Linux Kernel: how to use request_module() and try_module_get()

Question

I'm struggling to understand how to use in the proper way the

try_module_get()

I found this interesting post: How to put a check in the code to ensure the inter kernel module dependency - Linux Kernel?

But I miss one point: I can get the request_module working, but I don' get how to use the try_module_get

I explain:

If I use

ret = request_module("ipt_conntrack")

the module xt_conntrack is correctly inserted, but marked as no used, because accordingly to the above post I didn't used the try_module_get.

But how can I call the try_module_get? The function requires a struct module parameter, which I don't know how to populate for the xt_conntrack module. I found several examples, but all related to the "THIS_MODULE" parameter, which does not apply in this case. Can you point me in the right direction for this?

Thanks for your help

I'm not sure what you are trying to do but perhaps you should be using `symbol_request()` and `symbol_put()`? However, the "xt_conntrack" module doesn't export any symbols, so maybe not. — Ian Abbott, May 13 '20 at 14:41
Needs of `try_module_get` depends on what actually you want to do with the requested module (`ipt_conntrack`). Many use cases calls `try_module_get` automatically. Note also, that `request_module` way is used only in a very specific cases. So, make sure that you actually need to use this approach. — Tsyvarev, May 13 '20 at 15:56
Yes, I definitely need it: if the xt_conntrack is loaded but not used, not all the connection tracking information are available. For example the call to "nf_ct_get "returns NULL. If the module is loaded and "registered" to be used (for example using an iptables rule) the infos are available. An old doc I found (http://inai.de/documents/Netfilter_Modules.pdf) explain it at chapter 4.3, but this is an outdated info right now, not working with newer kernels. That's why I thought to use "try_module_get". If you have any other way to do it, any suggestion is welcome — cips, May 13 '20 at 16:08

score 1 · Accepted Answer · answered May 13 '20 at 16:01

Perhaps something like this would work. I haven't tested it.

/**
 * try_find_module_get() - Try and get reference to named module
 * @name: Name of module
 *
 * Attempt to find the named module.  If not found, attempt to request the module by
 * name and try to find it again.  If the module is found (possibly after requesting the
 * module), try and get a reference to it.
 *
 * Return:
 * * pointer to module if found and got a reference.
 * * NULL if module not found or failed to get a reference.
 */
static struct module *try_find_module_get(const char *name)
{
    struct module *mod;

    mutex_lock(&module_mutex);
    /* Try and find the module. */
    mod = find_module(name);
    if (!mod) {
        mutex_unlock(&module_mutex);
        /* Not found.  Try and request it. */
        if (request_module(name))
            return NULL;  /* Failed to request module. */
        mutex_lock(&module_mutex);
        /* Module requested.  Try and find it again. */
        mod = find_module(name);
    }
    /* Try and get a reference if module found. */
    if (mod && !try_module_get(mod))
        mod = NULL;  /* Failed to get a reference. */
    mutex_unlock(&module_mutex);
    return mod;
}

THANKS! I'm giving to this a first try and it seems to work....Now, I need to some some more extensive tests, but in any case that's exactly what I was looking for...even more to tell the true....;-) Thanks again!! I'll let you know anyway.... — cips, May 13 '20 at 16:31
I confirm it works perfectly. Than I now have another issue, but the load and the try_module_get works perfectly. Thanks again — cips, May 14 '20 at 11:52

Linux Kernel: how to use request_module() and try_module_get()

1 Answers1