No permissions to change tty mode inside a docker container - linux

Question

I'm trying to run some software inside a docker container, which wants to do VT_SETMODE for the tty currently in use. This will always fail with an error saying "Operation not permitted".
I've tried playing around with permissions/groups with no luck.
Finally I created a small snippet to reproduce the error:
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <linux/vt.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
int main() {
const char *tty_path;
tty_path = "/dev/tty1";
int fd = open(tty_path, O_RDWR | O_CLOEXEC);
if (fd < 0) {
printf("ERROR: Failed to open %s\n", strerror(errno));
return 1;
}
struct vt_mode mode = {
.mode = VT_AUTO,
};
errno = 0;
ioctl(fd, VT_SETMODE, &mode);
if (errno) {
printf("ERROR: %s\n", strerror(errno));
return 1;
}
return 0;
}
I ran the code inside a docker container with a simple dockerfile:
FROM archlinux/base
RUN pacman -Sy --noconfirm gcc
That was started with command:
docker build -f Dockerfile -t tty-test . && docker run --device /dev/tty1 -v $HOME/tty-test:/volume -it tty-test /bin/bash -c 'cd /volume && gcc tty_r.c && ./a.out ; /bin/bash'
Output is this:
ERROR: Operation not permitted
Can anyone explain why the tty is not accessible from the container, or is there some way to give more control to the container?

Answer 1

Okay it seems that I have solved my own problem. I was trying to run the code inside a container which had no privilege to do tty-configuration. Adding --cap-add SYS_TTY_CONFIG to the docker run command fixed the issue.