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Fbi is a framebuffer image viewer that does not need an X-server setup. It's perfect for what I want to achieve: a stand alone dedicated slide show.

I want to run the following command as a non-root user:

fbi -d /dev/fb0 -T 1 foo.jpg

When I run fbi as non-root, the result on tty1 is:

access /dev/tty1: Permission denied

I can perfectly run the command as root (sudo ...), but that's not desirable. My question is: how can I run fbi as non-root user on a tty?

The setup of the rpi4 is "headless": no X-server installed, the fbi command is issued from ssh or crontab, the output tty1 is a screen connected to a HDMI port on the rpi4.

I tried many things:

  • checked all possible forums, many of which address this problem without giving a satisfactory solution. The man page for fbi suggests adding the user to the vido and tty groups which I did but to no avail.
  • added the user to the groups tty and video
  • changed permissions on tty1 and fb0 to 666. Interestingly the error message changes to ...

ioctl VT_ACTIVATE: Operation not permitted.

Of course, after reboot the permissions of the /dev/tty1 and /dev/fb0 change back to normal. So changing these permissions is no good idea at all, even if it would work, which it doesn't.

Thanks you guys for your help!

HenkN
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  • Would it be possible to tweak the source code, more specifically https://git.kraxel.org/cgit/fbida/tree/vt.c line 162? (I'm just following a comment the author seems to have left there regarding radeonfb.) – Vicky Apr 11 '21 at 23:35

1 Answers1

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I'm running bullseye on a rpi3.

Here is how I got it to work (from inside a docker container fwiw):

  • [x] add the user to the tty and video groups
  • [x] change permission on /dev/tty1 to allow group read
  • [ ] grant capability CAP_SYS_TTY_CONFIG to the fbi binary

The last part is what you missed (the cap).

One liner to get it:

setcap 'cap_sys_tty_config+ep' $(which fbi)

[UPDATED]

Also, to persist the permission on the tty on raspbian, look into

cat /usr/lib/udev/rules.d/50-udev-default.rules | grep "\"tty\[0"

Mangled Deutz
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  • Yeah, works fine! However, the rw setting on /dev/tty1 does not persist after reboot. I tried different things, a.o.: 1. edit /etc/login.defs and change TTYPERM to 0660 2. add rule in /etc/udev/rules.d: ACTION=="add", KERNEL=="tty1", SUBSYSTEM=="tty", DRIVER=="", MODE="0660". Neither works: after reboot tty1 is write-only again for group. – HenkN Sep 19 '21 at 15:12
  • Let me look into that part (I will need it too). – Mangled Deutz Sep 20 '21 at 16:02
  • @HenkN I managed to have my perm persisted by tweaking udev default rules. Updated the answer. – Mangled Deutz Sep 20 '21 at 16:39
  • Still not working. When I change the rule in /etc/lib/udev/50-udev-defaults.rules into ... SUBSYSTEM=="tty", KERNEL=="tty[0-9]*", GROUP="tty", MODE="0660" ... all tty's get rw permission on group EXCEPT tty1!. Obviously there is some other part where the HDMI output is connected with tty1 and set to write-only. – HenkN Sep 25 '21 at 13:27
  • Yeah, just use any *other* tty with fbi (it does not matter). `fbi -a -noverbose -norandom -T 2 -d /dev/fb0 -once whatever.jpg` (<- this works for me - using a PI3 with a hat display over hdmi) – Mangled Deutz Sep 27 '21 at 18:55