Re: Cant fg a suspended su (4.1.0-dev-7)

[Philippe Troin <phil@fifi.org>]

Peter Whaite <peta@cortexmachina.com> writes:

> Philippe Troin said:
> > 
> > A few questions for Peter:
> > 
> >  - Which su are you using? GNU su, the shadow utilities's version of
> >    su or something else?
> 
> % ls -l =su
> -rwsr-xr-x    1 root     root        14112 Jan 16  2001 /bin/su*
> % /bin/su --version
> 
> su (GNU sh-utils) 2.0
> Written by David MacKenzie.
> 
> Copyright (C) 1999 Free Software Foundation, Inc.

Using the shadow utilities' su here... That's why I could not
reproduce the problem.
 
> >  - Can you send us the output of 'ps xajf' ran from within the su
> >    shell... You might want to trim it. I'm interested in the PGID of
> >    the the su'ed zsh and its parent... For me it looks like this:

8< snip >8

> 
> % /bin/su peta
> Password:
> % ps xajf
> 
>  PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND
> 20712 20728 20728 20728 pts/2    21112 S      501   0:00  |       \_ zsh
> 20728 21091 21091 20728 pts/2    21112 S        0   0:00  |           \_ /bin/su
> 21091 21095 21095 20728 pts/2    21112 S      501   0:00  |               \_ zsh
> 21095 21112 21112 20728 pts/2    21112 R      501   0:00  |                   \_

Now I know what the problem is... GNU su does not exec() the sub-shell
and hangs around. The zsh-workers/17859 patch (which I'm responsible
for) explicitly makes sure zsh is its own process group leader for
interactive sessions. This cures quite a few problems but introduced
this bug.

Before zsh-workers/17859 was applied:

  the upper zsh would have had its own process group

  both su and the lower zsh share their process group

  Pros: suspend (as coded currently) works

  Cons: terminal-related signals get sent to both su and zsh. Su must
  contain special logic to ignore these signals, otherwise a (eg.)
  CTRL-C will kill the su process leaving the lower zsh completely
  disconnected from the upper zsh, which leads to chaos.

After zsh-workers/17859 was applied:

  the upper zsh would have had its own process group

  su has its own process group

  at startup, zsh creates its own process group

  Pros: no need to add some special logic to su (or any other
  application)

  Cons: suspend (as currently coded) does not work.

> and also (leftovers?)...

8< snip >8

Not needed.
 
> I can confirm that it doesnt always happen.  On a FreeBSD system this
> version of su
> 
> $FreeBSD: src/usr.bin/su/su.c,v 1.34.2.3 2001/08/17 15:44:42 ru Exp $
> 
> works fine.

Probably because FreeBSD's su does exec() on $SHELL (shadow su's
behavior) rather than fork()+exec() (GNU su behavior).
 
> Hope this helps

It did.

I'll check out how the other shells (bash and tcsh) handle this case
and will post a patch for bin_suspend() later.

Phil.