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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 49755C00140 for ; Wed, 24 Aug 2022 16:31:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239642AbiHXQb2 (ORCPT ); Wed, 24 Aug 2022 12:31:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53832 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239934AbiHXQbQ (ORCPT ); Wed, 24 Aug 2022 12:31:16 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E901992F77 for ; Wed, 24 Aug 2022 09:31:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1661358674; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ccVMBc1bD/9NWDqLeOT+0fK/B1IszQ6Qg39orye31v0=; b=dgvABez0p3XYkLA9PjnCa+4TgrOQBxukLB5JFvYYsXC9gVyD9VPz3JKBks2clRo6l2EoJP hZCyMawYf3pyY3fvC8B+Au2p47IC0fBzpGcP2T/St3j7OttU2LxuxR+UWPoKSwU6LXgg46 N3ingq8aTFueAwxViD0sVxMBQ6fv4Tg= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-155-4RXzuCA-Oau5cEr7hNCxNQ-1; 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 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 ---------------------- -- 2.37.1