ARM assembly learning (IV)-EEWORLD

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1. Android JNI Implementation

1. Static implementation of jni: first get the declaration of the native method from Java, and then implement the declared method through JNI.
2. Dynamic implementation of jni: first implement the native method through JNI overloading JNI_OnLoad(), and then call the native method directly in Java.

If the static implementation is used, it is easy to search for the exported function in IDA without encryption. If the dynamic
implementation is used, it can also play a certain role in hiding. The exported part does not directly show the function name called by the Java layer, but the bound function name.

Native layer:
register related functions in JNI_OnLoad

JNIEXPORT jstring JNICALL native_hello(JNIEnv *env, jclass clazz)
{
return (*env)->NewStringUTF(env, "hello load jni.");
}

// Binding table for Java and JNI functions

static JNINativeMethod method_table[] = {
{ "HelloLoad", "()Ljava/lang/String;", (void*)native_hello },//绑定
};

Java layer:
//Methods registered in jni

public native String HelloLoad();

2. Calling Arm assembly code in Android

Android.mk file

# This file is jni/Android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)

LOCAL_ARM_MODE := arm

# Name of the local module
LOCAL_MODULE := hello-jni
# The files that make up the source code
LOCAL_SRC_FILES := hello-jni.c multiple.s

include $(BUILD_SHARED_LIBRARY)

multiple.s conforms to GNU syntax

@ This file is jni/multiple.s
.text
.align    2
.global    armFunction
.type    armFunction, %function
armFunction:
@ Multiply by 10. Input value and return value in r0
stmfd    sp!, {fp,ip,lr}
mov r3, r0, asl #3
add    r0, r3, r0, asl #1
ldmfd    sp!, {fp,ip,lr}
bx    lr
.size    armFunction, .-armFunction

Directly call armFunction(input) in c

Keywords：arm Reference address：ARM assembly learning (IV)

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