From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S934911AbcKPKLB (ORCPT <rfc822;w@1wt.eu>);
        Wed, 16 Nov 2016 05:11:01 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:60209 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S932249AbcKPKLA (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 16 Nov 2016 05:11:00 -0500
Date: Wed, 16 Nov 2016 11:10:42 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Kees Cook <keescook@chromium.org>, Will Deacon <will.deacon@arm.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        David Windsor <dave@progbits.org>, LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()
Message-ID: <20161116101042.GN3142@twins.programming.kicks-ass.net>
References: <20161114173946.501528675@infradead.org>
 <20161114174446.486581399@infradead.org>
 <20161115073322.GC28248@kroah.com>
 <20161115080314.GD3142@twins.programming.kicks-ass.net>
 <CAGXu5jLNduviFkM1A4ozub_n2uB2nAHrhE41BsPFJZq+uhn4Mg@mail.gmail.com>
 <20161116082151.GA24017@kroah.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161116082151.GA24017@kroah.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Nov 16, 2016 at 09:21:51AM +0100, Greg KH wrote:
> > What should we do about things like this (bpf_prog_put() and callbacks
> > from kernel/bpf/syscall.c):
> > 
> > 
> > static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
> > {
> >         struct user_struct *user = prog->aux->user;
> > 
> >         atomic_long_sub(prog->pages, &user->locked_vm);
> 
> Oh that's scary.  Let's just make one reference count rely on another
> one and not check things...

Its not a reference count, its a resource limit thingy. Also, isn't
stacking, or in general building an object graph, the entire point of
reference counts?

> >         free_uid(user);
> > }
> > 
> > static void __bpf_prog_put_rcu(struct rcu_head *rcu)
> > {
> >         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
> > 
> >         free_used_maps(aux);
> >         bpf_prog_uncharge_memlock(aux->prog);
> >         bpf_prog_free(aux->prog);
> > }
> > 
> > void bpf_prog_put(struct bpf_prog *prog)
> > {
> >         if (atomic_dec_and_test(&prog->aux->refcnt))
> >                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> > }
> > 
> > 
> > Not only do we want to protect prog->aux->refcnt, but I think we want
> > to protect user->locked_vm too ... I don't think it's sane for
> > user->locked_vm to be a stats_t ?
> 
> I don't think this is sane code...

I once again fail to see any problems. That code is fine.