About use the backup method to call the original method

[buffcow]

Hello every devs, I want to ask a question about development here.
Regarding the LspHooker class of handleHookedMethod method where the parameter is args, If the method is non-static the args will have a "thisObject". LspHooker class has a backup field. hook method if non-static the first parameter of the backup method is thisObject, but “param.args” is processed:
if (Modifier.isStatic(method.getModifiers())) {
....
} else {
param.thisObject = args[0];
param.args = new Object[args.length - 1];
System.arraycopy(args, 1, param.args, 0, args.length - 1);
}

But why in here is invoke by "param.args", not the "args" parameter of the handleHookedMethod function? Isn't “param.args" already subtracted a thisObject?

I invoke like you too but the carsh log prompt is one less parameter:
java.lang.IllegalArgumentException: Wrong number of arguments; expected 4, got 3
I invoke backup method by "args" is ok:
backup.invoke(param.thisObject, args)
Why?
@yujincheng08

[yujincheng08]

If the original method is static, then backup should be invoked by invoke(null, args).
If the original method is non-static, then the backup should be invoked by invoke(thisObject, args).

For Xposed interface, thisObject is always null (unless some modules set it) for static methods. So both invokes can be merged to invoke(thisObject, args)

[yujincheng08]

For example, you are hooking:

class Test {
  public void test(String str) {}
  public void testStatic(String str){}
}

If these two methods are not hooked, we reflectively invoke them like:

testMethod.invoke(testObject, someString);
testStaticMethod.invoke(null, someString);

The dex builder will generate the backup methods like:

static void backupTest(Test testObject, String str);
static void backupTestStatic(String str);

If these methods are not yet used to backup, reflectively you should invoke like:

backupTestMethod.invoke(null, testObject, someString);
backupTestStaticMethod.invoke(null, someString);

After they are used to backup methods, you should invoke them like my second code snippets as if they are the reflection of the hooked methods:

backupTestMethod.invoke(testObject, someString); // testMethod.invoke(testObject, someString);
backupTestStaticMethod.invoke(null, someString); // testStaticMethod.invoke(null, someString);

As you can see, before and after the backupTestMethod is used to backup, the invocation is different.

So if you use the backupTestMethod BEFORE it's used to backup by backupTestMethod.invoke(testObject, someString);, you will encounter IllegalArgumentException.

[buffcow]

So if you use the backupTestMethod BEFORE it's used to backup by backupTestMethod.invoke(testObject, someString);, you will encounter IllegalArgumentException.

oh!🧐 But where is the code of used as a backup?

[buffcow]

As you can see, before and after the backupTestMethod is used to backup, the invocation is different.

Is the used to backup yahfa's backupAndHook?

[yujincheng08]

Yup. But YAHFA is different from ours.

[buffcow]

ahhh.. I'm using yahfa's original code, it's not yours
Now I finally understand, thank you very much!😃