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Date:   Wed, 20 Jul 2022 14:58:42 -0600
From:   Tycho Andersen <tycho@tycho.pizza>
To:     "Serge E. Hallyn" <serge@hallyn.com>
Cc:     "Eric W . Biederman" <ebiederm@xmission.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: __fatal_signal_pending() should also check
 PF_EXITING
Message-ID: <YthsgqAZYnwHZLn+@tycho.pizza>
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 <20220720150328.GA30749@mail.hallyn.com>
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On Wed, Jul 20, 2022 at 10:03:28AM -0500, Serge E. Hallyn wrote:
> On Wed, Jul 13, 2022 at 11:53:05AM -0600, Tycho Andersen wrote:
> > The wait_* code uses signal_pending_state() to test whether a thread has
> > been interrupted, which ultimately uses __fatal_signal_pending() to detect
> > if there is a fatal signal.
> > 
> > When a pid ns dies, it does:
> > 
> >     group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
> > 
> > for all the tasks in the pid ns. That calls through:
> > 
> >     group_send_sig_info() ->
> >       do_send_sig_info() ->
> >         send_signal_locked() ->
> >           __send_signal_locked()
> > 
> > which does:
> > 
> >     pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
> > 
> > which puts sigkill in the set of shared signals, but not the individual
> > pending ones. When complete_signal() is called at the end of
> > __send_signal_locked(), if the task already had PF_EXITING (i.e. was
> > already waiting on something in its fd closing path like a fuse flush),
> > complete_signal() will not wake up the thread, since wants_signal() checks
> > PF_EXITING before testing for SIGKILL.
> > 
> > If tasks are stuck in a killable wait (e.g. a fuse flush operation), they
> > won't see this shared signal, and will hang forever, since TIF_SIGPENDING
> > is set, but the fatal signal can't be detected. So, let's also look for
> > PF_EXITING in __fatal_signal_pending().
> > 
> > Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
> 
> Cool, thanks for nailing this down!
> 
> I assume you've been running this on some boxes with no weird effects?

Yes, but I haven't tested all the paths.

> > ---
> >  include/linux/sched/signal.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> > index cafbe03eed01..c20b7e1d89ef 100644
> > --- a/include/linux/sched/signal.h
> > +++ b/include/linux/sched/signal.h
> > @@ -402,7 +402,8 @@ static inline int signal_pending(struct task_struct *p)
> >  
> >  static inline int __fatal_signal_pending(struct task_struct *p)
> >  {
> > -	return unlikely(sigismember(&p->pending.signal, SIGKILL));
> > +	return unlikely(sigismember(&p->pending.signal, SIGKILL) ||
> > +			p->flags & PF_EXITING);
> 
> Looking around at the callers this does seem safe, but the name does
> now seem misleading.  Should this be renamed to something like
> exiting_or_fatal_signal_pending()?  

This is why I like my original patch better: it is just expanding the
set of signals to include the shared signals, which are indeed still
fatal pending signals for the task. I don't really understand Eric's
argument about kernel threads ignoring SIGKILL, since kernel threads
can still ignore SIGKILL just fine after this patch.

But yes, assuming Eric is ok with this venison. I can send a v2 with
the name change as you suggest.

Thanks for looking.

Tycho