[client] Disable pidfd check on Android 11 and below (#4127)

Disable pidfd check on Android 11 and below

On Android 11 (SDK <= 30) and earlier, pidfd-related system calls
are blocked by seccomp policies, causing SIGSYS crashes.

This change overrides `checkPidfdOnce` to return an error on
affected versions, preventing the use of unsupported pidfd features.

.github/workflows/mobile-build-validation.yml

@@ -43,7 +43,7 @@ jobs:
       - name: gomobile init
         run: gomobile init
       - name: build android netbird lib
-        run: PATH=$PATH:$(go env GOPATH) gomobile bind -o $GITHUB_WORKSPACE/netbird.aar -javapkg=io.netbird.gomobile -ldflags="-X golang.zx2c4.com/wireguard/ipc.socketDirectory=/data/data/io.netbird.client/cache/wireguard -X github.com/netbirdio/netbird/version.version=buildtest"  $GITHUB_WORKSPACE/client/android
+        run: PATH=$PATH:$(go env GOPATH) gomobile bind -o $GITHUB_WORKSPACE/netbird.aar -javapkg=io.netbird.gomobile -ldflags="-checklinkname=0 -X golang.zx2c4.com/wireguard/ipc.socketDirectory=/data/data/io.netbird.client/cache/wireguard -X github.com/netbirdio/netbird/version.version=buildtest"  $GITHUB_WORKSPACE/client/android
         env:
           CGO_ENABLED: 0
           ANDROID_NDK_HOME: /usr/local/lib/android/sdk/ndk/23.1.7779620

client/android/client.go

@@ -64,7 +64,9 @@ type Client struct {
 }
 
 // NewClient instantiate a new Client
-func NewClient(cfgFile, deviceName string, uiVersion string, tunAdapter TunAdapter, iFaceDiscover IFaceDiscover, networkChangeListener NetworkChangeListener) *Client {
+func NewClient(cfgFile string, androidSDKVersion int, deviceName string, uiVersion string, tunAdapter TunAdapter, iFaceDiscover IFaceDiscover, networkChangeListener NetworkChangeListener) *Client {
+	execWorkaround(androidSDKVersion)
+
 	net.SetAndroidProtectSocketFn(tunAdapter.ProtectSocket)
 	return &Client{
 		cfgFile:               cfgFile,

client/android/exec.go

@@ -0,0 +1,26 @@
+//go:build android
+
+package android
+
+import (
+	"fmt"
+	_ "unsafe"
+)
+
+// https://github.com/golang/go/pull/69543/commits/aad6b3b32c81795f86bc4a9e81aad94899daf520
+// In Android version 11 and earlier, pidfd-related system calls
+// are not allowed by the seccomp policy, which causes crashes due
+// to SIGSYS signals.
+
+//go:linkname checkPidfdOnce os.checkPidfdOnce
+var checkPidfdOnce func() error
+
+func execWorkaround(androidSDKVersion int) {
+	if androidSDKVersion > 30 { // above Android 11
+		return
+	}
+
+	checkPidfdOnce = func() error {
+		return fmt.Errorf("unsupported Android version")
+	}
+}