Can't access gpio mapped memory on focal armhf server on pi

[Kyle Nitzsche]

On released (and apt updated) 20.04 armhf preinstalled server on Pi3B+, we cannot use pigpio C program to access memory map for gpio. Customer says the same code works fine on 18.04.

ubuntu@ubuntu:~$ sudo ./pigpio/kyle.o
2020-07-31 18:36:11 initPeripherals: mmap gpio failed (Operation not permitted)

Details on code below, but the executable is attached, and the customer says this fairly represents their code usage.

ubuntu@ubuntu:~/pigpio$ uname -a
Linux ubuntu 5.4.0-1015-raspi #15-Ubuntu SMP Fri Jul 10 05:37:25 UTC 2020 armv7l armv7l armv7l GNU/Linux

We thought perhaps these kernel configs were involved:

CONFIG_DEVMEM=y
CONFIG_ARCH_HAS_DEVMEM_IS_ALLOWED=y
CONFIG_STRICT_DEVMEM=y

... and we modified the kernel cmdline with the following, but the result was the same.
iomem=relaxed strict-devmem=0

The program is simply this https://github.com/joan2937/pigpio/ with make & make install
then this c file:

#include <pigpio.h>
#include <stdio.h>
#include <stdlib.h>

int main(void){

gpioInitialise();

}

built with gcc -Wall -I/home/ubuntu/pigpio -pthread -o kyle.o kyle.c -lpigpio -lrt

[Juerg Haefliger (juergh)]

I've confirmed that 5.4 gives 'permission denied' and 5.3 seems to work fine. I suspect that this is due to the lockdown patches in 5.4.

[Kyle Nitzsche (knitzsche)]

This is an important issue blocking the project. Can we please set the importance and assign this? Thank you.

[Juerg Haefliger (juergh)]

This is not a kernel problem. The problem is that the pigpio library is trying to mmap the GPIOs with PROT_EXEC which is a security risk. In Focal /dev is mounted with the noexec flag so that's no longer possible. They could patch the pigpio library to remove the PROT_EXEC flag from the mmap call but I don't know if the library relies on that in other places (it really shouldn't). There's probably a library in Ubuntu to access GPIOs which is really what they should use so they get support if something doesn't work. Closing as invalid as this is not a bug.

[Jon Watson (jwatson-kdp)]

We are going to try editing the pigpio library and rebuilding it and seeing if it works. There doesn't appear to be a clear requirement for pigpio to rely on PROT_EXEC and it has been removed in the past.

Conversation about this is happening here:
https://github.com/joan2937/pigpio/issues/375

[Jon Watson (jwatson-kdp)]

We completed a test this morning with the modified pigpio library and verified our applications can now read the GPIO signals once we removed the PROT_EXEC flags from the library init code.

We will pursue getting a change in with the pigpio library separately.