From: Tycho Andersen <tycho@tycho.pizza>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
Christian Brauner <brauner@kernel.org>,
fuse-devel <fuse-devel@lists.sourceforge.net>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: strange interaction between fuse + pidns
Date: Mon, 11 Jul 2022 16:53:36 -0600 [thread overview]
Message-ID: <Ysyp8Kbl8FzhApUb@netflix> (raw)
In-Reply-To: <877d4jbabb.fsf@email.froward.int.ebiederm.org>
On Mon, Jul 11, 2022 at 04:37:12PM -0500, Eric W. Biederman wrote:
> Tycho Andersen <tycho@tycho.pizza> writes:
>
> > Hi all,
> >
> > On Mon, Jul 11, 2022 at 03:59:15PM +0200, Miklos Szeredi wrote:
> >> On Mon, 11 Jul 2022 at 12:35, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >> >
> >> > Can you try the attached untested patch?
> >>
> >> Updated patch to avoid use after free on req->args.
> >>
> >> Still mostly untested.
> >
> > Thanks, when I applied your patch, I still ended up with tasks stuck
> > waiting with a SIGKILL pending. So I looked into that and came up with
> > the patch below. With both your patch and mine, my testcase exits
> > cleanly.
> >
> > Eric (or Christian, or anyone), can you comment on the patch below? I
> > have no idea what this will break. Maybe instead a better approach is
> > some additional special case in __send_signal_locked()?
> >
> > Tycho
> >
> > From b7ea26adcf3546be5745063cc86658acb5ed37e9 Mon Sep 17 00:00:00 2001
> > From: Tycho Andersen <tycho@tycho.pizza>
> > Date: Mon, 11 Jul 2022 11:26:58 -0600
> > Subject: [PATCH] sched: __fatal_signal_pending() should also check shared
> > signals
> >
> > 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, in zap_pid_ns_processes() 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. 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.
>
> Hmm.
>
> That is perplexing.
Thanks for taking a look.
> __send_signal_locked calls complete_signal. Then if any of the tasks of
> the process can receive the signal, complete_signal will loop through
> all of the tasks of the process and set the per thread SIGKILL. Pretty
> much by definition tasks can always receive SIGKILL.
>
> Is complete_signal not being able to do that?
In my specific case it was because my testcase was already trying to
exit and had set PF_EXITING when the signal is delivered, so
complete_signal() was indeed not able to do that since PF_EXITING is
checked before SIGKILL in wants_signal().
But I changed my testacase to sleep instead of exit, and I get the
same hang behavior, even though complete_signal() does add SIGKILL to
the set. So there's something else going on there...
> The patch below really should not be necessary, and I have pending work
> that if I can push over the finish line won't even make sense.
>
> As it is currently an abuse to use the per thread SIGKILL to indicate
> that a fatal signal has been short circuit delivered. That abuse as
> well as being unclean tends to confuse people reading the code.
How close is your work? I'm wondering if it's worth investigating the
non-PF_EXITING case further, or if we should just land this since it
fixes the PF_EXITING case as well. Or maybe just do something like
this in addition:
diff --git a/kernel/signal.c b/kernel/signal.c
index 6f86fda5e432..0f71dfb1c3d2 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -982,12 +982,12 @@ static inline bool wants_signal(int sig, struct task_struct *p)
if (sigismember(&p->blocked, sig))
return false;
- if (p->flags & PF_EXITING)
- return false;
-
if (sig == SIGKILL)
return true;
+ if (p->flags & PF_EXITING)
+ return false;
+
if (task_is_stopped_or_traced(p))
return false;
?
Tycho
next prev parent reply other threads:[~2022-07-11 22:54 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-23 17:21 strange interaction between fuse + pidns Tycho Andersen
2022-06-23 21:55 ` Vivek Goyal
2022-06-23 23:41 ` Tycho Andersen
2022-06-24 17:36 ` Vivek Goyal
2022-07-11 10:35 ` Miklos Szeredi
2022-07-11 13:59 ` Miklos Szeredi
2022-07-11 20:25 ` Tycho Andersen
2022-07-11 21:37 ` Eric W. Biederman
2022-07-11 22:53 ` Tycho Andersen [this message]
2022-07-11 23:06 ` Eric W. Biederman
2022-07-12 13:43 ` Tycho Andersen
2022-07-12 14:34 ` Eric W. Biederman
2022-07-12 15:14 ` Tycho Andersen
2022-07-13 17:53 ` [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING Tycho Andersen
2022-07-20 15:03 ` Serge E. Hallyn
2022-07-20 20:58 ` Tycho Andersen
2022-07-21 1:54 ` Serge E. Hallyn
2022-07-27 15:44 ` Tycho Andersen
2022-07-27 16:32 ` Eric W. Biederman
2022-07-27 17:55 ` Tycho Andersen
2022-07-28 18:48 ` Eric W. Biederman
2022-07-27 17:55 ` Oleg Nesterov
2022-07-27 18:18 ` Tycho Andersen
2022-07-27 19:19 ` Oleg Nesterov
2022-07-27 19:40 ` Tycho Andersen
2022-07-28 9:12 ` Oleg Nesterov
2022-07-28 21:20 ` Tycho Andersen
2022-07-29 5:04 ` Eric W. Biederman
2022-07-29 13:50 ` Tycho Andersen
2022-07-29 16:15 ` Eric W. Biederman
2022-07-29 16:48 ` Tycho Andersen
2022-07-29 17:40 ` [RFC][PATCH] fuse: In fuse_flush only wait if someone wants the return code Eric W. Biederman
2022-07-29 20:47 ` Oleg Nesterov
2022-07-30 0:15 ` Al Viro
2022-07-30 5:10 ` [RFC][PATCH v2] " Eric W. Biederman
2022-08-01 15:16 ` Tycho Andersen
2022-08-02 12:50 ` Miklos Szeredi
2022-08-15 13:59 ` Tycho Andersen
2022-08-15 17:55 ` Serge E. Hallyn
2022-09-01 14:06 ` [PATCH] " Tycho Andersen
2022-09-19 15:03 ` Tycho Andersen
2022-09-20 18:02 ` Serge E. Hallyn
2022-09-26 14:17 ` Tycho Andersen
2022-09-27 9:46 ` Miklos Szeredi
2022-09-29 14:05 ` [fuse-devel] " Stef Bon
2022-09-29 16:39 ` [PATCH v2] " Tycho Andersen
2022-09-30 13:35 ` Miklos Szeredi
2022-09-30 14:01 ` Tycho Andersen
2022-09-30 14:41 ` Miklos Szeredi
2022-09-30 16:09 ` Tycho Andersen
2022-10-26 9:01 ` Miklos Szeredi
2022-11-14 16:02 ` [PATCH v3] " Tycho Andersen
2022-11-28 15:00 ` Tycho Andersen
2022-12-08 14:26 ` Miklos Szeredi
2022-12-08 17:49 ` Tycho Andersen
2022-12-19 19:16 ` Tycho Andersen
2023-01-03 14:51 ` Tycho Andersen
2023-01-05 15:15 ` Serge E. Hallyn
2023-01-26 14:12 ` Miklos Szeredi
2022-09-30 19:47 ` [PATCH] " Serge E. Hallyn
2022-09-19 15:46 ` [RFC][PATCH v2] " Eric W. Biederman
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