathElemUnmarshalFunc{
+ {4, 1}: unmarshalHardDrivePath,
+ {4, 4}: unmarshalFilePath,
+}
+
+// UnknownPath is a generic structure for all types of path elements not
+// understood by this library. The UEFI-specified set of path element
+// types is vast and mostly unused, this generic type allows for parsing as
+// well as pass-through of not-understood path elements.
+type UnknownPath struct {
+ TypeVal uint8
+ SubTypeVal uint8
+ DataVal []byte
+}
+
+func (u UnknownPath) typ() uint8 {
+ return u.TypeVal
+}
+
+func (u UnknownPath) subType() uint8 {
+ return u.SubTypeVal
+}
+
+func (u UnknownPath) data() ([]byte, error) {
+ return u.DataVal, nil
+}
+
+// Marshal encodes the device path in binary form.
+func (d DevicePath) Marshal() ([]byte, error) {
+ var buf []byte
+ for _, p := range d {
+ buf = append(buf, p.typ(), p.subType())
+ elemBuf, err := p.data()
+ if err != nil {
+ return nil, fmt.Errorf("failed marshaling path element: %w", err)
+ }
+ // 4 is size of header which is included in length field
+ if len(elemBuf)+4 > math.MaxUint16 {
+ return nil, fmt.Errorf("path element payload over maximum size")
+ }
+ buf = append16(buf, uint16(len(elemBuf)+4))
+ buf = append(buf, elemBuf...)
+ }
+ // End of device path (Type 0x7f, SubType 0xFF)
+ buf = append(buf, 0x7f, 0xff, 0x04, 0x00)
+ return buf, nil
+}
+
+// UnmarshalDevicePath parses a binary device path.
+func UnmarshalDevicePath(data []byte) (DevicePath, error) {
+ rest := data
+ var p DevicePath
+ for {
+ if len(rest) < 4 {
+ if len(rest) != 0 {
+ return nil, fmt.Errorf("dangling bytes at the end of device path: %x", rest)
+ }
+ break
+ }
+ t := rest[0]
+ subT := rest[1]
+ dataLen := binary.LittleEndian.Uint16(rest[2:4])
+ if int(dataLen) > len(rest) {
+ return nil, fmt.Errorf("path element larger than rest of buffer: %d > %d", dataLen, len(rest))
+ }
+ if dataLen < 4 {
+ return nil, fmt.Errorf("path element must be at least 4 bytes (header), length indicates %d", dataLen)
+ }
+ elemData := rest[4:dataLen]
+ rest = rest[dataLen:]
+
+ unmarshal, ok := pathElementUnmarshalMap[[2]byte{t, subT}]
+ if !ok {
+ p = append(p, &UnknownPath{
+ TypeVal: t,
+ SubTypeVal: subT,
+ DataVal: elemData,
+ })
+ continue
+ }
+ elem, err := unmarshal(elemData)
+ if err != nil {
+ return nil, fmt.Errorf("failed decoding path element %d: %w", len(p), err)
+ }
+ p = append(p, elem)
+ }
+ var endOfPathIdx int
+ for i, e := range p {
+ if e.typ() == 0x7f && e.subType() == 0xff {
+ endOfPathIdx = i
+ break
+ }
+ }
+ switch {
+ case len(p) == 0:
+ return nil, errors.New("empty DevicePath without End Of Path element")
+ case endOfPathIdx == -1:
+ return nil, fmt.Errorf("got DevicePath with %d elements, but without End Of Path element", len(p))
+ case endOfPathIdx != len(p)-1:
+ return nil, fmt.Errorf("got DevicePath with %d elements with End Of Path element at %d (wanted as last element)", len(p), endOfPathIdx)
+ }
+ p = p[:len(p)-1]
+
+ return p, nil
+}
diff --git a/metropolis/pkg/efivarfs/devicepath_test.go b/metropolis/pkg/efivarfs/devicepath_test.go
new file mode 100644
index 0000000..b5823ac
--- /dev/null
+++ b/metropolis/pkg/efivarfs/devicepath_test.go
@@ -0,0 +1,89 @@
+package efivarfs
+
+import (
+ "bytes"
+ "testing"
+
+ "github.com/google/uuid"
+)
+
+func TestMarshalExamples(t *testing.T) {
+ cases := []struct {
+ name string
+ path DevicePath
+ expected []byte
+ expectError bool
+ }{
+ {
+ name: "TestNone",
+ path: DevicePath{},
+ expected: []byte{
+ 0x7f, 0xff, // End of HW device path
