How to change dlsym() return pointer value

Question

I am trying to find connect symbol in libcrypto.so file using dlsym() and want to replace this connect with my method.

void * lib_orig_tols  = dlopen(libcrypto.so, RTLD_GLOBAL);   

void * p_orig_connectOriginal;
     p_orig_connectOriginal = (void*)dlsym(lib_orig_tols, "connect");

    if (p_orig_connectOriginal != NULL)
        {
        LOGI(" is not NULL");

        void *p_orig_connect = (void*)*((unsigned *)p_orig_connectOriginal);

        *((unsigned *)p_orig_connectOriginal) = (unsigned)( my_crypto_connect);
        }


    int my_crypto_connect(int , const struct sockaddr , socklen_t){
        **
        ***********
        ***************
        **********
    }

My my app got crashed with error
is not NULL

Fatal signal 11 (SIGSEGV), code 2, fault addr 0xb6e4755d in tid 7909

do i am doing something wrong.

Please correct the indentation of your code. You must use function pointers. A unsigned* is not a valid function pointer. — RedX
– RedX, Commented Sep 27, 2016 at 5:54
1. You can't do that 2. This code doesn't make sense even if you could do that. — hobbs
– hobbs, Commented Sep 27, 2016 at 5:56
Redx, app did not crashed this point it crashed when i try to initialize the new value in ` *((unsigned *)p_orig_connectOriginal) = (unsigned)( my_crypto_connect); ` later i am typecasting p_orig_connectOriginal into function pointer. — vinit
– vinit, Commented Sep 27, 2016 at 5:59
@hobbs : i want to replace system SO function with my function. So when any app call this SO connect then i will get that call. — vinit
– vinit, Commented Sep 27, 2016 at 6:04

Lorinczy Zsigmond · Accepted Answer · 2016-09-27 13:15:30Z

1

Well, your code is incomplete, it is full with syntactical errors, also it contains bogus typecasts (use intptr_t)

Plus, there is no 'connect' in libcrypto.so. Pick one from these:

BIO_CONNECT_free
BIO_CONNECT_new
BIO_new_connect
BIO_s_connect

answered Sep 27, 2016 at 13:15

Lorinczy Zsigmond

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2 Comments

vinit Over a year ago

I want to do swizzling in this code. want to change system SO connect function with my connect function. So when any application call that connect i got the call. after some modification i am calling the SO connect using it original function pointer.

Lorinczy Zsigmond Over a year ago

I don't understand the "SO" part, but it is perfectly possible -- of course you are not supposed to overwrite share code and other nonsense. Here is an example: tsocks.sourceforge.net

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