From dc2fd4b00946751ebd222d366fc64550e4188dc2 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 10 Dec 2014 15:45:24 -0800 Subject: [PATCH] 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 Cc: Aaron Tomlin Cc: Alexey Dobriyan Cc: "Eric W. Biederman" , Cc: Sterling Alexander Cc: Peter Zijlstra Cc: Roland McGrath Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/exit.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 232c4bc8bcc9..0272305bf855 100644 --- a/kernel/exit.c +++ b/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); } } -- 2.20.1