load JNI ELF

elf_load_jni.c

#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "jni.h"

int __system_property_get(const char* name, char* value) { strcpy(value, "8.0.0"); return 0; }

const char* (JNICALL fakeGetStringUTFChars) (JNIEnv *env, jstring str, jboolean *isCopy) {
    if (strcmp((char*)str, "getPackageName") == 0) return "my.some_package";
    return (char*)str;
}

jmethodID(JNICALL fakeGetMethodID) (JNIEnv *env, jclass clazz, const char *name, const char *sig) {
    return (jmethodID)name;
}

jobject(JNICALL fakeCallObjectMethod) (JNIEnv *env, jobject obj, jmethodID methodID, ...) {
    return (jobject)methodID;
}

jstring(JNICALL fakeNewStringUTF) (JNIEnv *env, const char *utf) {
    auto s = (jstring)malloc(512);
    strcpy((char*)s, utf);
    return s;
}

jclass(JNICALL fakeGetObjectClass) (JNIEnv *env, jobject obj) {
    return nullptr;
}

extern "C" {
	int elf_main(int argc, char **argv, char **envp);
	void *elfload_dlopen(const char *filename, int flag);
	void *elfload_dlsym(void *handle, const char *symbol);
	void relocateELFs();
	int (*_native_jni)(JNINativeInterface_ **a1, void *a2, void *a3);

	void* __cdecl _malloc(size_t _Size) {
		return malloc(_Size);
	}

	char* __cdecl _strcpy(char* _Dest, char const* _Source) {
		return strcpy(_Dest, _Source);
	}

	void *elfload_static(const char *fname) {
		if (strcmp(fname, "__system_property_get") == 0) {
			return (void *)__system_property_get;
		}
		else if (strcmp(fname, "memset") == 0) {
			return (void *)memset;
		}
		else if (strcmp(fname, "strncat") == 0) {
			return (void *)strncat;
		}
		else if (strcmp(fname, "strchr") == 0) {
			typedef char *(*fn)(char* const, int const);
			return (void *)(fn)strchr;
		}
		else if (strcmp(fname, "strlen") == 0) {
			return (void *)strlen;
		}
		else if (strcmp(fname, "strcpy") == 0) {
			return (void *)_strcpy;
		}
		else if (strcmp(fname, "strcat") == 0) {
			return (void *)strcat;
		}
		else if (strcmp(fname, "strcmp") == 0) {
			return (void *)strcmp;
		}
		else if (strcmp(fname, "malloc") == 0) {
			return (void *)_malloc;
		}
		else if (strcmp(fname, "free") == 0) {
			return (void *)free;
		}
		return NULL;
	}
}

int main(int argc, char **argv) {
    JNINativeInterface_ n;
    JNINativeInterface_* pn = &n;
    n.GetStringUTFChars = fakeGetStringUTFChars;
    n.GetMethodID = fakeGetMethodID;
    n.CallObjectMethod = fakeCallObjectMethod;
    n.NewStringUTF = fakeNewStringUTF;
    n.GetObjectClass = fakeGetObjectClass;

	char* sargv[] = { argv[0], "" };
	elf_main(2, sargv, nullptr);

    auto dll = elfload_dlopen("lib-native.so", 0x29A);
	relocateELFs();
    _native_jni =
		(void * (*)(JNINativeInterface_ **, void *, void *))
        elfload_dlsym(dll, "_native_jni");
    
    return (*_native_jni)(&pn, nullptr, argv[1]);
}