exec: rework the group exit and fix the race with kill

As Roland pointed out, we have the very old problem with exec.  de_thread()
sets SIGNAL_GROUP_EXIT, kills other threads, changes ->group_leader and then
clears signal->flags.  All signals (even fatal ones) sent in this window
(which is not too small) will be lost.

With this patch exec doesn't abuse SIGNAL_GROUP_EXIT.  signal_group_exit(),
the new helper, should be used to detect exit_group() or exec() in progress.
It can have more users, but this patch does only strictly necessary changes.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Robin Holt <holt@sgi.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -760,7 +760,7 @@ static int de_thread(struct task_struct *tsk)
 	 */
 	read_lock(&tasklist_lock);
 	spin_lock_irq(lock);
-	if (sig->flags & SIGNAL_GROUP_EXIT) {
+	if (signal_group_exit(sig)) {
 		/*
 		 * Another group action in progress, just
 		 * return so that the signal is processed.
@@ -778,6 +778,7 @@ static int de_thread(struct task_struct *tsk)
 	if (unlikely(tsk->group_leader == task_child_reaper(tsk)))
 		task_active_pid_ns(tsk)->child_reaper = tsk;
 
+	sig->group_exit_task = tsk;
 	zap_other_threads(tsk);
 	read_unlock(&tasklist_lock);
 
@@ -802,7 +803,6 @@ static int de_thread(struct task_struct *tsk)
 	}
 
 	sig->notify_count = count;
-	sig->group_exit_task = tsk;
 	while (atomic_read(&sig->count) > count) {
 		__set_current_state(TASK_UNINTERRUPTIBLE);
 		spin_unlock_irq(lock);
@@ -871,15 +871,10 @@ static int de_thread(struct task_struct *tsk)
 		leader->exit_state = EXIT_DEAD;
 
 		write_unlock_irq(&tasklist_lock);
-        }
+	}
 
 	sig->group_exit_task = NULL;
 	sig->notify_count = 0;
-	/*
-	 * There may be one thread left which is just exiting,
-	 * but it's safe to stop telling the group to kill themselves.
-	 */
-	sig->flags = 0;
 
 no_thread_group:
 	exit_itimers(sig);
@@ -1549,7 +1544,7 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 	int err = -EAGAIN;
 
 	spin_lock_irq(&tsk->sighand->siglock);
-	if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
+	if (!signal_group_exit(tsk->signal)) {
 		tsk->signal->group_exit_code = exit_code;
 		zap_process(tsk);
 		err = 0;

include/linux/sched.h

@@ -555,6 +555,13 @@ struct signal_struct {
 #define SIGNAL_STOP_CONTINUED	0x00000004 /* SIGCONT since WCONTINUED reap */
 #define SIGNAL_GROUP_EXIT	0x00000008 /* group exit in progress */
 
+/* If true, all threads except ->group_exit_task have pending SIGKILL */
+static inline int signal_group_exit(const struct signal_struct *sig)
+{
+	return	(sig->flags & SIGNAL_GROUP_EXIT) ||
+		(sig->group_exit_task != NULL);
+}
+
 /*
  * Some day this will be a full-fledged user tracking system..
  */

kernel/exit.c

@@ -1083,11 +1083,12 @@ do_group_exit(int exit_code)
 		struct signal_struct *const sig = current->signal;
 		struct sighand_struct *const sighand = current->sighand;
 		spin_lock_irq(&sighand->siglock);
-		if (sig->flags & SIGNAL_GROUP_EXIT)
+		if (signal_group_exit(sig))
 			/* Another thread got here before we took the lock.  */
 			exit_code = sig->group_exit_code;
 		else {
 			sig->group_exit_code = exit_code;
+			sig->flags = SIGNAL_GROUP_EXIT;
 			zap_other_threads(current);
 		}
 		spin_unlock_irq(&sighand->siglock);

kernel/signal.c

@@ -957,7 +957,6 @@ void zap_other_threads(struct task_struct *p)
 {
 	struct task_struct *t;
 
-	p->signal->flags = SIGNAL_GROUP_EXIT;
 	p->signal->group_stop_count = 0;
 
 	for (t = next_thread(p); t != p; t = next_thread(t)) {
@@ -1697,7 +1696,8 @@ static int do_signal_stop(int signr)
 	} else {
 		struct task_struct *t;
 
-		if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED))
+		if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
+		    unlikely(sig->group_exit_task))
 			return 0;
 		/*
 		 * There is no group stop already in progress.