Re: More waitpid issues with CLONE_DETACHED/CLONE_THREAD

[Daniel Jacobowitz <dan@debian.org>]

On Sat, Jan 31, 2004 at 09:12:33PM -0800, Linus Torvalds wrote:
> 
> 
> On Sat, 31 Jan 2004, Daniel Jacobowitz wrote:
> > > 
> > > Can you verify that that process doesn't have any sub-threads? (Again, 
> > > that should be easily visible in /proc/<pid>/task/).
> > 
> > It is quite easily visible - in fact, it's hilarious.
> > 
> >  8454 pts/8    Z      0:00 [linux-dp] <defunct>
> > 
> > drow@nevyn:~% ls /proc/8454
> > auxv  cmdline  cwd@  environ  exe@  fd/  maps  mem  mounts  root@  stat
> > 
> > drow@nevyn:~% ls /proc/8454/task
> > ls: /proc/8454/task: No such file or directory
> 
> Oh damn. Yeah, we don't allow you to even see the threads in this case (it
> checks "pid_alive(task)". We don't want you to try to confuse things by 
> opening files of processes that we think are dead.
> 
> For the case of trying to figure out sub-threads of a dead thread group
> leader, that may actually be a bug.
> 
> Just for testing this, you might remove the check for
> 
>         if (!pid_alive(task))
>                 goto out;
> 
> in proc_pident_lookup() in fs/proc/base.c.
> 
> Ingo - comments? We really want the sysadmin to be able to find threads 
> that have a dead group leader, and right now that seems to be impossible.

I thought that a new group leader would be swapped in to that TID?  But
I was always confused by the mechanics of that.

Note that processes going invisible this way is, um, a bit of a
security problem.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer