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Wed, 24 Aug 2022 12:31:11 -0400 X-MC-Unique: 4RXzuCA-Oau5cEr7hNCxNQ-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5EDFF18A6580; Wed, 24 Aug 2022 16:31:10 +0000 (UTC) Received: from t480s.redhat.com (unknown [10.39.193.5]) by smtp.corp.redhat.com (Postfix) with ESMTP id 76918403348; Wed, 24 Aug 2022 16:31:06 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, kexec@lists.infradead.org, David Hildenbrand , Linus Torvalds , Andrew Morton , Ingo Molnar , David Laight , Jonathan Corbet , Andy Whitcroft , Joe Perches , Dwaipayan Ray , Lukas Bulwahn , Baoquan He , Vivek Goyal , Dave Young Subject: [PATCH RFC 1/2] coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel") Date: Wed, 24 Aug 2022 18:30:59 +0200 Message-Id: <20220824163100.224449-2-david@redhat.com> In-Reply-To: <20220824163100.224449-1-david@redhat.com> References: <20220824163100.224449-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 ARC-Seal: i=1; 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dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=dgvABez0; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf21.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com X-Stat-Signature: f9p5zfrkaipmqycqpxdeo4krgu8bi1xk X-Rspamd-Server: rspam05 X-HE-Tag: 1661358674-399945 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: Linus notes [1] that the introduction of new code that uses VM_BUG_ON() is just as bad as BUG_ON(), because it will crash the kernel on distributions that enable CONFIG_DEBUG_VM (like Fedora): VM_BUG_ON() has the exact same semantics as BUG_ON. It is literally no different, the only difference is "we can make the code smaller because these are less important". [2] This resulted in a more generic discussion about usage of BUG() and friends. While there might be corner cases that still deserve a BUG_ON(), most BUG_ON() cases should simply use WARN_ON_ONCE() and implement a recovery path if reasonable: The only possible case where BUG_ON can validly be used is "I have some fundamental data corruption and cannot possibly return an error". [2] As a very good approximation is the general rule: "absolutely no new BUG_ON() calls _ever_" [2] ... not even if something really shouldn't ever happen and is merely for documenting that an invariant always has to hold. There is only one good BUG_ON(): Now, that said, there is one very valid sub-form of BUG_ON(): BUILD_BUG_ON() is absolutely 100% fine. [2] While WARN will also crash the machine with panic_on_warn set, that's exactly to be expected: So we have two very different cases: the "virtual machine with good logging where a dead machine is fine" - use 'panic_on_warn'. And the actual real hardware with real drivers, running real loads by users. [3] The basic idea is that warnings will similarly get reported by users and be found during testing. However, in contrast to a BUG(), there is a way to actually influence the expected behavior (e.g., panic_on_warn) and to eventually keep the machine alive to extract some debug info. Ingo notes that not all WARN_ON_ONCE cases need recovery. If we don't ever expect this code to trigger in any case, recovery code is not really helpful. I'd prefer to keep all these warnings 'simple' - i.e. no attempted recovery & control flow, unless we ever expect these to trigger. [4] There have been different rules floating around that were never properly documented. Let's try to clarify. [1] https://lkml.kernel.org/r/CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com [2] https://lore.kernel.org/r/CAHk-=wg40EAZofO16Eviaj7mfqDhZ2gVEbvfsMf6gYzspRjYvw@mail.gmail.com [3] https://lore.kernel.org/r/CAHk-=wgF7K2gSSpy=m_=K3Nov4zaceUX9puQf1TjkTJLA2XC_g@mail.gmail.com Signed-off-by: David Hildenbrand --- Documentation/process/coding-style.rst | 27 ++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst index 03eb53fd029a..a6d81ff578fe 100644 --- a/Documentation/process/coding-style.rst +++ b/Documentation/process/coding-style.rst @@ -1186,6 +1186,33 @@ expression used. For instance: #endif /* CONFIG_SOMETHING */ +22) Do not crash the kernel +--------------------------- + +Do not add new code that uses BUG(), BUG_ON(), VM_BUG_ON(), ... to crash +the kernel if certain conditions are not met. Instead, use WARN_ON_ONCE() +with recovery code (if reasonable) instead. Unavoidable data corruption / +security issues might be a very rare exception to this rule and need good +justification. + +There is no need for WARN_ON_ONCE() recovery code if we never expect it to +possibly trigger unless something goes seriously wrong or some other code +is changed to break invariants. Note that VM_WARN_ON_ONCE() cannot be used +in conditionals. + +Usage of WARN() and friends is fine for something that is not expected to +be triggered easily. ``panic_on_warn`` users are not an excuse to not use +WARN(): whoever enables ``panic_on_warn`` explicitly asked the kernel to +crash in this case. + +However, WARN() and friends should not be used for something that is +expected to trigger easily, for example, by user space. pr_warn_once() +might be a reasonable replacement to notify the user. + +Note that BUILD_BUG_ON() is perfectly fine because it will make compilation +fail instead of crashing the kernel. + + Appendix I) References ---------------------- From patchwork Wed Aug 24 16:31:00 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 12953802 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 C99A7C00140 for ; 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dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="FHo4aXd/"; spf=pass (imf01.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam01 X-HE-Tag: 1661358680-113586 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: checkpatch does not point out that VM_BUG_ON() and friends should be avoided, however, Linus notes: VM_BUG_ON() has the exact same semantics as BUG_ON. It is literally no different, the only difference is "we can make the code smaller because these are less important". [1] So let's warn on VM_BUG_ON() and friends as well. While at it, make it clearer that the kernel really shouldn't be crashed. Note that there are some other *_BUG_ON flavors, but they are not all bad: for example, KVM_BUG_ON() only triggers a WARN_ON_ONCE and then flags KVM as being buggy, so we'll not care about them for now here. [1] https://lore.kernel.org/r/CAHk-=wg40EAZofO16Eviaj7mfqDhZ2gVEbvfsMf6gYzspRjYvw@mail.gmail.com Signed-off-by: David Hildenbrand --- scripts/checkpatch.pl | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl index 79e759aac543..4c18acf17032 100755 --- a/scripts/checkpatch.pl +++ b/scripts/checkpatch.pl @@ -4695,12 +4695,12 @@ sub process { } } -# avoid BUG() or BUG_ON() - if ($line =~ /\b(?:BUG|BUG_ON)\b/) { +# do not use BUG(), BUG_ON(), VM_BUG_ON() and friends. + if ($line =~ /\b(?:BUG|BUG_ON|VM_BUG_ON|VM_BUG_ON_[A-Z]+)\b/) { my $msg_level = \&WARN; $msg_level = \&CHK if ($file); &{$msg_level}("AVOID_BUG", - "Avoid crashing the kernel - try using WARN_ON & recovery code rather than BUG() or BUG_ON()\n" . $herecurr); + "Do not crash the kernel unless it is unavoidable - use WARN_ON_ONCE & recovery code (if reasonable) rather than BUG(), BUG_ON(), VM_BUG_ON(), ...\n" . $herecurr); } # avoid LINUX_VERSION_CODE