Can I hook functions in linked libraries?

Question

Using EasyHook I have successfully hooked both exported functions and known vtable functions for various C++ classes. In all these cases target programs have used DLLs.

Provided I know the address of a function's entry point, is it possible to do the same when a library has been linked into the target program as opposed to being a separate library?

Answer 1

It appears with EasyHook you can hook any subroutine whose address is calculable.

In my case hooking static linked SSL_read and SSL_write in OpenSSL was as simple as identifying the offsets with my favourite debugger and then installing the hooks.

// delegate for EasyHook:
[UnmanagedFunctionPointer(CallingConvention.Cdecl, 
    SetLastError = true, CharSet = CharSet.Ansi)]
delegate Int32 SLL_readDelegate(IntPtr SSL_ptr, IntPtr buffer, Int32 length);

// import SSL_read (I actually did it manually, but this will work in most cases)
/* proto from ssl_lib.c -> int SSL_read(SSL *s,void *buf,int num) */
[DllImport("ssleay32.dll", SetLastError = true)]
public static extern Int32 SSL_read(IntPtr ssl, IntPtr buffer, Int32 len);

// the new routine
static Int32 SSL_readCallback(IntPtr SSL_ptr, IntPtr buffer, Int32 length)
{
    /* call the imported SSL_read */
    int ret = SSL_read(SSL_ptr, buffer, length);
    /* TODO: your code here, e.g:
     * string log_me = Marshal.PtrToString(buffer, ret);
     */
    return ret;
}

Now all that's left is to install the hook:

private LocalHook sslReadHook;

public void Run(RemoteHooking.IContext InContext, String InArg1)
{
    // ... initialization code omitted for brevity

    /* the value for ssl_read_addr is made up in this example
     * you'll need to study your target and how it's loaded(?) to 
     * identify the addresses you want to hook
     */
    int ssl_read_addr = 0x12345678; /* made up for examples sake */
    sslReadHook = LocalHook.Create(new IntPtr(ssl_read_addr),
        new SSL_readDelegate(SSL_readCallback), this);

    // ...
}

I should mention that in this example you'll need libeay32.dll and ssleay32.dll as the latter depends on the former.

Happy hooking!