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17.12. Custom ioctl Commands
We have seen that the ioctl system call is implemented for sockets; SIOCSIFADDR and SIOCSIFMAP are examples of "socket ioctls." Now let's see how the third argument of the system call is used by networking code.
When the ioctl system call is invoked on a socket, the command number is one of the symbols defined in <linux/sockios.h>, and the sock_ioctl function directly invokes a protocol-specific function (where "protocol" refers to the main network protocol being used, for example, IP or AppleTalk).
Any ioctl command that is not recognized by the protocol layer is passed to the device layer. These device-related ioctl commands accept a third argument from user space, a struct ifreq *. This structure is defined in <linux/if.h>. The SIOCSIFADDR and SIOCSIFMAP commands actually work on the ifreq structure. The extra argument to SIOCSIFMAP, although defined as ifmap, is just a field of ifreq.
In addition to using the standardized calls, each interface can define its own ioctl commands. The plip interface, for example, allows the interface to modify its internal timeout values via ioctl. The ioctl implementation for sockets recognizes 16 commands as private to the interface: SIOCDEVPRIVATE through SIOCDEVPRIVATE+15.
int (*do_ioctl)(struct net_device *dev, struct ifreq *ifr, int cmd);
The ifr pointer points to a kernel-space address that holds a copy of the structure passed by the user. After do_ioctl returns, the structure is copied back to user space; Therefore, the driver can use the private commands to both receive and return data.
The device-specific commands can choose to use the fields in struct ifreq, but they already convey a standardized meaning, and it's unlikely that the driver can adapt the structure to its needs. The field ifr_data is a caddr_t item (a pointer) that is meant to be used for device-specific needs. The driver and the program used to invoke its ioctl commands should agree about the use of ifr_data. For example, pppstats uses device-specific commands to retrieve information from the ppp interface driver.
It's not worth showing an implementation of do_ioctl here, but with the information in this chapter and the kernel examples, you should be able to write one when you need it. Note, however, that the plip implementation uses ifr_data incorrectly and should not be used as an example for an ioctl implementation.
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