+ 0x04, 0x00, // Length: 4 bytes
+ },
+ },
+ {
+ // From UEFI Device Path Examples, extracted single entry
+ name: "TestHD",
+ path: DevicePath{
+ &HardDrivePath{
+ PartitionNumber: 1,
+ PartitionStartBlock: 0x22,
+ PartitionSizeBlocks: 0x2710000,
+ PartitionMatch: PartitionGPT{
+ PartitionUUID: uuid.MustParse("15E39A00-1DD2-1000-8D7F-00A0C92408FC"),
+ },
+ },
+ },
+ expected: []byte{
+ 0x04, 0x01, // Hard Disk type
+ 0x2a, 0x00, // Length
+ 0x01, 0x00, 0x00, 0x00, // Partition Number
+ 0x22, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // Part Start
+ 0x00, 0x00, 0x71, 0x02, 0x00, 0x00, 0x00, 0x00, // Part Size
+ 0x00, 0x9a, 0xe3, 0x15, 0xd2, 0x1d, 0x00, 0x10,
+ 0x8d, 0x7f, 0x00, 0xa0, 0xc9, 0x24, 0x08, 0xfc, // Signature
+ 0x02, // Part Format GPT
+ 0x02, // Signature GPT
+ 0x7f, 0xff, // End of HW device path
+ 0x04, 0x00, // Length: 4 bytes
+ },
+ },
+ {
+ name: "TestFilePath",
+ path: DevicePath{
+ FilePath("asdf"),
+ },
+ expected: []byte{
+ 0x04, 0x04, // File Path type
+ 0x0e, 0x00, // Length
+ 'a', 0x00, 's', 0x00, 'd', 0x00, 'f', 0x00,
+ 0x00, 0x00,
+ 0x7f, 0xff, // End of HW device path
+ 0x04, 0x00, // Length: 4 bytes
+ },
+ },
+ }
+
+ for _, c := range cases {
+ t.Run(c.name, func(t *testing.T) {
+ got, err := c.path.Marshal()
+ if err != nil && !c.expectError {
+ t.Fatalf("unexpected error: %v", err)
+ }
+ if err == nil && c.expectError {
+ t.Fatalf("expected error, got %x", got)
+ }
+ if err != nil && c.expectError {
+ // Do not compare result in case error is expected
+ return
+ }
+ if !bytes.Equal(got, c.expected) {
+ t.Fatalf("expected %x, got %x", c.expected, got)
+ }
+ if _, err := UnmarshalDevicePath(got); err != nil {
+ t.Errorf("failed to unmarshal value again: %v", err)
+ }
+ })
+ }
+}
diff --git a/metropolis/pkg/efivarfs/efivarfs.go b/metropolis/pkg/efivarfs/efivarfs.go
index 8132579..e751145 100644
--- a/metropolis/pkg/efivarfs/efivarfs.go
+++ b/metropolis/pkg/efivarfs/efivarfs.go
@@ -14,24 +14,36 @@
// See the License for the specific language governing permissions and
// limitations under the License.
-// This package was written with the aim of easing efivarfs integration.
+// Package efivarfs provides functions to read and manipulate UEFI runtime
+// variables. It uses Linux's efivarfs [1] to access the variables and all
+// functions generally require that this is mounted at
+// "/sys/firmware/efi/efivars".
//
-// https://www.kernel.org/doc/html/latest/filesystems/efivarfs.html
+// [1] https://www.kernel.org/doc/html/latest/filesystems/efivarfs.html
package efivarfs
import (
- "bytes"
+ "encoding/binary"
+ "errors"
"fmt"
+ "io/fs"
"os"
- "path/filepath"
+ "strings"
"github.com/google/uuid"
"golang.org/x/text/encoding/unicode"
)
const (
- Path = "/sys/firmware/efi/efivars"
- GlobalGuid = "8be4df61-93ca-11d2-aa0d-00e098032b8c"
+ Path = "/sys/firmware/efi/efivars"
+)
+
+var (
+ // ScopeGlobal is the scope of variables defined by the EFI specification
+ // itself.
+ ScopeGlobal = uuid.MustParse("8be4df61-93ca-11d2-aa0d-00e098032b8c")
+ // ScopeSystemd is the scope of variables defined by Systemd/bootspec.
+ ScopeSystemd = uuid.MustParse("4a67b082-0a4c-41cf-b6c7-440b29bb8c4f")
)
// Encoding defines the Unicode encoding used by UEFI, which is UCS-2 Little
@@ -39,79 +51,108 @@
// Spec 2.9, Sections 33.2.6 and 1.8.1.
var Encoding = unicode.UTF16(unicode.LittleEndian, unicode.IgnoreBOM)
-// ExtractString returns EFI variable data based on raw variable file contents.
