linux-kernel - Re: [PATCH] Convert struct pid count to refcount

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Date:   Thu, 28 Mar 2019 10:39:58 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [PATCH] Convert struct pid count to refcount_t

On Thu, Mar 28, 2019 at 03:26:19PM +0100, Oleg Nesterov wrote:
> On 03/27, Joel Fernandes wrote:
> >
> > Also, based on Kees comment, I think it appears to me that get_pid and
> > put_pid can race in this way in the original code right?
> >
> > get_pid			put_pid
> >
> > 			atomic_dec_and_test returns 1
> > atomic_inc
> > 			kfree
> >
> > deref pid /* boom */
> > -------------------------------------------------
> >
> > I think get_pid needs to call atomic_inc_not_zero()
> 
> No.
> 
> get_pid() should only be used if you already have a reference or you do
> something like
> 
> 	rcu_read_lock();
> 	pid = find_vpid();
> 	get_pid();
> 	rcu_read_lock();
> 
> in this case we rely on call_rcu(delayed_put_pid) which drops the initial
> reference.
> 
> If put_pid() sees pid->count == 1, then a) nobody else has a reference and
> b) nobody else can find this pid on rcu-protected lists, so it is safe to
> free it.

I agree. Check my reply to Jann, I already replied to him about this. thanks!