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Wed, 29 Apr 2020 05:09:17 +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> <65a83b48-3965-5ae9-fafd-3e8836b03d2c@fb.com> From: Yonghong Song Message-ID: <7f15274d-46dd-f43f-575e-26a40032f900@fb.com> Date: Tue, 28 Apr 2020 22:09:13 -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: <65a83b48-3965-5ae9-fafd-3e8836b03d2c@fb.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MWHPR20CA0034.namprd20.prod.outlook.com (2603:10b6:300:ed::20) 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:e9ab) by MWHPR20CA0034.namprd20.prod.outlook.com (2603:10b6:300:ed::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2958.19 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 mean always pass valid object pointer into the prog. > In above case 'map' will always be valid. > Consider prog that iterating all map elements. > It's weird that the prog would always need to do > if (map == 0) >   goto out; > even if it doesn't care about finding last. > All progs would have to have such extra 'if'. > If we always pass valid object than there is no need > for such extra checks inside the prog. > First and last element can be indicated via seq_num > or via another flag or via helper call like is_this_last_elem() > or something. Okay, I see what you mean now. Basically this means seq_ops->next() should try to get/maintain next two elements, otherwise, we won't know whether the one in seq_ops->show() is the last or not. We could do it in newly implemented iterator bpf_map/task/task_file. Let me check how I could make existing seq_ops (ipv6_route/netlink) works with minimum changes.