-// It returns string-represented data, or an error.
-func ExtractString(contents []byte) (string, error) {
- // Fail if total length is shorter than attribute length.
- if len(contents) < 4 {
- return "", fmt.Errorf("contents too short.")
- }
- // Skip attributes, see @linux//Documentation/filesystems:efivarfs.rst for format
- efiVarData := contents[4:]
- espUUIDNullTerminated, err := Encoding.NewDecoder().Bytes(efiVarData)
- if err != nil {
- // Pass the decoding error unwrapped.
- return "", err
- }
- // Remove the null suffix.
- return string(bytes.TrimSuffix(espUUIDNullTerminated, []byte{0})), nil
+// Attribute contains a bitset of EFI variable attributes.
+type Attribute uint32
+
+const (
+ // If set the value of the variable is is persistent across resets and
+ // power cycles. Variables without this set cannot be created or modified
+ // after UEFI boot services are terminated.
+ AttrNonVolatile Attribute = 1 << iota
+ // If set allows access to this variable from UEFI boot services.
+ AttrBootserviceAccess
+ // If set allows access to this variable from an operating system after
+ // UEFI boot services are terminated. Variables setting this must also
+ // set AttrBootserviceAccess. This is automatically taken care of by Write
+ // in this package.
+ AttrRuntimeAccess
+ // Marks a variable as being a hardware error record. See UEFI 2.10 section
+ // 8.2.8 for more information about this.
+ AttrHardwareErrorRecord
+ // Deprecated, should not be used for new variables.
+ AttrAuthenticatedWriteAccess
+ // Variable requires special authentication to write. These variables
+ // cannot be written with this package.
+ AttrTimeBasedAuthenticatedWriteAccess
+ // If set in a Write() call, tries to append the data instead of replacing
+ // it completely.
+ AttrAppendWrite
+ // Variable requires special authentication to access and write. These
+ // variables cannot be accessed with this package.
+ AttrEnhancedAuthenticatedAccess
+)
+
+func varPath(scope uuid.UUID, varName string) string {
+ return fmt.Sprintf("/sys/firmware/efi/efivars/%s-%s", varName, scope.String())
}
-// ReadLoaderDevicePartUUID reads the ESP UUID from an EFI variable. It
-// depends on efivarfs being already mounted.
-func ReadLoaderDevicePartUUID() (uuid.UUID, error) {
- // Read the EFI variable file containing the ESP UUID.
- espUuidPath := filepath.Join(Path, "LoaderDevicePartUUID-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f")
- efiVar, err := os.ReadFile(espUuidPath)
+// Write writes the value of the named variable in the given scope.
+func Write(scope uuid.UUID, varName string, attrs Attribute, value []byte) error {
+ // Write attributes, see @linux//Documentation/filesystems:efivarfs.rst for format
+ f, err := os.OpenFile(varPath(scope, varName), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
if err != nil {
- return uuid.Nil, fmt.Errorf("couldn't read the LoaderDevicePartUUID file at %q: %w", espUuidPath, err)
- }
- contents, err := ExtractString(efiVar)
- if err != nil {
- return uuid.Nil, fmt.Errorf("couldn't decode an EFI variable: %w", err)
- }
- out, err := uuid.Parse(contents)
- if err != nil {
- return uuid.Nil, fmt.Errorf("value in LoaderDevicePartUUID could not be parsed as UUID: %w", err)
- }
- return out, nil
-}
-
-// CreateBootEntry creates an EFI boot entry variable and returns its
-// non-negative index on success. It may return an io error.
-func CreateBootEntry(be *BootEntry) (int, error) {
- // Find the index by looking up the first empty slot.
- var ep string
- var n int
- for ; ; n++ {
- en := fmt.Sprintf("Boot%04x-%s", n, GlobalGuid)
- ep = filepath.Join(Path, en)
- _, err := os.Stat(ep)
- if os.IsNotExist(err) {
- break
+ e := err
+ // Unwrap PathError here as we wrap our own parameter message around it
+ var perr *fs.PathError
+ if errors.As(err, &perr) {
+ e = perr.Err
}
- if err != nil {
- return -1, err
- }
+ return fmt.Errorf("writing %q in scope %s: %w", varName, scope, e)
}
-
- // Create the boot variable.
- bem, err := be.Marshal()
- if err != nil {
- return -1, fmt.Errorf("while marshaling the EFI boot entry: %w", err)
+ // Required by UEFI 2.10 Section 8.2.3:
+ // Runtime access to a data variable implies boot service access. Attributes
+ // that have EFI_VARIABLE_RUNTIME_ACCESS set must also have
+ // EFI_VARIABLE_BOOTSERVICE_ACCESS set. The caller is responsible for
+ // following this rule.
