fix(windows): resolve kernel path via %SystemRoot%

.changelog/288.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+windows: Resolve kernel image path via %SystemRoot% so KernelVersion works on hosts whose system drive is not C:\.
+```

providers/windows/kernel_windows.go

@@ -18,13 +18,56 @@
 package windows
 
 import (
+	"os"
+	"path/filepath"
+
 	windows "github.com/elastic/go-windows"
+	"golang.org/x/sys/windows/registry"
 )
 
-const windowsKernelExe = `C:\Windows\System32\ntoskrnl.exe`
+// fallbackSystemRoot is the last-resort default when the registry query and
+// both environment variables are unavailable.
+const fallbackSystemRoot = `C:\Windows`
+
+// systemRootFromRegistry reads the SystemRoot value from
+// HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion, which reflects the
+// actual Windows directory regardless of the process environment. Returns ""
+// on any error so the caller can fall back gracefully.
+func systemRootFromRegistry() string {
+	k, err := registry.OpenKey(registry.LOCAL_MACHINE,
+		`SOFTWARE\Microsoft\Windows NT\CurrentVersion`,
+		registry.READ|registry.WOW64_64KEY)
+	if err != nil {
+		return ""
+	}
+	defer k.Close()
+	val, _, err := k.GetStringValue("SystemRoot")
+	if err != nil {
+		return ""
+	}
+	return val
+}
+
+// kernelExePath returns the absolute path to the running kernel image.
+// It prefers the registry (immune to a stripped process environment), then
+// falls back to %SystemRoot% / %WINDIR%, then to the hardcoded default.
+// See #287.
+func kernelExePath() string {
+	root := systemRootFromRegistry()
+	if root == "" {
+		root = os.Getenv("SystemRoot")
+	}
+	if root == "" {
+		root = os.Getenv("WINDIR")
+	}
+	if root == "" {
+		root = fallbackSystemRoot
+	}
+	return filepath.Join(root, "System32", "ntoskrnl.exe")
+}
 
 func KernelVersion() (string, error) {
-	versionData, err := windows.GetFileVersionInfo(windowsKernelExe)
+	versionData, err := windows.GetFileVersionInfo(kernelExePath())
 	if err != nil {
 		return "", err
 	}

providers/windows/kernel_windows_test.go

@@ -0,0 +1,95 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you 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 windows
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+// resolveKernelExePath is like kernelExePath but accepts an injected
+// systemRoot string so tests can exercise the env-var fallback chain
+// without touching the registry.
+func resolveKernelExePath(regRoot, systemRoot, windir string) string {
+	root := regRoot
+	if root == "" {
+		root = systemRoot
+	}
+	if root == "" {
+		root = windir
+	}
+	if root == "" {
+		root = fallbackSystemRoot
+	}
+	return filepath.Join(root, "System32", "ntoskrnl.exe")
+}
+
+func TestKernelExePath(t *testing.T) {
+	tests := []struct {
+		name    string
+		regRoot string // simulated registry value (empty = registry miss)
+		envRoot string // %SystemRoot%
+		windir  string // %WINDIR%
+		want    string
+	}{
+		{
+			// Registry wins even when env vars differ -- the primary path.
+			name:    "registry value used when present",
+			regRoot: `W:\Windows`,
+			envRoot: `C:\Windows`,
+			want:    `W:\Windows\System32\ntoskrnl.exe`,
+		},
+		{
+			// Regression for #287: registry absent, non-default drive via env.
+			name:    "SystemRoot env fallback",
+			envRoot: `W:\Windows`,
+			want:    `W:\Windows\System32\ntoskrnl.exe`,
+		},
+		{
+			name:   "WINDIR fallback when SystemRoot empty",
+			windir: `D:\WINNT`,
+			want:   `D:\WINNT\System32\ntoskrnl.exe`,
+		},
+		{
+			name: "hardcoded fallback when all sources absent",
+			want: `C:\Windows\System32\ntoskrnl.exe`,
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			got := resolveKernelExePath(tc.regRoot, tc.envRoot, tc.windir)
+			if got != tc.want {
+				t.Fatalf("resolveKernelExePath() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+// TestKernelExePathLive checks that the live kernelExePath (registry + env)
+// returns a non-empty path ending in ntoskrnl.exe.
+func TestKernelExePathLive(t *testing.T) {
+	p := kernelExePath()
+	if p == "" {
+		t.Fatal("kernelExePath() returned empty string")
+	}
+	if filepath.Base(p) != "ntoskrnl.exe" {
+		t.Fatalf("kernelExePath() = %q, want path ending in ntoskrnl.exe", p)
+	}
+}