How to export symbol from Linux kernel module in this case?

Question

I've got two kernel modules built, one of which is a net_device. My net_device module A depends on module B which provide some extra control mechanism to export device info.

I want module B to be able to call the "xmit" function which is in module A. As a result, module B will become dependent on module A if I simple export symbol from A. This, obviously, creates a "deadlock" dependency situation.

Does anyone have experience resolving this? How can I properly export the "xmit" function in A and let B use it?

0andriy 0andriy · Accepted Answer · 2015-01-04T19:42:18

You may provide the callback function from module A. In that case you don't need to export each function you need to the kernel namespace. I presume you just could supply some structure to the B. Something like:

internal header:

struct possible_ops {
    int (*xmit)(...);
};

A:

struct private {
    struct possible_ops *ops;
};
...  
ops = kzalloc(sizeof(*ops));
ops->xmit = xmit;

B:

whatever(struct possible_ops *ops) {
    if (ops && ops->xmit) {
        ret = ops->xmit();
        ...
    }
}

How to export symbol from Linux kernel module in this case?

1 Answers