Dynamically load a function while cross compiling in Linux

Question

If I have a function that is in one Linux Kernel, but not others, how would I go about using the same source code to compile for different kernels?

For example, if functionA is an old function which both kernels support, but functionB is only supported in newer kernels and I want to use functionB when I can for the systems that support it, how do I have only one piece of code?

Is there a way to dynamically try to load the function by name functionB and then if it fails, go ahead and dynamically load functionA?

Answer 1

You can only compile a module for a specific kernel version, other kernels will refuse to load it. You will therefore need to compile it once for every kernel version that you need to support, thus you can simply use compile-time macros such as LINUX_VERSION_CODE and KERNEL_VERSION():

static void do_something(void)
{
#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,0,0)
        functionB();
#else
        functionA();
#endif
}

Furthermore, make sure to check whether functionB() is behind some CONFIG_ option: in such case you also want to wrap everything in a #ifdef CONFIG_THAT_ENABLES_FUNCTION_B.