Commit d80f7d7b authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

signal: Guarantee that SIGNAL_GROUP_EXIT is set on process exit 

Track how many threads have not started exiting and when the last
thread starts exiting set SIGNAL_GROUP_EXIT.

This guarantees that SIGNAL_GROUP_EXIT will get set when a process
exits.  In practice this achieves nothing as glibc's implementation of
_exit calls sys_group_exit then sys_exit.  While glibc's implemenation
of pthread_exit calls exit (which cleansup and calls _exit) if it is
the last thread and sys_exit if it is the last thread.

This means the only way the kernel might observe a process that does
not set call exit_group is if the language runtime does not use glibc.

With more cleanups I hope to move the decrement of quick_threads
earlier.

Link: https://lkml.kernel.org/r/87bkukd4tc.fsf_-_@email.froward.int.ebiederm.org


Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent cbe9dac3

include/linux/sched/signal.h

+1 −0
Original line number Diff line number Diff line
@@ -94,6 +94,7 @@ struct signal_struct { 
	refcount_t		sigcnt;

	atomic_t		live;

	int			nr_threads;

	int			quick_threads;

	struct list_head	thread_head;



	wait_queue_head_t	wait_chldexit;	/* for wait4() */

kernel/exit.c

+18 −0
Original line number Diff line number Diff line
@@ -733,11 +733,29 @@ static void check_stack_usage(void) 
static inline void check_stack_usage(void) {}

#endif



static void synchronize_group_exit(struct task_struct *tsk, long code)

{

	struct sighand_struct *sighand = tsk->sighand;

	struct signal_struct *signal = tsk->signal;



	spin_lock_irq(&sighand->siglock);

	signal->quick_threads--;

	if ((signal->quick_threads == 0) &&

	    !(signal->flags & SIGNAL_GROUP_EXIT)) {

		signal->flags = SIGNAL_GROUP_EXIT;

		signal->group_exit_code = code;

		signal->group_stop_count = 0;

	}

	spin_unlock_irq(&sighand->siglock);

}



void __noreturn do_exit(long code)

{

	struct task_struct *tsk = current;

	int group_dead;



	synchronize_group_exit(tsk, code);



	WARN_ON(tsk->plug);



	kcov_task_exit(tsk);

kernel/fork.c

+2 −0
Original line number Diff line number Diff line
@@ -1692,6 +1692,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) 
		return -ENOMEM;



	sig->nr_threads = 1;

	sig->quick_threads = 1;

	atomic_set(&sig->live, 1);

	refcount_set(&sig->sigcnt, 1);
@@ -2444,6 +2445,7 @@ static __latent_entropy struct task_struct *copy_process( 
			__this_cpu_inc(process_counts);

		} else {

			current->signal->nr_threads++;

			current->signal->quick_threads++;

			atomic_inc(&current->signal->live);

			refcount_inc(&current->signal->sigcnt);

			task_join_group_stop(p);