+ if attrs&AttrRuntimeAccess != 0 {
+ attrs |= AttrBootserviceAccess
}
- if err := os.WriteFile(ep, bem, 0644); err != nil {
- return -1, fmt.Errorf("while creating a boot entry variable: %w", err)
+ // Linux wants everything in on write, so assemble an intermediate buffer
+ buf := make([]byte, len(value)+4)
+ binary.LittleEndian.PutUint32(buf[:4], uint32(attrs))
+ copy(buf[4:], value)
+ _, err = f.Write(buf)
+ if err1 := f.Close(); err1 != nil && err == nil {
+ err = err1
}
- return n, nil
+ return err
}
-// SetBootOrder replaces contents of the boot order variable with the order
-// specified in ord. It may return an io error.
-func SetBootOrder(ord *BootOrder) error {
- op := filepath.Join(Path, fmt.Sprintf("BootOrder-%s", GlobalGuid))
- if err := os.WriteFile(op, ord.Marshal(), 0644); err != nil {
- return fmt.Errorf("while creating a boot order variable: %w", err)
+// Read reads the value of the named variable in the given scope.
+func Read(scope uuid.UUID, varName string) ([]byte, Attribute, error) {
+ val, err := os.ReadFile(varPath(scope, varName))
+ if err != nil {
+ e := err
+ // Unwrap PathError here as we wrap our own parameter message around it
+ var perr *fs.PathError
+ if errors.As(err, &perr) {
+ e = perr.Err
+ }
+ return nil, Attribute(0), fmt.Errorf("reading %q in scope %s: %w", varName, scope, e)
}
- return nil
+ if len(val) < 4 {
+ return nil, Attribute(0), fmt.Errorf("reading %q in scope %s: malformed, less than 4 bytes long", varName, scope)
+ }
+ return val[4:], Attribute(binary.LittleEndian.Uint32(val[:4])), nil
+}
+
+// List lists all variable names present for a given scope sorted by their names
+// in Go's "native" string sort order.
+func List(scope uuid.UUID) ([]string, error) {
+ vars, err := os.ReadDir(Path)
+ if err != nil {
+ return nil, fmt.Errorf("failed to list variable directory: %w", err)
+ }
+ var outVarNames []string
+ suffix := fmt.Sprintf("-%v", scope)
+ for _, v := range vars {
+ if v.IsDir() {
+ continue
+ }
+ if !strings.HasSuffix(v.Name(), suffix) {
+ continue
+ }
+ outVarNames = append(outVarNames, strings.TrimSuffix(v.Name(), suffix))
+ }
+ return outVarNames, nil
}
diff --git a/metropolis/pkg/efivarfs/format.go b/metropolis/pkg/efivarfs/format.go
deleted file mode 100644
index a71dbf9..0000000
--- a/metropolis/pkg/efivarfs/format.go
+++ /dev/null
@@ -1,33 +0,0 @@
-// Copyright 2020 The Monogon Project Authors.
-//
-// SPDX-License-Identifier: Apache-2.0
-//
-// 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 efivarfs
-
-import (
- "github.com/google/uuid"
-)
-
-// MarshalEFIGUID returns src represented in the EFI GUID format, as defined in
-// UEFI specification, version 2.9, appendix A:
-// https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf
-func MarshalEFIGUID(src uuid.UUID) []byte {
- efi := make([]byte, 16)
- transform := []int{3, 2, 1, 0, 5, 4, 7, 6, 8, 9, 10, 11, 12, 13, 14, 15}
- for dest, from := range transform {
- efi[dest] = src[from]
- }
- return efi
-}
diff --git a/metropolis/pkg/efivarfs/variables.go b/metropolis/pkg/efivarfs/variables.go
new file mode 100644
index 0000000..876173c
--- /dev/null
+++ b/metropolis/pkg/efivarfs/variables.go
@@ -0,0 +1,136 @@
+package efivarfs
+
+import (
+ "bytes"
+ "encoding/binary"
+ "errors"
+ "fmt"
+ "io/fs"
+ "math"
+ "regexp"
+ "strconv"
+
+ "github.com/google/uuid"
+)
+
+func decodeString(varData []byte) (string, error) {
+ efiStringRaw, err := Encoding.NewDecoder().Bytes(varData)
+ if err != nil {
+ // Pass the decoding error unwrapped.
+ return "", err
+ }
+ // Remove the null suffix.
