Fix: Handle missing GRUB variables in /etc/default/grub gracefully (#711)

`updateDefaultGrubFileKernelCommandLineArgs()` hard-errors when a
variable like `GRUB_CMDLINE_LINUX` is missing from `/etc/default/grub`,
blocking all kernel command-line modifications (SELinux, verity, etc.)
on images that use `grub-mkconfig` but lack the file or variable.

The fix introduces a sentinel error `errDefaultGrubFileVarNotFound` and
a tolerant wrapper `getDefaultGrubFileLinuxArgsOrEmpty()` that treats a
missing variable as empty. `updateDefaultGrubFileKernelCommandLineArgs`,
`addExtraCommandLineToDefaultGrubFile` now uses this wrapper and
`UpdateDefaultGrubFileVariable` (which creates the variable if absent)
instead of `replaceDefaultGrubFileVarAssign` (which assumed it exists).
Behavior is unchanged when the variable is present.

Author: vinceaperri

toolkit/tools/pkg/imagecustomizerlib/defaultgrubutils.go

@@ -4,6 +4,7 @@
 package imagecustomizerlib
 
 import (
+	"errors"
 	"fmt"
 	"path/filepath"
 	"strings"
@@ -33,6 +34,8 @@ const (
 	defaultGrubFileVarNameCmdlineForSELinux = defaultGrubFileVarNameCmdlineLinux
 )
 
+var errDefaultGrubFileVarNotFound = errors.New("default grub variable not found")
+
 type defaultGrubFileVarAssign struct {
 	Token grub.Token
 	Line  grub.Line
@@ -122,7 +125,8 @@ func findDefaultGrubFileVarAssign(varAssigns []defaultGrubFileVarAssign, name de
 		}
 	}
 
-	err := fmt.Errorf("failed to find %s variable assignment (%s)", installutils.GrubDefFile, name)
+	err := fmt.Errorf("failed to find %s variable assignment (%s): %w", installutils.GrubDefFile, name,
+		errDefaultGrubFileVarNotFound)
 	return defaultGrubFileVarAssign{}, err
 }
 
@@ -211,7 +215,7 @@ func GetDefaultGrubFileLinuxArgsFromMultipleVars(defaultGrubFileContent string)
 
 // Takes the string contents of /etc/default/grub file and inserts the provided command-line args.
 func addExtraCommandLineToDefaultGrubFile(defaultGrubFileContent string, extraCommandLine string) (string, error) {
-	cmdLineVarAssign, _, insertAt, err := GetDefaultGrubFileLinuxArgs(defaultGrubFileContent,
+	cmdLineVarAssign, _, insertAt, err := getDefaultGrubFileLinuxArgsOrEmpty(defaultGrubFileContent,
 		defaultGrubFileVarNameCmdlineLinuxDefault)
 	if err != nil {
 		return "", err
@@ -222,9 +226,23 @@ func addExtraCommandLineToDefaultGrubFile(defaultGrubFileContent string, extraCo
 	// Add the extra command-line args.
 	argsString = argsString[:insertAt] + " " + extraCommandLine + " " + argsString[insertAt:]
 
-	// Rewrite GRUB_CMDLINE_LINUX_DEFAULT line.
-	defaultGrubFileContent = replaceDefaultGrubFileVarAssign(defaultGrubFileContent, cmdLineVarAssign, argsString)
-	return defaultGrubFileContent, nil
+	// Write the variable, creating it if it doesn't exist.
+	return UpdateDefaultGrubFileVariable(defaultGrubFileContent, string(defaultGrubFileVarNameCmdlineLinuxDefault), argsString)
+}
+
+// getDefaultGrubFileLinuxArgsOrEmpty is like GetDefaultGrubFileLinuxArgs but treats a missing variable as having an
+// empty value instead of returning an error. Real parse errors are still returned.
+func getDefaultGrubFileLinuxArgsOrEmpty(defaultGrubFileContent string, varName defaultGrubFileVarName,
+) (defaultGrubFileVarAssign, []grubConfigLinuxArg, int, error) {
+	cmdLineVarAssign, args, insertAt, err := GetDefaultGrubFileLinuxArgs(defaultGrubFileContent, varName)
+	if errors.Is(err, errDefaultGrubFileVarNotFound) {
+		// Variable doesn't exist — treat as empty.
+		return defaultGrubFileVarAssign{}, nil, 0, nil
+	} else if err != nil {
+		return defaultGrubFileVarAssign{}, nil, 0, err
+	}
+
+	return cmdLineVarAssign, args, insertAt, nil
 }
 
