"Read only" error in assigning driver function to netdev_ops function pointer

Question

I'm experimenting with network drivers in a qemu VM running Ubuntu 16.04.1. uname -r gives 4.4.0-31-generic as the kernel version.

In linux/netdevice.h for that kernel version, I found the comment

"@netdev_ops: Includes several pointers to callbacks, if one wants to override the ndo_*() functions"

However, in my code, an assignment such as dev->netdev_ops->ndo_open = netmod_open (where netmod_open is the open method in my own driver) gives the compiler error:

assignment of member 'ndo_open' in read-only object

Further, netdev_ops is declared as const in the source:

const struct net_device_ops *netdev_ops;

How can I "override the ndo_*() functions" if the struct containing their pointers is const?

It looks like I need to create my own struct net_device_ops and then assign dev->netdev_ops the address of that struct. So in general I need to learn more about how data structures are passed around in the kernel. Any links to such info would be appreciated. It seems like most of the tutorials I find have outdated kernel APIs. — Vale132
– Vale132, Commented Dec 21, 2016 at 3:11

Tsyvarev · Accepted Answer · 2016-12-21 07:49:40Z

2

How can I "override the ndo_*() functions"

Define your own variable of type struct net_device_ops, setup its fields(hooks actually) which you want to override, and assign this variable to dev->netdev_ops.

Note, that according to description for struct net_device_ops, the only field .ndo_start_xmit is required to be set, other fields you may leave non-initialized (that is NULL).

answered Dec 21, 2016 at 7:49

Tsyvarev

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