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Wed, 29 Apr 2020 01:15:17 +0000 Subject: Re: [PATCH bpf-next v1 03/19] bpf: add bpf_map iterator To: Alexei Starovoitov , Martin KaFai Lau CC: Andrii Nakryiko , , , Daniel Borkmann , References: <20200427201235.2994549-1-yhs@fb.com> <20200427201237.2994794-1-yhs@fb.com> <20200429003738.pv4flhdaxpg66wiv@kafai-mbp> <3df31c9a-2df7-d76a-5e54-b2cd48692883@fb.com> From: Yonghong Song Message-ID: Date: Tue, 28 Apr 2020 18:15:14 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 In-Reply-To: <3df31c9a-2df7-d76a-5e54-b2cd48692883@fb.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: CO2PR06CA0071.namprd06.prod.outlook.com (2603:10b6:104:3::29) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from macbook-pro-52.local.dhcp.thefacebook.com (2620:10d:c090:400::5:2258) by CO2PR06CA0071.namprd06.prod.outlook.com (2603:10b6:104:3::29) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2937.13 via Frontend Transport; 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>>  From looking at seq_file.c, when will show() be called with "v == NULL"? >> > > that v == NULL here and the whole verifier change just to allow NULL... > may be use seq_num as an indicator of the last elem instead? > Like seq_num with upper bit set to indicate that it's last? We could. But then verifier won't have an easy way to verify that. For example, the above is expected: int prog(struct bpf_map *map, u64 seq_num) { if (seq_num >> 63) return 0; ... map->id ... ... map->user_cnt ... } But if user writes int prog(struct bpf_map *map, u64 seq_num) { ... map->id ... ... map->user_cnt ... } verifier won't be easy to conclude inproper map pointer tracing here and in the above map->id, map->user_cnt will cause exceptions and they will silently get value 0. I do have another potential use case for this ptr_to_btf_id_or_null, e.g., for tcp6, instead of pointer casting, I could have bpf_prog like int prog(..., struct tcp6_sock *tcp_sk, struct timewait_sock *tw_sk, struct request_sock *req_sk) { if (tcp_sk) { /* dump tcp_sk ... */ } if (tw_sk) { /* dump tw_sk ... */ } if (req_sk) { /* dump req_sk ... */ } } The kernel infrastructure will ensure at any time only one of tcp_sk/tw_sk/req_sk is valid and the other two is NULL.