exit: reparent: use ->ptrace_entry rather than ->sibling for EXIT_DEAD tasks

reparent_leader() reuses ->sibling as a list node to add an EXIT_DEAD task
into dead_children list we are going to release.  This obviously removes
the dead task from its real_parent->children list and this is even good;
the parent can do nothing with the EXIT_DEAD reparented zombie, it only
makes do_wait() slower.

But, this also means that it can not be reparented once again, so if its
new parent dies too nobody will update ->parent/real_parent, they can
point to the freed memory even before release_task() we are going to call,
this breaks the code which relies on pid_alive() to access
->real_parent/parent.

Fortunately this is mostly theoretical, this can only happen if init or
PR_SET_CHILD_SUBREAPER process ignores SIGCHLD and the new parent
sub-thread exits right after we drop tasklist_lock.

Change this code to use ->ptrace_entry instead, we know that the child is
not traced so nobody can ever use this member.  This also allows to unify
this logic with exit_ptrace(), see the next changes.

Note: we really need to change release_task() to nullify real_parent/
parent/group_leader pointers, but we need to change the current users
first somehow.  And it would be better to reap this zombie immediately but
release_task_locked() we need is complicated by proc_flush_task().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
Cc: Sterling Alexander <stalexan@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roland McGrath <roland@hack.frob.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -548,7 +548,7 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
 	    p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
 		if (do_notify_parent(p, p->exit_signal)) {
 			p->exit_state = EXIT_DEAD;
-			list_move_tail(&p->sibling, dead);
+			list_add(&p->ptrace_entry, dead);
 		}
 	}
 
@@ -587,8 +587,8 @@ static void forget_original_parent(struct task_struct *father)
 
 	BUG_ON(!list_empty(&father->children));
 
-	list_for_each_entry_safe(p, n, &dead_children, sibling) {
-		list_del_init(&p->sibling);
+	list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {
+		list_del_init(&p->ptrace_entry);
 		release_task(p);
 	}
 }