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Date:   Tue, 8 Jan 2019 17:50:43 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Andrei Vagin <avagin@gmail.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH] kernel: release ptraced tasks before zap_pid_ns_processes
Message-ID: <20190108165043.GA10240@redhat.com>
References: <20190102205939.26231-1-avagin@gmail.com>
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Sorry for delay, vacation,

On 01/02, Andrei Vagin wrote:
>
> zap_pid_ns_processes() can stuck on waiting tasks from the dead list. In
> this case, we will have one unkillable process with one or more dead
> children.

Thanks!

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -664,9 +664,6 @@ static void forget_original_parent(struct task_struct *father,
>  {
>  	struct task_struct *p, *t, *reaper;
>  
> -	if (unlikely(!list_empty(&father->ptraced)))
> -		exit_ptrace(father, dead);
> -
>  	/* Can drop and reacquire tasklist_lock */
>  	reaper = find_child_reaper(father);
>  	if (list_empty(&father->children))
> @@ -705,8 +702,18 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  	LIST_HEAD(dead);
>  
>  	write_lock_irq(&tasklist_lock);
> -	forget_original_parent(tsk, &dead);
> +	if (unlikely(!list_empty(&tsk->ptraced)))
> +		exit_ptrace(tsk, &dead);
> +	write_unlock_irq(&tasklist_lock);
> +
> +	/* Ptraced tasks have to be released before zap_pid_ns_processes(). */
> +	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
> +		list_del_init(&p->ptrace_entry);
> +		release_task(p);
> +	}
>  
> +	write_lock_irq(&tasklist_lock);
> +	forget_original_parent(tsk, &dead);
>  	if (group_dead)
>  		kill_orphaned_pgrp(tsk->group_leader, NULL);

How about a different fix below? It avoids additional write_lock/unlock(tasklist),
and another list_for_each_entry_safe(dead) loop is called only if it is actually
needed.

Or I missed something?

Oleg.


--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -558,12 +558,14 @@ static struct task_struct *find_alive_th
 	return NULL;
 }
 
-static struct task_struct *find_child_reaper(struct task_struct *father)
+static struct task_struct *find_child_reaper(struct task_struct *father,
+						struct list_head *dead)
 	__releases(&tasklist_lock)
 	__acquires(&tasklist_lock)
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(father);
 	struct task_struct *reaper = pid_ns->child_reaper;
+	struct task_struct *p, *n;
 
 	if (likely(reaper != father))
 		return reaper;
@@ -579,6 +581,11 @@ static struct task_struct *find_child_re
 		panic("Attempted to kill init! exitcode=0x%08x\n",
 			father->signal->group_exit_code ?: father->exit_code);
 	}
+
+	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
+		list_del_init(&p->ptrace_entry);
+		release_task(p);
+	}
 	zap_pid_ns_processes(pid_ns);
 	write_lock_irq(&tasklist_lock);
 
@@ -668,7 +675,7 @@ static void forget_original_parent(struc
 		exit_ptrace(father, dead);
 
 	/* Can drop and reacquire tasklist_lock */
-	reaper = find_child_reaper(father);
+	reaper = find_child_reaper(father, dead);
 	if (list_empty(&father->children))
 		return;