linux-kernel - Re: [PATCH 06/26] rlimit: Remove unnecessary grab of tasklist

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Date:   Wed, 07 Jun 2017 09:08:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Roland McGrath <roland@...k.frob.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 06/26] rlimit: Remove unnecessary grab of tasklist_lock

Oleg Nesterov <oleg@...hat.com> writes:

> Hi Eric,
>
> I'll try very much to read this series tomorrow, can't do this today...
>
> On 06/06, Eric W. Biederman wrote:
>>
>> @@ -1380,13 +1380,6 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
>>  			return -EPERM;
>>  	}
>>  
>> -	/* protect tsk->signal and tsk->sighand from disappearing */
>> -	read_lock(&tasklist_lock);
>> -	if (!tsk->sighand) {
>> -		retval = -ESRCH;
>> -		goto out;
>> -	}
>
> Yes, the comment is wrong.
>
> However we do need read_lock(tasklist_lock) to access ->group_leader. And the
> ->sighand != NULL check ensures that ->group_leader is the valid
> pointer.

As of 4.12-rc1 The code does not access group_leader anymore.

> Also, update_rlimit_cpu() is not safe without tasklist / sighand-check.
>
> We can probably change this code to rely on rcu.

Good point a NULL sighand will cause update_rlimit_cpu to OOPS.

Grr.  There is a point in my tree where this is perfectly safe.  But not
at this point.  Consider this patch dropped for the moment.

Eric