modinfo() equivalent INSIDE kernel?

Question

I have two modules A, B. A has a function f() that is globally acessible, i.e. the f() symbol is exported. B may want to call f() occasionally. But B should only call f() if module A is loaded. What is the best way for B to tell if A is loaded?

Part b to this question is there is a way to check if f() is exported?

I'm not sure which method is more effecient.

Answer 1

If B uses at least one of A's symbols, then by the time B is loaded, a module providing the required symbol(s) (in other words, a module like A) is also already loaded.

Part b to this question is there is a way to check if f() is exported?

If the symbol were not available, you would not be able to load the module (B) requesting it.

Answer 2

I assume you load module B first, then optionally module A. My strategy would be to have A register a set of functions with B when A first initializes. B keeps a statically allocated function pointer (or a pointer to a struct full of function pointers) and provides exported functions to register and unregisters a handler. When A loads, it registers its function (or struct of functions) with B. When A unloads, it unregisters its functions.

It might go something like this:

B.h

typedef int (*foo_t)(int);
int B_register_foo(foo_t);
int B_unregister_foo(foo_t);

B.c

static foo_t foo = NULL;

int B_register_foo(foo_t f) {
    if (!foo) {
        foo = f;
        return 0;
    }
    return -EFOO;
}

void B_unregister_foo(foo_t f) {
    if (foo == f)
        foo = NULL;
}

EXPORT_SYMBOL(B_register_foo);
EXPORT_SYMBOL(B_unregister_foo);

void B_maybe_call_foo(int arg) {
    return (foo) ? foo(arg) : 0;
}

A.c

static int A_foo(int arg);

static int __init init_A(void) {
    if (B_register_foo(A_foo))
        return -EFOO;
    return 0;
}

static void __exit exit_A(void) {
    B_unregister_foo(A_foo);
}