[PATCH] proc: Report SIGEV_NONE in /proc/pid/timers if target task has died

[Andrei Vagin <avagin@google.com>]

When a posix timer is created targeting a specific thread (using
SIGEV_SIGNAL | SIGEV_THREAD_ID), it takes a reference to the target
struct pid in timer->it_pid. If the target thread subsequently
terminates, its numeric tid is freed and can be recycled for an
unrelated task. However, the timer holds its reference to the original
struct pid.

show_timer() in /proc/[pid]/timers previously called pid_nr_ns()
directly on timer->it_pid without checking whether any task remained
attached to that struct pid. As a result:
1. It reported the stale tid, which could mistakenly refer to a recycled
   pid.
2. In the kernel, expired signals for dead target threads are dropped by
   posixtimer_send_sigqueue() because posixtimer_get_target() returns
   NULL, so the timer functionally acts as SIGEV_NONE.
3. Checkpoint/restore tools (CRIU) parsing /proc/[pid]/timers would try
   to restore a timer with SIGEV_SIGNAL | SIGEV_THREAD_ID targeting a
   non-existent or unrelated thread.

Check pid_has_task(timer->it_pid, timer->it_pid_type) in show_timer().
If the target task has died, override notify to SIGEV_NONE and report
PID 0 (e.g., 'notify: none/pid.0').

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrei Vagin <avagin@google.com>
---
 fs/proc/base.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 780f81259052..e3a6ea4fffea 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2519,17 +2519,23 @@ static int show_timer(struct seq_file *m, void *v)
 	struct k_itimer *timer = hlist_entry((struct hlist_node *)v, struct k_itimer, list);
 	struct timers_private *tp = m->private;
 	int notify = timer->it_sigev_notify;
+	pid_t nr = 0;
 
 	guard(spinlock_irq)(&timer->it_lock);
 	if (!posixtimer_valid(timer))
 		return 0;
 
+	if (timer->it_pid && pid_has_task(timer->it_pid, timer->it_pid_type))
+		nr = pid_nr_ns(timer->it_pid, tp->ns);
+	else
+		notify = SIGEV_NONE;
+
 	seq_printf(m, "ID: %d\n", timer->it_id);
 	seq_printf(m, "signal: %d/%px\n", timer->sigq.info.si_signo,
 		   timer->sigq.info.si_value.sival_ptr);
 	seq_printf(m, "notify: %s/%s.%d\n", nstr[notify & ~SIGEV_THREAD_ID],
 		   (notify & SIGEV_THREAD_ID) ? "tid" : "pid",
-		   pid_nr_ns(timer->it_pid, tp->ns));
+		   nr);
 	seq_printf(m, "ClockID: %d\n", timer->it_clock);
 
 	return 0;
-- 
2.55.0.691.gc56d675ccc-goog