 // Takes the string contents of the /etc/default/grub file and replaces a set of command-line args.
@@ -241,7 +259,7 @@ func addExtraCommandLineToDefaultGrubFile(defaultGrubFileContent string, extraCo
 func updateDefaultGrubFileKernelCommandLineArgs(defaultGrubFileContent string, varName defaultGrubFileVarName,
 	argsToRemove []string, newArgs []string,
 ) (string, error) {
-	cmdLineVarAssign, args, insertAt, err := GetDefaultGrubFileLinuxArgs(defaultGrubFileContent, varName)
+	cmdLineVarAssign, args, insertAt, err := getDefaultGrubFileLinuxArgsOrEmpty(defaultGrubFileContent, varName)
 	if err != nil {
 		return "", err
 	}
@@ -252,9 +270,8 @@ func updateDefaultGrubFileKernelCommandLineArgs(defaultGrubFileContent string, v
 		return "", err
 	}
 
-	// Rewrite GRUB_CMDLINE_LINUX line.
-	defaultGrubFileContent = replaceDefaultGrubFileVarAssign(defaultGrubFileContent, cmdLineVarAssign, value)
-	return defaultGrubFileContent, nil
+	// Write the variable, creating it if it doesn't exist.
+	return UpdateDefaultGrubFileVariable(defaultGrubFileContent, string(varName), value)
 }
 
 // Takes the string contents of the /etc/default/grub file and rewrites one of the variable assignments lines.

toolkit/tools/pkg/imagecustomizerlib/defaultgrubutils_test.go

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+package imagecustomizerlib
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestUpdateDefaultGrubFileKernelCommandLineArgsMissingVar(t *testing.T) {
+	// Empty file: GRUB_CMDLINE_LINUX_DEFAULT does not exist.
+	content := ""
+
+	result, err := updateDefaultGrubFileKernelCommandLineArgs(content,
+		defaultGrubFileVarNameCmdlineLinuxDefault,
+		nil, []string{"security=selinux", "selinux=1"})
+	assert.NoError(t, err)
+	assert.Equal(t, "GRUB_CMDLINE_LINUX_DEFAULT=\" security=selinux selinux=1 \"\n", result)
+}
+
+func TestUpdateDefaultGrubFileKernelCommandLineArgsExistingVar(t *testing.T) {
+	content := `GRUB_CMDLINE_LINUX_DEFAULT="rd.auto=1"` + "\n"
+
+	result, err := updateDefaultGrubFileKernelCommandLineArgs(content,
+		defaultGrubFileVarNameCmdlineLinuxDefault,
+		nil, []string{"security=selinux"})
+	assert.NoError(t, err)
+	assert.Equal(t, "GRUB_CMDLINE_LINUX_DEFAULT=\"rd.auto=1 security=selinux \"\n", result)
+}
+
+func TestAddExtraCommandLineToDefaultGrubFileMissingVar(t *testing.T) {
+	// Empty file: GRUB_CMDLINE_LINUX_DEFAULT does not exist.
+	content := ""
+
+	result, err := addExtraCommandLineToDefaultGrubFile(content, "console=tty0 console=ttyS0")
+	assert.NoError(t, err)
+	assert.Equal(t, "GRUB_CMDLINE_LINUX_DEFAULT=\" console=tty0 console=ttyS0 \"\n", result)
+}
+
+func TestAddExtraCommandLineToDefaultGrubFileExistingVar(t *testing.T) {
+	content := `GRUB_CMDLINE_LINUX_DEFAULT=" $kernelopts"` + "\n"
+
+	result, err := addExtraCommandLineToDefaultGrubFile(content, "console=tty0")
+	assert.NoError(t, err)
+	assert.Equal(t, "GRUB_CMDLINE_LINUX_DEFAULT=\"  console=tty0 \\$kernelopts\"\n", result)
+}