Re: supporting terminal ownership assignment (tcsetpgrp()) in posix_spawn

[Godmar Back <godmar@gmail.com>]

On Mon, Jun 7, 2021 at 5:04 PM Adhemerval Zanella <
adhemerval.zanella@linaro.org> wrote:

>
>
> On 03/06/2021 10:58, Godmar Back via Libc-help wrote:
> > I've recently been playing with posix_spawn() and noticed that it lacks
> > support for assigning terminal ownership (as would be done via
> > tcsetpgrp()). As a consequence, it cannot be used for job control shells
> > when starting jobs in the foreground [1].
> >
> > IBM's implementation of spawn [2] for zOS supports SPAWN_SETTCPGRP for
> this
> > reason.
> >
> > Even though the POSIX spec states "Future Directions: None" I came across
> > this Austin group issue [3] which proposes to add a way to set the child
> > process's session id, and which appears to have been accepted in 2016.
> This
> > is an example of evolution of posix_spawn.
> >
> > Is anyone aware of efforts to add something similar to POSIX, and more
> > specifically, to Linux?
> >
> > If the current implementation of posix_spawn is library based (I'm
> guessing
> > it is) then there would be nothing to prevent Linux from adding
> additional
> > flags, just like QNX or Blackberry have already done ([3]).
>
> Other OS might provide a syscall for posix_spawn (I know MacOSX does and
> I think maybe Solaris as well), but for Linux is done on top of clone
> syscall.
>
> And glibc alread supports POSIX_SPAWN_SETSID since 2.26, so why can't you
> use it along with with tcsetpgrp() called from the parent?


SETSID sets a new session, which is a different concept than the foreground
process group of a
controlling terminal.

tcsetpgrp() informs the OS which process group shall be the foreground
process group of the
terminal to which it is applied.  However, this can't be called by the
parent since the parent
doesn't know the process group id of the process that will be spawned by
the call to posix_spawn().

This process id (and the corresponding process group id) is known to the
parent only after the
call to posix_spawn() was made (and has returned). By that point, it's too
late to set the controlling
terminal's foreground process group. Specifically, by the time posix_spawn
returns, the child process
may have already acted under the assumption that it has ownership of the
terminal. Since the
child process is in a separate process group (because POSIX_SPAWN_SETPGROUP
was given to
posix_spawn()), this may lead to the suspension of the child process via
SIGTTIN/SIGTTOU.

Thus, calling tcsetpgrp() from the parent after the fact cannot be done in
a race-free fashion, one
needs to ensure that it's done after placing the child in a new process
group but before exec'ing the
child's executable.

In Linux's library-based implementation of posix_spawn(), support
for POSIX_SPAWN_TCSETPGROUP
could be easily added - my question is why hasn't it?

 - Godmar