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=-8.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 45677C433E0 for ; Tue, 16 Feb 2021 19:56:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ECDE064E79 for ; Tue, 16 Feb 2021 19:56:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230006AbhBPT41 (ORCPT ); Tue, 16 Feb 2021 14:56:27 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:19776 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229811AbhBPT41 (ORCPT ); Tue, 16 Feb 2021 14:56:27 -0500 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 11GJW3Ds165880; Tue, 16 Feb 2021 14:55:33 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=message-id : subject : from : to : cc : date : in-reply-to : references : content-type : mime-version : content-transfer-encoding; s=pp1; bh=FJasGMz6yQ2mJBAMr8M5KHdBzgAua7aGkxTsa/2cCog=; b=N61vDsbTWf3iUTNkdkO1Qz4O1f0fN1rvkD+C2PGVLqsxddBf4Lsz1p4ygjIZQrAB/Nyn /+PlQgBqfX18eWiRWbh+Ajgb/DrhS8GbkAXW3Hfv7fF7MGPqcXhz0ZNxV4UE90qgMaY6 vQA/i/DDHDrYWnV0fX6bE5qrhrycT/ultjzm5u7WsWZNZk6/zZoi/ASgF5fxozIvHtKW NAPmPAi7+logn/lcxjkd/VHVRFt8jkX7XVWvECpEpib43/g6c5WSGxbnx1VhH03Poj99 Rzj9oG0VNPBkpsxFD6AaPfpAYFJId8SQQdfXS7fWR1q0dJaYDlHV7+DxCmm1dEbOdYfr KQ== Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 36rm0ph36d-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 16 Feb 2021 14:55:33 -0500 Received: from m0098404.ppops.net (m0098404.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.36/8.16.0.36) with SMTP id 11GJYWsb182787; Tue, 16 Feb 2021 14:55:32 -0500 Received: from ppma04ams.nl.ibm.com (63.31.33a9.ip4.static.sl-reverse.com [169.51.49.99]) by mx0a-001b2d01.pphosted.com with ESMTP id 36rm0ph35p-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 16 Feb 2021 14:55:32 -0500 Received: from pps.filterd (ppma04ams.nl.ibm.com [127.0.0.1]) by ppma04ams.nl.ibm.com (8.16.0.42/8.16.0.42) with SMTP id 11GJleDE003317; Tue, 16 Feb 2021 19:55:29 GMT Received: from b06cxnps3075.portsmouth.uk.ibm.com (d06relay10.portsmouth.uk.ibm.com [9.149.109.195]) by ppma04ams.nl.ibm.com with ESMTP id 36p6d8b27s-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 16 Feb 2021 19:55:29 +0000 Received: from d06av21.portsmouth.uk.ibm.com (d06av21.portsmouth.uk.ibm.com [9.149.105.232]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 11GJtRVe48693514 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 16 Feb 2021 19:55:27 GMT Received: from d06av21.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7647B52052; Tue, 16 Feb 2021 19:55:27 +0000 (GMT) Received: from [9.171.64.123] (unknown [9.171.64.123]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id 142CB5204E; Tue, 16 Feb 2021 19:55:27 +0000 (GMT) Message-ID: <7bcfe4bfd5a2c4768fb07908b709e10ec089903b.camel@linux.ibm.com> Subject: Re: [PATCH v2 bpf-next] bpf: Explicitly zero-extend R0 after 32-bit cmpxchg From: Ilya Leoshkevich To: Brendan Jackman , bpf@vger.kernel.org Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , KP Singh , Florent Revest Date: Tue, 16 Feb 2021 20:55:26 +0100 In-Reply-To: <20210216141925.1549405-1-jackmanb@google.com> References: <20210216141925.1549405-1-jackmanb@google.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.38.3 (3.38.3-1.fc33) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-02-16_09:2021-02-16,2021-02-16 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 mlxlogscore=999 suspectscore=0 phishscore=0 priorityscore=1501 impostorscore=0 malwarescore=0 bulkscore=0 adultscore=0 spamscore=0 lowpriorityscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2102160158 Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, 2021-02-16 at 14:19 +0000, Brendan Jackman wrote: > As pointed out by Ilya and explained in the new comment, there's a > discrepancy between x86 and BPF CMPXCHG semantics: BPF always loads > the value from memory into r0, while x86 only does so when r0 and the > value in memory are different. The same issue affects s390. > > At first this might sound like pure semantics, but it makes a real > difference when the comparison is 32-bit, since the load will > zero-extend r0/rax. > > The fix is to explicitly zero-extend rax after doing such a > CMPXCHG. Since this problem affects multiple archs, this is done in > the verifier by patching in a BPF_ZEXT_REG instruction after every > 32-bit cmpxchg. Any archs that don't need such manual zero-extension > can do a look-ahead with insn_is_zext to skip the unnecessary mov. > > Reported-by: Ilya Leoshkevich > Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg") > Signed-off-by: Brendan Jackman > --- > > Difference from v1[1]: Now solved centrally in the verifier instead > of >   specifically for the x86 JIT. Thanks to Ilya and Daniel for the > suggestions! > > [1] > https://lore.kernel.org/bpf/d7ebaefb-bfd6-a441-3ff2-2fdfe699b1d2@iogearbox.net/T/#t > >  kernel/bpf/verifier.c                         | 36 > +++++++++++++++++++ >  .../selftests/bpf/verifier/atomic_cmpxchg.c   | 25 +++++++++++++ >  .../selftests/bpf/verifier/atomic_or.c        | 26 ++++++++++++++ >  3 files changed, 87 insertions(+) I tried this with my s390 atomics patch, and it's working, thanks! I was thinking whether this could go through the existing zext_dst flag infrastructure, but it probably won't play too nicely with the x86_64 JIT, which doesn't override bpf_jit_needs_zext(). Acked-by: Ilya Leoshkevich Tested-by: Ilya Leoshkevich [...]