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From:   Tycho Andersen <tycho@tycho.pizza>
To:     "Eric W . Biederman" <ebiederm@xmission.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>, linux-kernel@vger.kernel.org,
        Tycho Andersen <tycho@tycho.pizza>
Subject: [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING
Date:   Wed, 13 Jul 2022 11:53:05 -0600
Message-Id: <20220713175305.1327649-1-tycho@tycho.pizza>
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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>
---
 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);
 }
 
 static inline int fatal_signal_pending(struct task_struct *p)

base-commit: 32346491ddf24599decca06190ebca03ff9de7f8
-- 
2.34.1