Question

I am trying to make a c program to access GPIOs on an embedded linux system which will be run by a non root user. I can already access the GPIOs through sysfs (/sys/class/gpio) and have made a simple program that used mmap (through /dev/mem/) to control the GPIOs. However to write to /sys/class/gpio/ and /dev/mem/ you must have root privileges. What would be the most “correct” or standard way to access the GPIO in a program run as a non-root user?
Writing a device driver?

Giving the user read/write access to /sys/class/gpio/ so the program can use sysfs?
Or Giving the user read/write access to /dev/mem/ so the program can use mmap()?

Thanks

Answer 1

One potential option is to make a process setuid by setting the s bit.

e.g.

chmod +s myExectuable

However, this has terrible security implications as the process then runs as root – with all the hazards that entails. Only an option if you really trust the user-space app, and even then, risky.

I don’t think changing the default ownership and permissions of sysfs is possible without hacking up your kernel, and even then it would be tricky: sysfs is intricately connected with object model of the the Linux Driver model.

You may have more luck with the permissions on /dev/.

Ultimately, the correct way of solving this problem is a kernel-mode driver – in which you can implement whatever finely grained security (or lack thereof) you wish. Furthermore, you can implement mitigation against any potential ill-effects of allowing a user-mode application to control hardware.