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Fri, 28 May 2021 22:16:16 +0000 To: bpf@vger.kernel.org From: Eugene Loh Subject: using skip>0 with bpf_get_stack() Message-ID: <30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@oracle.com> Date: Fri, 28 May 2021 18:16:15 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [148.87.23.10] X-ClientProxiedBy: SJ0PR13CA0044.namprd13.prod.outlook.com (2603:10b6:a03:2c2::19) To CO1PR10MB4644.namprd10.prod.outlook.com (2603:10b6:303:99::24) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from localhost.localdomain (148.87.23.10) by SJ0PR13CA0044.namprd13.prod.outlook.com (2603:10b6:a03:2c2::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4195.10 via Frontend Transport; Fri, 28 May 2021 22:16:16 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: dcd5c4f5-28c0-4d3f-ef2c-08d922263655 X-MS-TrafficTypeDiagnostic: CO1PR10MB4532: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:8882; 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which means that get_perf_callchain() will return "num_elem" stack frames.  Then, since we skip "skip" frames, we'll fill the user buffer with only "num_elem - skip" frames, the remaining frames being filled zero. For example, let's say the call stack is         leaf <- caller <- foo1 <- foo2 <- foo3 <- foo4 <- foo5 <- foo6 Let's say I pass bpf_get_stack() a buffer with num_elem==4 and ask skip==2.  I would expect to skip 2 frames then get 4 frames, getting back:         foo1  foo2  foo3  foo4 Instead, I get         foo1  foo2  0  0 skipping 2 frames but also leaving frames zeroed out. I think the init_nr computation should be: -       if (sysctl_perf_event_max_stack < num_elem) +       if (sysctl_perf_event_max_stack <= num_elem + skip)                 init_nr = 0;         else -               init_nr = sysctl_perf_event_max_stack - num_elem; +               init_nr = sysctl_perf_event_max_stack - num_elem - skip; Incidentally, the return value of the function is presumably the size of the returned data.  Would it make sense to say so in include/uapi/linux/bpf.h?