From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F03C6C2D0B1 for ; Thu, 6 Feb 2020 11:29:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B9B3720730 for ; Thu, 6 Feb 2020 11:29:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="QrFQwEl7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727606AbgBFL3K (ORCPT ); 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-Google-Smtp-Source: APXvYqzlXUwReQiQ7QaDb2/khWr0uJbvXUqawl258pPAvjPXGpxxs29kYGBokCGXHPHAd2kkH1Do9TH/W4iGNWCQt58= 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. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 559FFC352A2 for ; Thu, 6 Feb 2020 11:29:13 +0000 (UTC) Received: from whitealder.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1BCE320730 for ; Thu, 6 Feb 2020 11:29:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=google.com header.i=@google.com header.b="QrFQwEl7" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1BCE320730 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lists.linuxfoundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linux-kernel-mentees-bounces@lists.linuxfoundation.org Received: from localhost (localhost [127.0.0.1]) by whitealder.osuosl.org (Postfix) with ESMTP id CA8B98160B; Thu, 6 Feb 2020 11:29:12 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from whitealder.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id CTvHPFO6aq2a; Thu, 6 Feb 2020 11:29:12 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by whitealder.osuosl.org (Postfix) with ESMTP id 1C6F98160A; Thu, 6 Feb 2020 11:29:12 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 0094CC1D81; Thu, 6 Feb 2020 11:29:12 +0000 (UTC) Received: from fraxinus.osuosl.org (smtp4.osuosl.org [140.211.166.137]) by lists.linuxfoundation.org (Postfix) with ESMTP id 63F81C013E for ; Thu, 6 Feb 2020 11:29:10 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by fraxinus.osuosl.org (Postfix) with ESMTP id 5AB9985F34 for ; Thu, 6 Feb 2020 11:29:10 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from fraxinus.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id mRoLXKwI3ysf for ; Thu, 6 Feb 2020 11:29:09 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mail-oi1-f193.google.com (mail-oi1-f193.google.com [209.85.167.193]) by fraxinus.osuosl.org (Postfix) with ESMTPS id 87BC985ECE for ; Thu, 6 Feb 2020 11:29:09 +0000 (UTC) Received: by mail-oi1-f193.google.com with SMTP id c16so4246077oic.3 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=FAHZCLjcgwyBczi30m33Gj7lPuyz50tz1NZSON63uhvCldzPhvkjm9Kouw9A8jzhry +/wg2a6onJcObzrQqIU7gtwIruk/x5FJBjP8Y/3jHOU++Eq22pRJL/3ZxDbC1uO+gcok 70sZOle+CA0Zdq6zPe2BRkVE3aJRi3w2bdw2vlxHPfv+w2B5B9oywfxpj/dQclanKWgK xl08BVi4kAHafF5y4pjSpuk6RerZ3kxiC8iKoQuBuciWuVxLYYa2dMfAjOmgwEpJTnZF ppWndsfoWgiScs1ByVwxieNcwSQwBjfogZwbXWqWwks8NTgJgusy7luRcLkkM68KHsVD 2pIQ== X-Gm-Message-State: APjAAAWZRGd5ggbFESC1uDElkbHurRN2XvzX3Z+tKyU8xLpdnvCnyFGJ ThYjYb8DvF7Xq1JSTykocZBR5/lN8Kj2G6NYw4QEFQ== X-Google-Smtp-Source: APXvYqzlXUwReQiQ7QaDb2/khWr0uJbvXUqawl258pPAvjPXGpxxs29kYGBokCGXHPHAd2kkH1Do9TH/W4iGNWCQt58= 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> Date: Thu, 6 Feb 2020 12:28:42 +0100 Message-ID: To: Joel Fernandes Cc: Kees Cook , "Paul E . McKenney" , Oleg Nesterov , James Morris , kernel list , David Howells , Shakeel Butt , Thomas Gleixner , linux-kernel-mentees@lists.linuxfoundation.org Subject: Re: [Linux-kernel-mentees] [PATCH] cred: Use RCU primitives to access RCU pointers X-BeenThere: linux-kernel-mentees@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Jann Horn via Linux-kernel-mentees Reply-To: Jann Horn Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-kernel-mentees-bounces@lists.linuxfoundation.org Sender: "Linux-kernel-mentees" 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. _______________________________________________ Linux-kernel-mentees mailing list Linux-kernel-mentees@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees