linux-kernel - Re: [PATCH] cred: Use RCU primitives to access RCU pointers

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Message-ID: <20200128114818.GA17943@redhat.com>
Date:   Tue, 28 Jan 2020 12:48:18 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Amol Grover <frextrite@...il.com>,
        David Howells <dhowells@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] cred: Use RCU primitives to access RCU pointers

On 01/28, Jann Horn wrote:
>
> On Tue, Jan 28, 2020 at 8:28 AM Amol Grover <frextrite@...il.com> wrote:
> > task_struct.cred and task_struct.real_cred are annotated by __rcu,
>
> task_struct.cred doesn't actually have RCU semantics though, see
> commit d7852fbd0f0423937fa287a598bfde188bb68c22. For task_struct.cred,
> it would probably be more correct to remove the __rcu annotation?

Yes, but get_task_cred() assumes it has RCU semantics... To be honest
I didn't know we have this helper. Can't it race with revert_creds() in
do_faccessat() ?

Oleg.