From patchwork Tue Feb 14 19:20:48 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: John Moon X-Patchwork-Id: 13140753 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 A1BCBC61DA4 for ; Tue, 14 Feb 2023 19:21:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229510AbjBNTVq (ORCPT ); Tue, 14 Feb 2023 14:21:46 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36150 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229460AbjBNTVq (ORCPT ); Tue, 14 Feb 2023 14:21:46 -0500 Received: from mx0b-0031df01.pphosted.com (mx0b-0031df01.pphosted.com [205.220.180.131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5F8D1105; Tue, 14 Feb 2023 11:21:44 -0800 (PST) Received: from pps.filterd (m0279873.ppops.net [127.0.0.1]) by mx0a-0031df01.pphosted.com (8.17.1.19/8.17.1.19) with ESMTP id 31E7hMd5031905; Tue, 14 Feb 2023 19:21:33 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; h=from : to : cc : subject : date : message-id : mime-version : content-type; s=qcppdkim1; bh=rVFs+DSj0sFGO78OF5BmcVkZIkrjvvBZ8DIKrGxxl0E=; b=R0ouuPFEO7u8Z7fqIgfHQ6JIDeIRP0c6Pj4IsC/PN9q7utFDPH5IaeUfsu/L0QH5KfgS CtekIDoPWCmKDWAZcnr2vLkhIo2TiXsQ1ze+mkYLRPeUFo/bvc+bQyuFiWPyn3B26GZs 6APBO2iUpl8lBYYz9HJ/hZrjRDXuY1IgKwKS6GBTU5Xt6ueCxE1iMBYHDK9QKoiomGVw jkpszzbMzJsRMy6UFQEwwNImFJD257R4xmiKKMBK6EhpW1Ddf0svu9FKuFS5WcWXW2b/ K+kTh9Gi1bwCqdC1LZlDVWijCuJCprIvgY8EaVlxh+Lvwj1eqOWThfxOfoTYOy+65Dsl PA== Received: from nalasppmta05.qualcomm.com (Global_NAT1.qualcomm.com [129.46.96.20]) by mx0a-0031df01.pphosted.com (PPS) with ESMTPS id 3nr4kpa02k-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 14 Feb 2023 19:21:33 +0000 Received: from nalasex01c.na.qualcomm.com (nalasex01c.na.qualcomm.com [10.47.97.35]) by NALASPPMTA05.qualcomm.com (8.17.1.5/8.17.1.5) with ESMTPS id 31EJLVO1018148 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 14 Feb 2023 19:21:31 GMT Received: from hu-johmoo-lv.qualcomm.com (10.49.16.6) by nalasex01c.na.qualcomm.com (10.47.97.35) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.36; Tue, 14 Feb 2023 11:21:31 -0800 From: John Moon To: Masahiro Yamada , Nathan Chancellor , Nick Desaulniers , "Nicolas Schier" CC: John Moon , , , , Trilok Soni , Bjorn Andersson , Todd Kjos , Matthias Maennich , Giuliano Procida , Jordan Crouse , Subject: [PATCH 0/1] Validating UAPI backwards compatibility Date: Tue, 14 Feb 2023 11:20:48 -0800 Message-ID: <20230214192049.14726-1-quic_johmoo@quicinc.com> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 X-Originating-IP: [10.49.16.6] X-ClientProxiedBy: nalasex01c.na.qualcomm.com (10.47.97.35) To nalasex01c.na.qualcomm.com (10.47.97.35) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-GUID: gvfb0zRubit0eBKj7q2WM4Vn0ctizjwJ X-Proofpoint-ORIG-GUID: gvfb0zRubit0eBKj7q2WM4Vn0ctizjwJ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.219,Aquarius:18.0.930,Hydra:6.0.562,FMLib:17.11.170.22 definitions=2023-02-14_13,2023-02-14_01,2023-02-09_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 suspectscore=0 spamscore=0 priorityscore=1501 phishscore=0 bulkscore=0 clxscore=1011 impostorscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2212070000 definitions=main-2302140168 Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org Hi all, The kernel community has rigorously enforced a policy of backwards compatibility in its UAPI headers for a long time. This has allowed user applications to enjoy stability across kernel upgrades without recompiling. In the vendor driver community (out-of-tree modules), there's been a lack of discipline when it comes to maintaining UAPI backwards compatibility. This has been a maintenance burden and limits our options for long-term support of older devices. Our goal is to add tooling for vendor driver developers because the upstream model of expert maintainer code review can be difficult to replicate in-house. Tools may help developers catch simple UAPI incompatibilities that could be easily overlooked by in-house review. We see in the kernel documentation: "Kernel headers are backwards compatible, but not forwards compatible. This means that a program built against a C library using older kernel headers should run on a newer kernel (although it may not have access to new features), but a program built against newer kernel headers may not work on an older kernel."[1] How does the kernel enforce this guarantee? We would be interested to learn about any tools or methods used by kernel developers to make sure the above statement remains true. Could the documentation on UAPI maintenance (from a developer's point of view) be expanded? Internally, we have a set of guidelines for our kernel developers regarding UAPI compatibility techniques. If there's interest in supplying a document on this topic with the kernel, we'd be happy to submit a draft detailing what we have so far as a jumping off point. Additionally, I've attached a shell script we've been using internally to validate changes to our UAPI headers are backwards compatible. The script uses libabigail's[2] tool abidiff[3] to compare a modified header's ABI before and after a patch is applied. If an existing UAPI is modified, the script exits non-zero. We use this script in our CI system to block changes that fail the check. Currently, the script works with gcc. It generates output like this when a backwards-incompatible change is made to a UAPI header: !!! ABI differences detected in include/uapi/linux/acct.h (compared to file at HEAD^1) !!! [C] 'struct acct' changed: type size changed from 512 to 544 (in bits) 1 data member insertion: '__u32 new_val', at offset 512 (in bits) at acct.h:71:1 0/1 UAPI header file changes are backwards compatible UAPI header ABI check failed However, we have not had success with clang. It seems clang is more aggressive in optimizing dead code away (no matter which options we pass). Therefore, no ABI differences are found. We wanted to share with the community to receive feedback and any advice when it comes to tooling/policy surrounding this issue. Our hope is that the script will help all kernel UAPI authors (even those that haven't upstreamed yet) maintain good discipline and avoid breaking userspace. [1] Documentation/kbuild/headers_install.rst [2] https://sourceware.org/libabigail/manual/libabigail-overview.html [3] https://sourceware.org/libabigail/manual/abidiff.html P.S. While at Qualcomm, Jordan Crouse authored the original version of the UAPI checker script. Thanks Jordan! John Moon (1): check-uapi: Add UAPI check script scripts/check-uapi.sh | 240 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 240 insertions(+) create mode 100755 scripts/check-uapi.sh base-commit: 0983f6bf2bfc0789b51ddf7315f644ff4da50acb --- 2.17.1