Saturday, August 2, 2014

Problem on linking OpenSSL into your NDK application

This week, I tried to compile a simple NDK application and link it with the OpenSSL library. Most of libraries (including OpenSSL) are not supported by the NDK, what makes it a bit more complicated to use. So, in this post, I describe what I usually do to properly compile applications that need external libs.

The project structure is as follows:

my_project/
    + jni/my_code.c
    + jni/Android.mk
    + jni/Application.mk
    + libs/system

Problem #01: How my Android.mk looks like?

In this example, I need two libraries: libcrypto.so and libssl.so. So, the final Android.mk looks like this

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

LOCAL_MODULE := my_exec
LOCAL_MODULE_TAGS := optional
LOCAL_SRC_FILES := my_code.c
LOCAL_C_INCLUDES += $(ANDROID_SRC)/external/openssl/include \ 
                    $(ANDROID_SRC)/external/openssl/crypto \ 
                    $(KERNEL_SRC)/kernel/module
LOCAL_LDLIBS += -L$(LOCAL_PATH)/../libs/system
LOCAL_SHARED_LIBRARIES := libandroid libdl libz libcrypto libssl
LOCAL_LDLIBS += -landroid -ldl -lz


include $(BUILD_EXECUTABLE) 

See, as an example, that we also include the headers for the library (in ANDROID_SRC/external/openssl/include).

NDK does not provide support for libcrypto.so and libssl.so -- we need to have access to the libraries somehow. So, you should create a folder (for example, my_project/libs/system) and push the files /system/lib/libcrypto.so and /system/lib/libssl.so from the device to such folder.

Problem #02: Where do I get the libs from?

You shall get all of them (including libc.so) from your rooted device. The point is that, as I said, NDK does not provide support for libcrypto.so and libssl.so. Therefore, you need to get such libraries from the device. However, there's another problem: most likely, the libcrypto.so and libssl.so libraries don't recognize the symbols from the NDK libc.so and libstdc++.so libraries. Regarding this problem, it will be discussed on item #04.

Problem #03: Where do I get the headers from?

Usually, you get them from the android source code. For OpenSSL, they are located inside the folder ANDROID_SRC/external.

Problem #04: Why does my lib complain about libc symbols?

As I described in topic #02, you need to copy the libraries from the device. However, libcrypto.so and libssl.so do not recognize some symbols from the libc.so and libstdc++.so libraries (most likely, the libraries provided by NDK are not compatible with the ones in the device). Usually, you will have the following compilation error:

/home/raul/android-ndk-r10/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/raul/my_project/jni/../libs/system/libcrypto.so: error: undefined reference to '__strlen_chk'
/home/raul/android_development/android-ndk-r10/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/raul/my_project/jni/../libs/system/libcrypto.so: error: undefined reference to '__memcpy_chk'
/home/raul/android_development/android-ndk-r10/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/raul/my_project/jni/../libs/system/libcrypto.so: error: undefined reference to '__memset_chk'
/home/raul/android_development/android-ndk-r10/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: 
/home/raul/my_project/jni/../libs/system/libcrypto.so: error: undefined reference to '__strchr_chk'
/home/raul/android_development/android-ndk-r10/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-
x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/raul/my_project/jni/../libs/system/libcrypto.so: error: undefined reference to '__strcat_chk'

That's easy to solve. Here comes the trick: you have to replace NDK's libraries by those ones from the device. NDK contains several libraries for distinct platforms, as we can see from the path NDK_FOLDER/platforms:

@desktop:$ ls NDK_FOLDER/platforms
android-12  android-13  android-14  android-15  android-16  android-17  
android-18  android-19  android-3   android-4   android-5   android-8  android-9

 Considering that your application is using NDK for platform android-17 (you can define that in file Application.mk), replace the main libraries of folder NDK_PATH/platforms/android-17

@desktop:$ ls NDK_FOLDER/platforms/android-17/arch-arm/usr/lib
crtbegin_dynamic.o  crtend_android.o  libc.a    libEGL.so       libjnigraphics.so libm_hard.a         libOpenSLES.so    libthread_db.so
crtbegin_so.o       crtend_so.o       libc.so   libGLESv1_CM.so  liblog.so       libm.so             libstdc++.a       libz.so

crtbegin_static.o   libandroid.so     libdl.so  libGLESv2.so     libm.a           libOpenMAXAL.so     libstdc++.so

So, push the libraries /system/lib/libc.so, and /system/lib/libstdc++.so from the device to the folder NDK_FOLDER/platforms/android-17/arch-arm/usr/lib. After that, compile the application again and voilĂ  -- all problems solved :-) 


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