linux-kernel - Re: [PATCH] pid: Add a judgment for ns null in pid_nr

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Message-ID: <20250805124300.GB27131@redhat.com>
Date: Tue, 5 Aug 2025 14:43:01 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: 高翔 <gaoxiang17@...omi.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Xiang Gao <gxxa03070307@...il.com>,
	"mjguzik@...il.com" <mjguzik@...il.com>,
	"Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>,
	"joel.granados@...nel.org" <joel.granados@...nel.org>,
	"lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pid: Add a judgment for ns null in pid_nr_ns

On 08/04, Oleg Nesterov wrote:
>
> But. I need to re-check, but I just realized that pid_alive() can't
> really help, tsk->thread_pid is not stable even if tsk == current.
>
> This means that, say, task_ppid_nr_ns() is buggy.

After the quick grep I don't see the problematic users, but if a zombie
task T does task_ppid_nr_ns(current, NULL) the kernel can crash:

	- pid_alive() succeeds, the task is not reaped yet
	
	- the parent/debugger does wait()->release_task(T), T->thread_pid
	  is NULL now

	- T calls task_tgid_nr_ns()-> ... pid_nr_ns(ns => NULL) because
	  task_active_pid_ns(T) returns NULL

Do you think this worth fixing?


In any case, I think that task_state(), sched_show_task(), bacct_add_tsk()
should be changed to use task_ppid_nr_ns(tsk).

Oleg.