linux-kernel - [PATCH 06/17] signal: Add JOBCTL_WILL

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Date: Tue, 18 Jun 2024 23:07:26 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  Tejun Heo <tj@...nel.org>,
  linux-kernel@...r.kernel.org
Subject: [PATCH 06/17] signal: Add JOBCTL_WILL_EXIT to mark exiting tasks


Mark tasks that need to exit with JOBCTL_WILL_EXIT instead of reusing
the per thread SIGKILL.

A jobctl flag was choosen for this purpose because jobctl changes are
protected by siglock, and updates are already careful not to change or
clear other bits in jobctl.  Protection by a lock when changing the
value is necessary as JOBCTL_WILL_EXIT will not be limited to being
set by the current task.  That task->jobctl is protected by siglock is
convenient as siglock is already held everywhere I want to set or
reset JOBCTL_WILL_EXIT.

Instead of having __fatal_signal_pending test JOBCTL_WILL_EXIT
directly add a more accurately named helper task_will_exit to test
JOBCTL_WILL_EXIT that __fatal_signal_pending calls.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/coredump.c                |  4 ++--
 include/linux/sched/jobctl.h |  2 ++
 include/linux/sched/signal.h |  7 ++++++-
 kernel/signal.c              | 11 ++++++-----
 4 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index be0405346882..bec9e290802a 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -373,7 +373,7 @@ static int zap_process(struct task_struct *start, int exit_code)
 	for_each_thread(start, t) {
 		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
 		if (!(t->flags & PF_POSTCOREDUMP)) {
-			sigaddset(&t->pending.signal, SIGKILL);
+			t->jobctl |= JOBCTL_WILL_EXIT;
 			signal_wake_up(t, 1);
 		}
 		nr += (t != current) && !(t->flags & PF_POSTCOREDUMP);
@@ -396,7 +396,7 @@ static int zap_threads(struct task_struct *tsk,
 
 		/* Allow SIGKILL, see prepare_signal() */
 		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
-		sigdelset(&tsk->pending.signal, SIGKILL);
+		tsk->jobctl &= ~JOBCTL_WILL_EXIT;
 		tsk->flags |= PF_DUMPCORE;
 	}
 	spin_unlock_irq(&tsk->sighand->siglock);
diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index 68876d0a7ef9..2e840f2db746 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -23,6 +23,7 @@ struct task_struct;
 
 #define JOBCTL_STOPPED_BIT	26	/* do_signal_stop() */
 #define JOBCTL_TRACED_BIT	27	/* ptrace_stop() */
+#define JOBCTL_WILL_EXIT_BIT	31	/* task will exit */
 
 #define JOBCTL_STOP_DEQUEUED	(1UL << JOBCTL_STOP_DEQUEUED_BIT)
 #define JOBCTL_STOP_PENDING	(1UL << JOBCTL_STOP_PENDING_BIT)
@@ -36,6 +37,7 @@ struct task_struct;
 
 #define JOBCTL_STOPPED		(1UL << JOBCTL_STOPPED_BIT)
 #define JOBCTL_TRACED		(1UL << JOBCTL_TRACED_BIT)
+#define JOBCTL_WILL_EXIT	(1UL << JOBCTL_WILL_EXIT_BIT)
 
 #define JOBCTL_TRAP_MASK	(JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
 #define JOBCTL_PENDING_MASK	(JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 0a0e23c45406..33bf363a3354 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -393,9 +393,14 @@ static inline int signal_pending(struct task_struct *p)
 	return task_sigpending(p);
 }
 
+static inline bool task_exit_pending(struct task_struct *p)
+{
+	return unlikely(READ_ONCE(p->jobctl) & JOBCTL_WILL_EXIT);
+}
+
 static inline int __fatal_signal_pending(struct task_struct *p)
 {
-	return unlikely(sigismember(&p->pending.signal, SIGKILL));
+	return task_exit_pending(p);
 }
 
 static inline int fatal_signal_pending(struct task_struct *p)
diff --git a/kernel/signal.c b/kernel/signal.c
index 269ec88f650d..ea7753b31be7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -156,7 +156,8 @@ static inline bool has_pending_signals(sigset_t *signal, sigset_t *blocked)
 
 static bool recalc_sigpending_tsk(struct task_struct *t)
 {
-	if ((t->jobctl & (JOBCTL_PENDING_MASK | JOBCTL_TRAP_FREEZE)) ||
+	if ((t->jobctl & (JOBCTL_PENDING_MASK | JOBCTL_TRAP_FREEZE |
+			  JOBCTL_WILL_EXIT)) ||
 	    PENDING(&t->pending, &t->blocked) ||
 	    PENDING(&t->signal->shared_pending, &t->blocked) ||
 	    cgroup_task_frozen(t)) {
@@ -910,7 +911,7 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
 		if (signal->core_state && (sig == SIGKILL)) {
 			struct task_struct *dumper =
 				signal->core_state->dumper.task;
-			sigaddset(&dumper->pending.signal, SIGKILL);
+			dumper->jobctl |= JOBCTL_WILL_EXIT;
 			signal_wake_up(dumper, 1);
 		}
 		/*
@@ -1054,7 +1055,7 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
 			signal->group_stop_count = 0;
 			__for_each_thread(signal, t) {
 				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
-				sigaddset(&t->pending.signal, SIGKILL);
+				t->jobctl |= JOBCTL_WILL_EXIT;
 				signal_wake_up(t, 1);
 			}
 			return;
@@ -1383,7 +1384,7 @@ int zap_other_threads(struct task_struct *p)
 		/* Don't bother with already dead threads */
 		if (t->exit_state)
 			continue;
-		sigaddset(&t->pending.signal, SIGKILL);
+		t->jobctl |= JOBCTL_WILL_EXIT;
 		signal_wake_up(t, 1);
 	}
 
@@ -2745,7 +2746,7 @@ bool get_signal(struct ksignal *ksig)
 		if ((signal->flags & SIGNAL_GROUP_EXIT) ||
 		     signal->group_exec_task) {
 			signr = SIGKILL;
-			sigdelset(&current->pending.signal, SIGKILL);
+			current->jobctl &= ~JOBCTL_WILL_EXIT;
 			trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
 					     &sighand->action[SIGKILL-1]);
 			recalc_sigpending();
-- 
2.41.0