Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 21 Jul 2020 21:51:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     David Howells <dhowells@redhat.com>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        David Windsor <dwindsor@gmail.com>,
        Hans Liljestrand <ishkamiel@gmail.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Paul Moore <paul@paul-moore.com>, edumazet@google.com,
        paulmck@kernel.org, shakeelb@google.com,
        James Morris <jamorris@linux.microsoft.com>,
        alex.huangjianhui@huawei.com, dylix.dailei@huawei.com,
        chenzefeng2@huawei.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] Convert nsproxy, groups, and creds to refcount_t
Message-ID: <20200721195132.GJ10769@hirez.programming.kicks-ass.net>
References: <202006142054.C00B3E9C9@keescook>
 <20200612183450.4189588-1-keescook@chromium.org>
 <7be4d56b-0406-099b-e505-02e074c5173e@huawei.com>
 <544539.1595328664@warthog.procyon.org.uk>
 <202007211144.A68C31D@keescook>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <202007211144.A68C31D@keescook>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jul 21, 2020 at 11:44:53AM -0700, Kees Cook wrote:
> On Tue, Jul 21, 2020 at 11:51:04AM +0100, David Howells wrote:
> > Kees Cook <keescook@chromium.org> wrote:
> > 
> > > > Should mm->mm_users also be replaced by refcount_t?
> > > 
> > > I'll say "yes". :)
> > > https://lore.kernel.org/lkml/1487671124-11188-1-git-send-email-elena.reshetova@intel.com/
> > > 
> > > > In addition, is it better to change all variables that use
> > > > atomic_dec_and_test to control the release process to refconut_t?
> > > 
> > > For the most part, yes. The following may find a lot of them:
> > > scripts/coccinelle/api/atomic_as_refcounter.cocci
> > 
> > I've been gradually undoing some of the conversions as there's no equivalent
> > of atomic_add_return() and atomic_dec_return() that allow me to log the
> > altered refcount through a tracepoint.
> 
> Please do not _undo_ the changes; just add the API you need.

add_return and sub_return are horrible interface for refcount, which is
the problem.

If you meant: refcount_dec(), but want the old value for tracing, you
want a different ordering than if you wanted to do
refcount_dec_and_test(); dec_return can't know this.

David, would something like a __refcount_*() API work where there is a
3rd argument (int *), which, if !NULL, will be assigned the old value?