m/p/efivarfs: init
This adds a package supporting efivarfs operations.
Change-Id: Ib0d0713a121efaa0ecdd7e70d8c9d27f4697f958
Reviewed-on: https://review.monogon.dev/c/monogon/+/426
Reviewed-by: Leopold Schabel <leo@nexantic.com>
diff --git a/metropolis/pkg/efivarfs/BUILD.bazel b/metropolis/pkg/efivarfs/BUILD.bazel
new file mode 100644
index 0000000..8ef16fb
--- /dev/null
+++ b/metropolis/pkg/efivarfs/BUILD.bazel
@@ -0,0 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+ name = "go_default_library",
+ srcs = ["efivarfs.go"],
+ importpath = "source.monogon.dev/metropolis/pkg/efivarfs",
+ visibility = ["//visibility:public"],
+ deps = ["@org_golang_x_text//encoding/unicode:go_default_library"],
+)
diff --git a/metropolis/pkg/efivarfs/efivarfs.go b/metropolis/pkg/efivarfs/efivarfs.go
new file mode 100644
index 0000000..90c4519
--- /dev/null
+++ b/metropolis/pkg/efivarfs/efivarfs.go
@@ -0,0 +1,74 @@
+// 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.
+
+// This package was written with the aim of easing efivarfs integration.
+//
+// https://www.kernel.org/doc/html/latest/filesystems/efivarfs.html
+package efivarfs
+
+import (
+ "bytes"
+ "fmt"
+ "io/ioutil"
+ "path/filepath"
+ "strings"
+
+ "golang.org/x/text/encoding/unicode"
+)
+
+const (
+ Path = "/sys/firmware/efi/efivars"
+ GlobalGuid = "8be4df61-93ca-11d2-aa0d-00e098032b8c"
+)
+
+// 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.")
+ }
+ // efiUnicode defines the Unicode encoding used by UEFI which is UCS-2
+ // Little Endian. For BMP characters UTF-16 is equivalent to UCS-2.
+ // See the UEFI Spec 2.9, Sections 33.2.6 and 1.8.1.
+ efiUnicode := unicode.UTF16(unicode.LittleEndian, unicode.IgnoreBOM)
+ // Skip attributes, see @linux//Documentation/filesystems:efivarfs.rst for format
+ efiVarData := contents[4:]
+ espUUIDNullTerminated, err := efiUnicode.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
+}
+
+// ReadLoaderDevicePartUUID reads the ESP UUID from an EFI variable. It
+// depends on efivarfs being already mounted. It returns a correct lowercase
+// UUID, or an error.
+func ReadLoaderDevicePartUUID() (string, error) {
+ // Read the EFI variable file containing the ESP UUID.
+ espUuidPath := filepath.Join(Path, "LoaderDevicePartUUID-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f")
+ efiVar, err := ioutil.ReadFile(espUuidPath)
+ if err != nil {
+ return "", fmt.Errorf("couldn't read the LoaderDevicePartUUID file at %q: %w", espUuidPath, err)
+ }
+ contents, err := ExtractString(efiVar)
+ if err != nil {
+ return "", fmt.Errorf("couldn't decode an EFI variable: %w", err)
+ }
+ return strings.ToLower(contents), nil
+}