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Wed, 2 Sep 2020 05:43:04 +0000 Subject: Re: [PATCH bpf-next 1/2] bpf: fix a verifier failure with xor To: John Fastabend , Andrii Nakryiko CC: bpf , Alexei Starovoitov , Daniel Borkmann , Kernel Team References: <20200825064608.2017878-1-yhs@fb.com> <20200825064608.2017937-1-yhs@fb.com> <465da51a-793e-5ea0-85dc-56ab4f36ae34@fb.com> <5f4f2d24485ac_5f398208c@john-XPS-13-9370.notmuch> From: Yonghong Song Message-ID: Date: Tue, 1 Sep 2020 22:43:03 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 In-Reply-To: <5f4f2d24485ac_5f398208c@john-XPS-13-9370.notmuch> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR11CA0085.namprd11.prod.outlook.com (2603:10b6:a03:f4::26) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from 255.255.255.255 (255.255.255.255) by BYAPR11CA0085.namprd11.prod.outlook.com (2603:10b6:a03:f4::26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3326.19 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: c08981a5-8818-4870-646f-08d84f030fc0 X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB4088.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Sep 2020 05:43:04.2267 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: RUbpBuv3nhlXuLxmBaGKg5x7VRFxT4iARTxm2LUEotGX9WjiLQl16cmVFz/+nNF9 X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB3510 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-09-02_03:2020-09-01,2020-09-02 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 lowpriorityscore=0 mlxscore=0 suspectscore=0 bulkscore=0 priorityscore=1501 mlxlogscore=999 phishscore=0 adultscore=0 spamscore=0 impostorscore=0 malwarescore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009020053 X-FB-Internal: deliver Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 9/1/20 10:27 PM, John Fastabend wrote: > Yonghong Song wrote: >> >> >> On 9/1/20 1:07 PM, Andrii Nakryiko wrote: >>> On Mon, Aug 24, 2020 at 11:47 PM Yonghong Song wrote: >>>> >>>> bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12. >>>> Compared to llvm 10, llvm 11 and 12 generates xor instruction which >>> >>> Does this mean that some perfectly working BPF programs will now fail >>> to verify on older kernels, if compiled with llvm 11 or llvm 12? If >> >> Right. >> >>> yes, is there something that one can do to prevent Clang from using >>> xor in such situations? >> >> The xor is generated by the combination of llvm simplifyCFG and >> instrCombine phase. > > Another option would be to move it out of the isAsCheapAsAMove on the John, do you mean the following change? diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td b/llvm/lib/Target/BPF/BPFInstrInfo.td index 4298e2eaec04..7448a2499d40 100644 --- a/llvm/lib/Target/BPF/BPFInstrInfo.td +++ b/llvm/lib/Target/BPF/BPFInstrInfo.td @@ -293,9 +293,9 @@ let isAsCheapAsAMove = 1 in { defm AND : ALU>=", srl>; - defm XOR : ALU>=", sra>; } + defm XOR : ALU; } Tried the above change with latest trunk. xor still generated :-( I did not trace down to exact llvm optimization location for this particular optimization instance. > llvm side. But, probably better to force the workaround until kernels > get support. Even with it being more expensive it wouldn't mean we never > get it so likely not a great idea. Just thought it might be worth > mentioning. If you have your own llvm and don't have these kernels yet > it looks like a win. > >> >> The following is a hack to prevent compiler from generating xor's.