[nycbug-talk] Understanding sys/module.h , *module_t and *modeventhand_t

[Tim A.]

I'm working through "Designing FreeBSD Rootkits" as a means
of deepening my understanding of both FreeBSD and C.
I'm understanding it well enough (I hope), examples all work,
experimenting has worked. Up to hooking syscalls now.

There's this one part from the beginning though, concerning
FreeBSD source from sys/module.h that I can't seem to wrap my head
around and it's bothering me.
Understanding it isn't essential to making things work.
But, I feel like if I can just understand this, it's a personal break
through for me. It's probably some simple explanation that's right
in front of my face but eluding me.

We had debated the meaning of the *modeventhand_t in discussion after
the last meeting. And after closer examination I felt I understood it
for the most part.

This question partly involves that line but more so the line
immediately preceding it, which declares *module_t and how we see 
*module_t is used.

Using cscope, I could find no definition for the "struct module" structure.
*module_t points to "struct module" (which is, apparently, undefined).

If I have to guess, I'd say the module structure then becomes defined by 
whatever
we make module_t point to, so long as it's a structure.

Later in hello.c
modeventhand_t is set to point to our event handler function "load".
*modeventhand_t expects for it's first argument (module_t), a pointer 
variable pointing to a structure type.
but load expects for it's first argument (struct module *module), a 
dereferrenced pointer
to a module structure that exists, but isn't defined?

What's really going on here?
And, here's a good question: What *is* the first argument being passed 
to load in execution?

I didn't see source for kldload, but I guess I'll hit the KLD man page 
and maybe that will
explain things more.

#######################################################################

#/sys/sys/module.h

...snip...

typedef struct module *module_t; /* Tim: Where is this module struct 
defined? */
typedef int (*modeventhand_t)(module_t, int /* modeventtype_t */, void *);

/*
 * Struct for registering modules statically via SYSINIT.
 */
typedef struct moduledata {
        const char      *name;          /* module name */
        modeventhand_t  evhand;         /* event handler */
        void            *priv;          /* extra data */
} moduledata_t;

...snip...

#./hello.c

...snip...
/* Event Handler */
static int
load(struct module *module, int cmd, void *arg)
{...snip...}

/* Module data structure, 2nd argument of DECLARE_MODULE */
static moduledata_t hello_mod = {
    "hello",
    load,
    NULL,
};

DECLARE_MODULE(hello, hello_mod, SI_SUB_DRIVERS, SI_ORDER_MIDDLE);

#######################################################################