signal: requeuing undeliverable signals

Kyle Huey recently reported[1] that rr gets confused if SIGKILL prevents
ptrace_signal from delivering a signal, as the kernel setups up a signal
frame for a signal that rr did not have a chance to observe with ptrace.

In looking into it I found a couple of bugs and a quality of
implementation issue.

- The test for signal_group_exit should be inside the for loop in get_signal.
- Signals should be requeued on the same queue they were dequeued from.
- When a fatal signal is pending ptrace_signal should not return another
  signal for delivery.

Kyle Huey has verified[2] an earlier version of this change.

I have reworked things one more time to completely fix the issues
raised, and to keep the code maintainable long term.

I have smoke tested this code and combined with a careful review I
expect this code to work fine.  Kyle if you can double check that
my last round of changes still works for rr I would appreciate it.

Eric W. Biederman (3):
      signal: In get_signal test for signal_group_exit every time through the loop
      signal: Requeue signals in the appropriate queue
      signal: Requeue ptrace signals

 fs/signalfd.c                |  5 +++--
 include/linux/sched/signal.h |  7 ++++---
 kernel/signal.c              | 44 ++++++++++++++++++++++++++------------------
 3 files changed, 33 insertions(+), 23 deletions(-)

[1] https://lkml.kernel.org/r/20211101034147.6203-1-khuey@kylehuey.com
[2] https://lkml.kernel.org/r/CAP045ApAX725ZfujaK-jJNkfCo5s+oVFpBvNfPJk+DKY8K7d=Q@mail.gmail.com
Tested-by: Kyle Huey <khuey@kylehuey.com>
Link: https://lkml.kernel.org/r/87bl2kekig.fsf_-_@email.froward.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 23505394ef70..167995d471da 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -286,17 +286,18 @@ static inline int signal_group_exit(const struct signal_struct *sig)
 extern void flush_signals(struct task_struct *);
 extern void ignore_signals(struct task_struct *);
 extern void flush_signal_handlers(struct task_struct *, int force_default);
-extern int dequeue_signal(struct task_struct *task,
-			  sigset_t *mask, kernel_siginfo_t *info);
+extern int dequeue_signal(struct task_struct *task, sigset_t *mask,
+			  kernel_siginfo_t *info, enum pid_type *type);
 
 static inline int kernel_dequeue_signal(void)
 {
 	struct task_struct *task = current;
 	kernel_siginfo_t __info;
+	enum pid_type __type;
 	int ret;
 
 	spin_lock_irq(&task->sighand->siglock);
-	ret = dequeue_signal(task, &task->blocked, &__info);
+	ret = dequeue_signal(task, &task->blocked, &__info, &__type);
 	spin_unlock_irq(&task->sighand->siglock);
 
 	return ret;