Odtnhj

I am working on an update application that must be able to run on an existing system and is also started by another application on this existing system.

I am using a shared library that itself uses OpenSSL, but didn't link against it. Previously my update application linked against OpenSSL to work around this. But now I cannot link against OpenSSL any longer due to binary incompatibilities. Now when my application is started on the existing system, I get an undefined symbol error, because OpenSSL is not loaded. Note that I don't have the option to load it through LD_PRELOAD, because I cannot change the application that starts my update application.

I thought about loading OpenSSL through dlopen hoping that the shared library would be able to use it. But the shared library uses OpenSSL in a C constructor (__attribute__((constructor (101))) void myConstructor()) and even if I call dlopen inside another constructor with higher priority, the dynamic loader already seems to have searched (and failed) for OpenSSL. My constructor is not even being executed.

LD_BIND_NOW is not set, so I wonder why the undefined symbol error occurs before my constructor is being called.

I found a solution myself.

The problem was that shared libraries are initialised (i.e. C constructors are being called) before the main application. You can see the order when you set the environment variable LD_DEBUG=libs. So I created a shared library myself. I linked this fix library after the buggy library (-lbuggy -lfix), because initialisation roughly (I don't really understand the rules.) takes place in reverse loading order.

Now I was able to load OpenSSL inside the C constructor of my fix library like this:

void *p = dlopen("libssl.so.1.0.0", RTLD_LAZY | RTLD_GLOBAL | RTLD_NODELETE);
if(p)
 dlclose(p);

The flag RTLD_GLOBAL assures that OpenSSL can also be used by other libraries.
The flag RTLD_NODELETE assures that the library is not unloaded on dlclose.

Note: When my update application runs on a newer system with OpenSSL 1.1 and a shared library that correctly links against OpenSSL, loading libssl.so.1.0.0 will fail silently, because the old OpenSSL version is not present on this system (embedded system, need to save disk space). If OpenSSL 1.0 was still present, this could be a problem.