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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id CD8EAC19F2D for ; Tue, 9 Aug 2022 18:45:44 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 2BBB58E0003; Tue, 9 Aug 2022 14:45:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 26B2A8E0001; Tue, 9 Aug 2022 14:45:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 10D8B8E0003; Tue, 9 Aug 2022 14:45:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id F25678E0001 for ; Tue, 9 Aug 2022 14:45:43 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id B2A3D80450 for ; Tue, 9 Aug 2022 18:45:43 +0000 (UTC) X-FDA: 79780932966.02.D84D774 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf14.hostedemail.com (Postfix) with ESMTP id C7D881000A1 for ; Tue, 9 Aug 2022 18:45:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1660070737; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8IbhLYjobqT2aTBvz3U46stbRUAh6ZahUb2ByDCNf0A=; b=TNum+KWLYec3Di+h32/Isu0qH5SxlmanT4GbdNIxh3jmBA884JDTCtS5w9vjNYDwMHMmMI uoO28ARsiR+cLEhQcRJi/M6dfkWixFbMQPwPEOxGstNyrd4sl3+uBS8BqdAvdjKBkr6up2 isY4LCfCaxaIvJuUKfg0dbRd9g0HP+M= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-599-zKLXJuGdN-GecVuxgUrxvQ-1; Tue, 09 Aug 2022 14:45:36 -0400 X-MC-Unique: zKLXJuGdN-GecVuxgUrxvQ-1 Received: by mail-wm1-f70.google.com with SMTP id h62-20020a1c2141000000b003a4f57eaeaaso1124113wmh.8 for ; Tue, 09 Aug 2022 11:45:36 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:in-reply-to:subject:organization:from :references:cc:to:content-language:user-agent:mime-version:date :message-id:x-gm-message-state:from:to:cc; bh=8IbhLYjobqT2aTBvz3U46stbRUAh6ZahUb2ByDCNf0A=; b=bLyxU23fsyrf//eWQSlS9XV7A0iXa1YnN1YGKVJUplPH3khwlSDkZHgZ3jyC9u6VKD Cke1jEs8xreKRkUOWO4I4nMv8e2n3Nf/Z1qrjsHkseCWQqn+zDkbXPCo17/JA/jn8bux TiHYT/qO2r8L89Y6dodDo/d0TRS/H5zH7eG1tZbuI8tR+wOsa+oNnHqO1EXH8YIK8DUC D6oUxUlNj10CWGo6q3JZsV/urOhZqDK2OhHDAOROkSyzwAn6Lzxhn9aiuu2Z+GGkLmZ4 pvvW0ti7R3E9V5WXNdWC6tAk+rDBR2bcsahls0rNF7QRRqt6mBEPu/wTPBYkurXIzI5N sdrw== X-Gm-Message-State: ACgBeo3XE7OElbIYiZ2L+alyLe9jZJT+/S8rjxEdHS1WtD0pG27yeYIE KeTOFaMz2+cPcxhTrFTOVIxSPzPuK8g4p34YCzJYMIY+HHA06FXfEFk78Ic56b+Uv2HPYErCpKe 8lKxgEzWh3Tc= X-Received: by 2002:a05:6000:1b88:b0:21f:eae8:d695 with SMTP id r8-20020a0560001b8800b0021feae8d695mr15071132wru.26.1660070735052; Tue, 09 Aug 2022 11:45:35 -0700 (PDT) X-Google-Smtp-Source: AA6agR69MLCunWf5SVWzymZ3UNdd8BL3fDtH5qkRFmVvHOW7S1grl/vbJS0Sb54Gqln+3VynlhGkbA== X-Received: by 2002:a05:6000:1b88:b0:21f:eae8:d695 with SMTP id r8-20020a0560001b8800b0021feae8d695mr15071118wru.26.1660070734738; Tue, 09 Aug 2022 11:45:34 -0700 (PDT) Received: from ?IPV6:2003:cb:c705:3700:aed2:a0f8:c270:7f30? (p200300cbc7053700aed2a0f8c2707f30.dip0.t-ipconnect.de. [2003:cb:c705:3700:aed2:a0f8:c270:7f30]) by smtp.gmail.com with ESMTPSA id i2-20020a05600c14c200b003a53ec3d847sm7225855wmh.29.2022.08.09.11.45.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Aug 2022 11:45:34 -0700 (PDT) Message-ID: <1a48d71d-41ee-bf39-80d2-0102f4fe9ccb@redhat.com> Date: Tue, 9 Aug 2022 20:45:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 To: Linus Torvalds Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, Andrew Morton , Greg Kroah-Hartman , Axel Rasmussen , Peter Xu , Hugh Dickins , Andrea Arcangeli , Matthew Wilcox , Vlastimil Babka , John Hubbard , Jason Gunthorpe References: <20220808073232.8808-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660070743; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=8IbhLYjobqT2aTBvz3U46stbRUAh6ZahUb2ByDCNf0A=; b=7neB1e286QrTTYQQG4A61h3vw7vV7SMJaru/gxCm8CiGErrmfqhp5591u2o/HGRkVQzIEp Ncq8qNU43rWgaRH/yzS+4I3BF0C65eFAkw2hmcbffJ3FPPTOcyuCN7xQCPQ541xJKFvhKZ t+YMVFW9wXzuz21FzZGQODVpmjNryhg= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=TNum+KWL; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf14.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660070743; a=rsa-sha256; cv=none; b=xYW+qXu3zItFrtOzFNaA0xFlAeQ+GtMDttcQzA49AtJXFD9J+fO4gjiw8IvNnP8YkpV12K rOP3eyWis/HdW0xSTLDcXjEpSAhxu1f0CdFtrSLsIppG6l2DpDsJDExb7MRJshrHW0gM9A RsO/RU9ASZQtpPxC8C7t0IvJP4U3le4= X-Rspamd-Queue-Id: C7D881000A1 Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=TNum+KWL; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf14.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: kbzmaebgddkb7gjxeu95yrx9u1dxtgw9 X-HE-Tag: 1660070737-571175 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 09.08.22 20:27, Linus Torvalds wrote: > I'm still reading through this, but > > STOP DOING THIS > > On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand wrote: >> >> + VM_BUG_ON(!is_cow_mapping(vma->vm_flags)); > > Using BUG_ON() for debugging is simply not ok. > > And saying "but it's just a VM_BUG_ON()" does not change *anything*. > At least Fedora enables that unconditionally for normal people, it is > not some kind of "only VM people do this". > > Really. BUG_ON() IS NOT FOR DEBUGGING. I totally agree with BUG_ON ... but if I get talked to in all-caps on a Thursday evening and feel like I just touched the forbidden fruit, I have to ask for details about VM_BUG_ON nowadays. VM_BUG_ON is only active with CONFIG_DEBUG_VM. ... which indicated some kind of debugging at least to me. I *know* that Fedora enables it and I *know* that this will make Fedora crash. I know why Fedora enables this debug option, but it somewhat destorys the whole purpose of VM_BUG_ON kind of nowadays? For this case, this condition will never trigger and I consider it much more a hint to the reader that we can rest assured that this condition holds. And on production systems, it will get optimized out. Should we forbid any new usage of VM_BUG_ON just like we mostly do with BUG_ON? > > Stop it. Now. > > If you have a condition that must not happen, you either write that > condition into the code, or - if you are convinced it cannot happen - > you make it a WARN_ON_ONCE() so that people can report it to you. I can just turn that into a WARN_ON_ONCE() or even a VM_WARN_ON_ONCE(). -- Thanks, David / dhildenb