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From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 12 Jun 2020 09:32:39 -0700
Message-ID: <CANn89i+yWQ_rO29NDuXZ7XWho_gTShUpujUN+jZop46W7e2qyw@mail.gmail.com>
Subject: Re: [PATCH RFC] cred: Add WARN to detect wrong use of get/put_cred
To:     Xiaoming Ni <nixiaoming@huawei.com>
Cc:     Paul Moore <paul@paul-moore.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        David Howells <dhowells@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Shakeel Butt <shakeelb@google.com>,
        jamorris@linux.microsoft.com, alex.huangjianhui@huawei.com,
        dylix.dailei@huawei.com, chenzefeng2@huawei.com,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Jun 12, 2020 at 3:28 AM Xiaoming Ni <nixiaoming@huawei.com> wrote:
>
> Cred release and usage check code flow:
>         1. put_cred()
>                 if (atomic_dec_and_test(&(cred)->usage))
>                         __put_cred(cred);
>
>         2. __put_cred()
>                 BUG_ON(atomic_read(&cred->usage) != 0);
>                 call_rcu(&cred->rcu, put_cred_rcu);
>
>         3. put_cred_rcu()
>                 if (atomic_read(&cred->usage) != 0)
>                         panic("CRED: put_cred_rcu() sees %p with usage %d\n",
>                                cred, atomic_read(&cred->usage));
>                 kmem_cache_free(cred_jar, cred);
>
> If panic is triggered on put_cred_rcu(), there are two possibilities
>         1. Call get_cred() after __put_cred(), usage > 0
>         2. Call put_cred() after __put_cred(), usage < 0
> Since put_cred_rcu is an asynchronous behavior, it is no longer the first
> scene when panic, there is no information about the murderer in the panic
> call stack...
>
> So, add WARN() in get_cred()/put_cred(), and pray to catch the murderer
> at the first scene.
>
> Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
> ---


It seems you reinvented refcount_t ?