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Fri, 18 Dec 2020 16:38:43 +0000 Subject: Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter To: Alexei Starovoitov , Song Liu CC: "bpf@vger.kernel.org" , "netdev@vger.kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "john.fastabend@gmail.com" , "kpsingh@chromium.org" , Kernel Team References: <20201215233702.3301881-1-songliubraving@fb.com> <20201215233702.3301881-2-songliubraving@fb.com> <20201217190308.insbsxpf6ujapbs3@ast-mbp> <20201218023444.i6hmdi3bp5vgxou2@ast-mbp> From: Yonghong Song Message-ID: <231d0521-62a7-427b-5351-359092e73dde@fb.com> Date: Fri, 18 Dec 2020 08:38:39 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:8460] X-ClientProxiedBy: MW4PR03CA0331.namprd03.prod.outlook.com (2603:10b6:303:dc::6) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21c1::119f] (2620:10d:c090:400::5:8460) by MW4PR03CA0331.namprd03.prod.outlook.com (2603:10b6:303:dc::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3654.12 via Frontend Transport; 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I missed that. Makes sense. >>> vm_file needs to be accurate, but vm_area_struct should be accessed as ptr_to_btf_id. >> >> Passing pointer of vm_area_struct into BPF will be tricky. For example, shall we >> allow the user to access vma->vm_file? IIUC, with ptr_to_btf_id the verifier will >> allow access of vma->vm_file as a valid pointer to struct file. However, since the >> vma might be freed, vma->vm_file could point to random data. > > I don't think so. The proposed patch will do get_file() on it. > There is actually no need to assign it into a different variable. > Accessing it via vma->vm_file is safe and cleaner. I did not check the code but do you have scenarios where vma is freed but old vma->vm_file is not freed due to reference counting, but freed vma area is reused so vma->vm_file could be garbage? > >>>> [1] ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock") >>> >>> Thanks for this link. With "if (mmap_lock_is_contended())" check it should work indeed. >> >> To make sure we are on the same page: I am using slightly different mechanism in >> task_vma_iter, which doesn't require checking mmap_lock_is_contended(). In the >> smaps_rollup case, the code only unlock mmap_sem when the lock is contended. In >> task_iter, we always unlock mmap_sem between two iterations. This is because we >> don't want to hold mmap_sem while calling the BPF program, which may sleep (calling >> bpf_d_path). > > That part is clear. I had to look into mmap_read_lock_killable() implementation > to realize that it's checking for lock_is_contended after acquiring > and releasing > if there is a contention. So it's the same behavior at the end. >