Linux Device Drivers, Second Edition

Access Control on a Device File

Offering access control is sometimes vital for the reliability of a device node. Not only should unauthorized users not be permitted to use the device (a restriction is enforced by the filesystem permission bits), but sometimes only one authorized user should be allowed to open the device at a time.

The problem is similar to that of using ttys. In that case, the login process changes the ownership of the device node whenever a user logs into the system, in order to prevent other users from interfering with or sniffing the tty data flow. However, it’s impractical to use a privileged program to change the ownership of a device every time it is opened, just to grant unique access to it.

None of the code shown up to now implements any access control beyond the filesystem permission bits. If the open system call forwards the request to the driver, open will succeed. We now introduce a few techniques for implementing some additional checks.

Every device shown in this section has the same behavior as the bare scull device (that is, it implements a persistent memory area) but differs from scull in access control, which is implemented in the open and close operations.

Single-Open Devices

The brute-force way to provide access control is to permit a device to be opened by only one process at a time (single openness). This technique is best avoided because it inhibits user ingenuity. A user might well want to run different processes on the same device, ...