flatpak-spawn --host and tcsetpgrp issue

[valentindavid]

Linux distribution and version

GNOME OS master

Flatpak version

Flatpak 1.4.3

Description of the problem

It seems bash has troubles to tcsetprg when spawning host bash with flatpak-spawn --host. This is unexpected since the in source code of HostCommand it should set the console right.

When using debug mode, we get the error:

F: ioctl(0, TIOCSCTTY, 0) failed: Operation not permitted

Maybe setsid and setpgid should not be called in child_setup_func from session-helper/flatpak-session-helper.c if data->set_tty.

Steps to reproduce

bash-5.0$ flatpak run --talk-name=org.freedesktop.Flatpak --command=bash org.freedesktop.Sdk//19.08
bash-5.0$ flatpak-spawn --host bash -i
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
bash-5.0$ 

[valentindavid]

From the man page: If this terminal is already the controlling terminal of a different session group then the ioctl fails with EPERM, unless the caller is root (more precisely: has the CAP_SYS_ADMIN capability)

[valentindavid]

I suppose ioctl TIOCSCTTY should be called from flatpak-spawn after flatpak-session-helper's child process has created the group.

[Erick555]

Isn't this caused by TIOCSTI being blocked in flatpak?

[valentindavid]

No the reason is the according to the man page of tcsetpgrp (called from bash):

ENOTTY
The calling process does not have a controlling terminal, or it has one but it is not described by fd, or, for tcsetpgrp(), this controlling terminal is no longer associated with the session of the calling process.

... And ioctl TIOCSCTTY called flatpak-session-helper:

If this terminal is already the controlling terminal of a different session group then the ioctl fails with EPERM, unless the caller is root (more precisely: has the CAP_SYS_ADMIN capability)

I do not think there is a way as a user to be able to move the terminal to control another session. And there is no way to move a process to another session. So I do not think it is possible.

We would need to create a pty and proxy it, GNU screen style.

[Erick555]

Ok, I thought this is caused by sandbox because I see same message when I call bubblwrap with --new-session arg which is equivalent of what flatpak does to block TIOCSTI:

bwrap --bind / / --new-session bash
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell 

[valentindavid]

Maybe the correct way to do it, is when calling flatpak, if it detects it is running in a tty, it should spawn a helper process. Maybe that registers to the main flatpak session helper. When flatpak-spawn tries to spawn a new process, instead of creating a process as a child of flatpak-spawn, it redirects it to the helper for the session. That helper will just take care of creating a new process, it will be automatically using the right terminal. Probably we can also setpgid the new process to the process group of flatpak-spawn so that CTRL-C is properly handled.

[akvadrako]

See also: #3285 and flatpak/flatpak-xdg-utils#15

[akvadrako]

When flatpak-spawn tries to spawn a new process, instead of creating a process as a child of flatpak-spawn, it redirects it to the helper for the session.

I'm not sure that'll work in many cases. What if a new pty is allocated in the sandbox, say by expect or gnome-terminal? Then when the user calls flatpak-spawn --host bash it won't work at all.

[akvadrako]

Here is a rough example for creating a pty, which I think is the correct approach: https://gist.github.com/akvadrako/24691578799e86c9d8303b854a5cf957

It doesn't handle redirected std{in,out,err} or window resizing.

[owtaylor]

Point to some old code that does allocates a PTY and handles forwarding when using the org.freedesktop.Flatpak.Development.HostCommand interface. I forget at this point how complete it is - it may not handle resizing properly.

https://github.com/owtaylor/PurpleEgg/blob/master/common/host-command.c

[TingPing]

I just want to add yet another example of a solution for this, used by gnome-builder.

https://gitlab.gnome.org/chergert/flatterm/-/blob/master/fp-vte-util.c