Re: [PATCH] pidhash: Kill switch_exec_pids

[Oleg Nesterov <oleg@tv-sign.ru>]

"Eric W. Biederman" wrote:
> 
> Oleg Nesterov <oleg@tv-sign.ru> writes:
> 
> > Eric W. Biederman wrote:
> >>
> >> +            detach_pid(current, PIDTYPE_PID);
> >> +            current->pid = leader->pid;
> >> +            attach_pid(current, PIDTYPE_PID,  current->pid);
> >
> > What happens after de_thread() unlocks tasklist_lock and before
> > it is taken again in release_task() ?
> >
> > In that window find_task_by_pid() will return dead leader, not
> > the new leader of thread group. This means we can miss tkill()
> > or ptrace(), for example.
> 
> All I have done is enlarged the window where this
> race is possible.  So for tkill I am not concerned,
> as it wants a particular thread.  Nor am I concerned
> about anything else that wants a particular thread.

Yes, you are right, sorry for noise. We have exactly same situation
before de_thread() locks tasklist after killing the leader.

> The fact that the group_leader does not point
> at the actual thread group leader might be a problem,
> as I have opened a window where that is now the case.
> 
> For signals that is not a problem as signals are still shared.
> This applies to most other resources as well.

Actually, now I think this patch fixes a small theoretical bug.
Currently we have a tiny window in switch_exec_pids() when it
detaches ->pid from PIDTYPE_PID namespace. RCU based kill_proc_info()
does not take tasklist, so we can miss a signal.

I have added Paul to the CC: list.

> So until we spot that case I'm ready to put this down
> of one of those cases in de_thread that looks wrong
> but happens to work.  Now if there is a way to make
> it work more cleanly that may be worth looking at.

I think you are right.

Andrew, please drop this one:

	dont-touch-current-tasks-in-de_thread.patch

Eric's patch includes this cleanup.

Oleg.
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