+ return string(bytes.TrimSuffix(efiStringRaw, []byte{0})), nil
+}
+
+// ReadLoaderDevicePartUUID reads the ESP UUID from an EFI variable.
+func ReadLoaderDevicePartUUID() (uuid.UUID, error) {
+ efiVar, _, err := Read(ScopeSystemd, "LoaderDevicePartUUID")
+ if err != nil {
+ return uuid.Nil, err
+ }
+ strContent, err := decodeString(efiVar)
+ if err != nil {
+ return uuid.Nil, fmt.Errorf("decoding string failed: %w", err)
+ }
+ out, err := uuid.Parse(strContent)
+ if err != nil {
+ return uuid.Nil, fmt.Errorf("value in LoaderDevicePartUUID could not be parsed as UUID: %w", err)
+ }
+ return out, nil
+}
+
+// Technically UEFI mandates that only upper-case hex indices are valid, but in
+// practice even vendors themselves ship firmware with lowercase hex indices,
+// thus accept these here as well.
+var bootVarRegexp = regexp.MustCompile(`^Boot([0-9A-Fa-f]{4})$`)
+
+// AddBootEntry creates an new EFI boot entry variable and returns its
+// non-negative index on success.
+func AddBootEntry(be *LoadOption) (int, error) {
+ varNames, err := List(ScopeGlobal)
+ if err != nil {
+ return -1, fmt.Errorf("failed to list EFI variables: %w", err)
+ }
+ presentEntries := make(map[int]bool)
+ // Technically these are sorted, but due to the lower/upper case issue
+ // we cannot rely on this fact.
+ for _, varName := range varNames {
+ s := bootVarRegexp.FindStringSubmatch(varName)
+ if s == nil {
+ continue
+ }
+ idx, err := strconv.ParseUint(s[1], 16, 16)
+ if err != nil {
+ // This cannot be hit as all regexp matches are parseable.
+ // A quick fuzz run agrees.
+ panic(err)
+ }
+ presentEntries[int(idx)] = true
+ }
+ idx := -1
+ for i := 0; i < math.MaxUint16; i++ {
+ if !presentEntries[i] {
+ idx = i
+ break
+ }
+ }
+ if idx == -1 {
+ return -1, errors.New("all 2^16 boot entry variables are occupied")
+ }
+
+ err = SetBootEntry(idx, be)
+ if err != nil {
+ return -1, fmt.Errorf("failed to set new boot entry: %w", err)
+ }
+ return idx, nil
+}
+
+// GetBootEntry returns the boot entry at the given index.
+func GetBootEntry(idx int) (*LoadOption, error) {
+ raw, _, err := Read(ScopeGlobal, fmt.Sprintf("Boot%04X", idx))
+ if errors.Is(err, fs.ErrNotExist) {
+ // Try non-spec-conforming lowercase entry
+ raw, _, err = Read(ScopeGlobal, fmt.Sprintf("Boot%04x", idx))
+ }
+ if err != nil {
+ return nil, err
+ }
+ return UnmarshalLoadOption(raw)
+}
+
+// SetBootEntry writes the given boot entry to the given index.
+func SetBootEntry(idx int, be *LoadOption) error {
+ bem, err := be.Marshal()
+ if err != nil {
+ return fmt.Errorf("while marshaling the EFI boot entry: %w", err)
+ }
+ return Write(ScopeGlobal, fmt.Sprintf("Boot%04X", idx), AttrNonVolatile|AttrRuntimeAccess, bem)
+}
+
+// SetBootOrder replaces contents of the boot order variable with the order
+// specified in ord.
+func SetBootOrder(ord *BootOrder) error {
+ return Write(ScopeGlobal, "BootOrder", AttrNonVolatile|AttrRuntimeAccess, ord.Marshal())
+}
+
+// GetBootOrder returns the current boot order of the system.
+func GetBootOrder() (*BootOrder, error) {
+ raw, _, err := Read(ScopeGlobal, "BootOrder")
+ if err != nil {
+ return nil, err
+ }
+ ord, err := UnmarshalBootOrder(raw)
+ if err != nil {
+ return nil, fmt.Errorf("invalid boot order structure: %w", err)
+ }
+ return ord, nil
+}
+
+// SetBootNext sets the boot entry used for the next boot only. It automatically
+// resets after the next boot.
+func SetBootNext(entryIdx uint16) error {
+ data := make([]byte, 2)
+ binary.LittleEndian.PutUint16(data, entryIdx)
+ return Write(ScopeGlobal, "BootNext", AttrNonVolatile|AttrRuntimeAccess, data)
+}