linux-kernel - Re: [PATCH] kernel/pid.c: check pid whether be NULL in __change

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Date:	Tue, 08 Oct 2013 05:53:56 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/pid.c: check pid whether be NULL in __change_pid()

On 10/07/2013 08:43 PM, Oleg Nesterov wrote:
> On 10/07, Chen Gang wrote:
>>
>> Within __change_pid(), 'new' may be NULL if it comes from detach_pid(),
>> and 'link->pid' also may be NULL ("link->pid = new"), so theoretically,\
>> the original 'link->pid' may be NULL, too.
> 
> I don't really understand this "theoretically",
> 
>> In real world, at least now, all callers which will call detach_pid()
>> or change_pid() will not cause issue,
> 
> Yes,
> 
>> but still recommend to check it
>> in __change_pid() to let itself consistency.
> 
> Why?
> 
> Contrary, I think we should not hide the problem. If __change_pid() is
> called when task->pids[type].pid is already NULL there is something
> seriously wrong.
> 

Hmm... In my opinion, it means need BUG_ON() for original 'link->pid'.

--------------------------------patch begin-----------------------------

[PATCH] kernel/pid.c: add BUG_ON() for "!pid" in __change_pid()

  Within __change_pid(), 'new' may be NULL if it comes from detach_pid(),
  and 'link->pid' also may be NULL ("link->pid = new"), so theoretically,
  the original 'link->pid' may be NULL, too.

  But in real world, all related extern functions always assume "if
  'link->pid' is already NULL, there must be something seriously wrong",
  although __change_pid() can accept parameter 'new' as NULL.

  So in __change_pid(), need add BUG_ON() for it: "it should not happen,
  when it really happen, OS must be continuing blindly, and next will
  cause serious issue".

Signed-off-by: Chen Gang <gang.chen@...anux.com>
---
 kernel/pid.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 9b9a266..8fc87f1 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -396,6 +396,12 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
 	link = &task->pids[type];
 	pid = link->pid;
 
+	/*
+	 * If task->pids[type].pid is already NULL, there must be something
+	 * seriously wrong
+	 */
+	BUG_ON(!pid);
+
 	hlist_del_rcu(&link->node);
 	link->pid = new;
 
-- 
1.7.7.6

--------------------------------patch end-------------------------------

> Oleg.
> 
> 
> 


-- 
Chen Gang
--
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