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[209.132.180.67]) by mx.google.com with ESMTP id w13si1985304oiw.106.2020.02.06.03.47.11; Thu, 06 Feb 2020 03:47:25 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=QrFQwEl7; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727594AbgBFL3K (ORCPT + 99 others); Thu, 6 Feb 2020 06:29:10 -0500 Received: from mail-oi1-f194.google.com ([209.85.167.194]:40672 "EHLO mail-oi1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726538AbgBFL3K (ORCPT ); Thu, 6 Feb 2020 06:29:10 -0500 Received: by mail-oi1-f194.google.com with SMTP id a142so4218972oii.7 for ; Thu, 06 Feb 2020 03:29:09 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=1LOcb8MY6mnGMOkw5o9OHiVJZqqoYe5yVwQN2zYkKLg=; b=QrFQwEl7g9MnWr9VvEcb/sCcX4RY/t3UHs5G2mHmckEcgWv9ZjbrTiqjg4+pY/aPF0 xbDHD74WDaBaQ0XIXaSFrZhM8HGOFuTdhlzhYHjoIze1f2vT0JQLzn4x+SPw9qpukcU1 K09tRWDU6NRRCZQI0k8q3KgFBQXfZSHGqpJniU7ng9+I+1nf/j9EppB0onpSSsn7k3Nt +8EZTW2qF1u6K/k3LkEX5LtVrtqV4AMOIqD0IJB/3V39Nf68AKXh/MiF9aVk6oyoE7pu 4gDTgbzlK2CwQE+/DUfkUSwCjo4HEhypduKjERXUnaA/UG66109Z/rc/h+aWN8mDzVXt Lcmg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=1LOcb8MY6mnGMOkw5o9OHiVJZqqoYe5yVwQN2zYkKLg=; b=SW7+K9/rZy4ekK3V6dtTCN/GSsNnWinQ0SBHeZOgRXCKufC/R9XJnJMObDpEbiKaKQ 5bkviGSzYAw3OFgf1IodOi2FarOHY3m5jqA3ZBxG78bqtif2g+6a5UshjTCocycaowji OUuUJBd+9+cvnpjb0WuB/S5gnzGMj2Mm6sRiOlPq0WAhplZN0KcfCUAOOVYNsIsLv2Pi trTef5r0mmhpe+HlwH3n3j782oqrOQ6VTZxT9JWikrkf9DW9YGg7RJHcCmTRl48M8FO+ G/j5DuRB/NLb2krQWgGINyTOEm/JTbKbbCw2D4YbtEBsXiO87ll47uxwTNApv5cbsROd np7w== X-Gm-Message-State: APjAAAXmjst4siFOiQRVu1dx5uekCrK/CWjBXLu/TFVnI9Ai+x7sKyUM WRCdZLGE6cQELKXwL2RL1tzx1/p6ZHlL5a6FbM7N7Q== X-Received: by 2002:aca:c7cb:: with SMTP id x194mr6575954oif.157.1580988548436; Thu, 06 Feb 2020 03:29:08 -0800 (PST) MIME-Version: 1.0 References: <20200128072740.21272-1-frextrite@gmail.com> <20200128170426.GA10277@workstation-portable> <20200129065738.GA17486@workstation-portable> <20200206013251.GC55522@google.com> In-Reply-To: <20200206013251.GC55522@google.com> From: Jann Horn Date: Thu, 6 Feb 2020 12:28:42 +0100 Message-ID: Subject: Re: [PATCH] cred: Use RCU primitives to access RCU pointers To: Joel Fernandes Cc: Amol Grover , David Howells , Shakeel Butt , James Morris , Oleg Nesterov , Kees Cook , Thomas Gleixner , kernel list , linux-kernel-mentees@lists.linuxfoundation.org, Madhuparna Bhowmik , "Paul E . McKenney" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 6, 2020 at 2:32 AM Joel Fernandes wrote: > On Wed, Jan 29, 2020 at 03:14:56PM +0100, Jann Horn wrote: > > On Wed, Jan 29, 2020 at 7:57 AM Amol Grover wrote: > > > On Tue, Jan 28, 2020 at 08:09:17PM +0100, Jann Horn wrote: > > > > On Tue, Jan 28, 2020 at 6:04 PM Amol Grover wrote: > > > > > On Tue, Jan 28, 2020 at 10:30:19AM +0100, Jann Horn wrote: > > > > > > On Tue, Jan 28, 2020 at 8:28 AM Amol Grover 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? > > > > > > > > > > Hi Jann, > > > > > > > > > > I went through the commit you mentioned. If I understand it correctly, > > > > > ->cred was not being accessed concurrently (via RCU), hence, a non_rcu > > > > > flag was introduced, which determined if the clean-up should wait for > > > > > RCU grace-periods or not. And since, the changes were 'thread local' > > > > > there was no need to wait for an entire RCU GP to elapse. > > > > > > > > Yeah. > > > > > > > > > The commit too, as you said, mentions the removal of __rcu annotation. > > > > > However, simply removing the annotation won't work, as there are quite a > > > > > few instances where RCU primitives are used. Even get_current_cred() > > > > > uses RCU APIs to get a reference to ->cred. > > > > > > > > Luckily, there aren't too many places that directly access ->cred, > > > > since luckily there are helper functions like get_current_cred() that > > > > will do it for you. Grepping through the kernel, I see: > > [...] > > > > So actually, the number of places that already don't use RCU accessors > > > > is much higher than the number of places that use them. > > > > > > > > > So, currently, maybe we > > > > > should continue to use RCU APIs and leave the __rcu annotation in? > > > > > (Until someone who takes it on himself to remove __rcu annotation and > > > > > fix all the instances). Does that sound good? Or do you want me to > > > > > remove __rcu annotation and get the process started? > > > > > > > > I don't think it's a good idea to add more uses of RCU APIs for > > > > ->cred; you shouldn't "fix" warnings by making the code more wrong. > > > > > > > > If you want to fix this, I think it would be relatively easy to fix > > > > this properly - as far as I can tell, there are only seven places that > > > > you'll have to change, although you may have to split it up into three > > > > patches. > > > > > > Thank you for the detailed analysis. I'll try my best and send you a > > > patch. > > Amol, Jann, if I understand the discussion correctly, objects ->cred > point (the subjective creds) are never (or never need to be) RCU-managed. > This makes sense in light of the commit Jann pointed out > (d7852fbd0f0423937fa287a598bfde188bb68c22). > > How about the following diff as a starting point? > > 1. Remove all ->cred accessing happening through RCU primitive. Sounds good. > 2. Remove __rcu from task_struct ->cred Sounds good. > 3. Also I removed the whole non_rcu flag, and introduced a new put_cred_non_rcu() API > which places that task-synchronously use ->cred can overwrite. Callers > doing such accesses like access() can use this API instead. That's wrong, don't do that. ->cred is a reference without RCU semantics, ->real_cred is a reference with RCU semantics. If there have never been any references with RCU semantics to a specific instance of struct cred, then that instance can indeed be freed without an RCU grace period. But it would be possible for some filesystem code to take a reference to current->cred, and assign it to some pointer with RCU semantics somewhere, then drop that reference with put_cred() immediately before you reach put_cred_non_rcu(); with the result that despite using put_cred(), the other side doesn't get RCU semantics. Just leave the whole ->non_rcu thing exactly as it was.