Merge "Preserve x18 while preloading SP-HALs in the zygote."

core/java/com/android/internal/os/ZygoteInit.java

@@ -173,12 +173,13 @@ public class ZygoteInit {
     }
 
     native private static void nativePreloadAppProcessHALs();
+    native private static void nativePreloadOpenGL();
 
     private static void preloadOpenGL() {
         String driverPackageName = SystemProperties.get(PROPERTY_GFX_DRIVER);
         if (!SystemProperties.getBoolean(PROPERTY_DISABLE_OPENGL_PRELOADING, false) &&
                 (driverPackageName == null || driverPackageName.isEmpty())) {
-            EGL14.eglGetDisplay(EGL14.EGL_DEFAULT_DISPLAY);
+            nativePreloadOpenGL();
         }
     }
 

core/jni/com_android_internal_os_ZygoteInit.cpp

@@ -16,21 +16,58 @@
 
 #define LOG_TAG "Zygote"
 
+#include <EGL/egl.h>
 #include <ui/GraphicBufferMapper.h>
 
 #include "core_jni_helpers.h"
 
 namespace {
 
+// Shadow call stack (SCS) is a security mitigation that uses a separate stack
+// (the SCS) for return addresses. In versions of Android newer than P, the
+// compiler cooperates with the system to ensure that the SCS address is always
+// stored in register x18, as long as the app was compiled with a new enough
+// compiler and does not use features that rely on SP-HALs (this restriction is
+// because the SP-HALs might not preserve x18 due to potentially having been
+// compiled with an old compiler as a consequence of Treble; it generally means
+// that the app must be a system app without a UI). This struct is used to
+// temporarily store the address on the stack while preloading the SP-HALs, so
+// that such apps can use the same zygote as everything else.
+struct ScopedSCSExit {
+#ifdef __aarch64__
+    void* scs;
+
+    ScopedSCSExit() {
+        __asm__ __volatile__("str x18, [%0]" ::"r"(&scs));
+    }
+
+    ~ScopedSCSExit() {
+        __asm__ __volatile__("ldr x18, [%0]; str xzr, [%0]" ::"r"(&scs));
+    }
+#else
+    // Silence unused variable warnings in non-SCS builds.
+    ScopedSCSExit() {}
+    ~ScopedSCSExit() {}
+#endif
+};
+
 void android_internal_os_ZygoteInit_nativePreloadAppProcessHALs(JNIEnv* env, jclass) {
+    ScopedSCSExit x;
     android::GraphicBufferMapper::preloadHal();
     // Add preloading here for other HALs that are (a) always passthrough, and
     // (b) loaded by most app processes.
 }
 
+void android_internal_os_ZygoteInit_nativePreloadOpenGL(JNIEnv* env, jclass) {
+    ScopedSCSExit x;
+    eglGetDisplay(EGL_DEFAULT_DISPLAY);
+}
+
 const JNINativeMethod gMethods[] = {
     { "nativePreloadAppProcessHALs", "()V",
       (void*)android_internal_os_ZygoteInit_nativePreloadAppProcessHALs },
+    { "nativePreloadOpenGL", "()V",
+      (void*)android_internal_os_ZygoteInit_nativePreloadOpenGL },
 };
 
 }  // anonymous namespace