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Creating a device-file node

Creating a device-file node presents A device-driver example, The Linux commands, Removing a device-file, LKM’s have full privileges, Developers disapprove, ‘Open Source’ philosophy, How to invoke ‘unlink’, Use ‘man’ for prototypes. | Creating a device-file node An introduction to some privileged Linux system-calls (needed for an upcoming programming exercise) A device-driver example • We want to build a simple device-driver, one that would let ordinary applications read and/or write to the CMOS memory (e.g. to adjust the date or time-of-day) • This will require that we have a so-called device ‘special’ file in the ‘/dev’ directory • Creating such a file normally requires the privileges of a System Administrator – but could be accomplished via a suitable LKM The Linux commands • If a System Administrator wanted to setup a device-file in the ‘/dev’ directory named ‘cmos’ that has read-and-write privileges, the following commands would be used: root# mknod /dev/cmos c 70 0 root# chmod a+rw /dev/cmos • Here the ‘c’ argument indicates that the file-node is for a ‘character-mode’ device • Values 70 and 0 are arbitrary id-numbers Removing a device-file • To delete a file from the ‘/dev’ directory, the SysAdmin uses the ‘unlink’ command: root# unlink /dev/cmos • Because of standard privilege-restrictions on the ‘/dev’ directory, these commands will fail if executed by unprivileged users • Students have not been given sufficient privileges to successfully execute these ‘mknod’, ‘chmod’ and ‘unlink’ commands LKM’s have full privileges • But CS students in this class DO possess privileges that allow them to install, and to remove, Linux kernel modules: user$ /sbin/insmod cmos.ko user$ /sbin/rmmod cmos.ko • While installed, modules execute inside the kernel itself, with no privilege barriers • Hence we should be able to perform the steps necessary to create a .