From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 935B61DD0C7 for ; Tue, 18 Mar 2025 10:54:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742295283; cv=none; b=tvu6pgAt7KKQTw4DtKZRAaDnczBQpK4I/DgupA32t+XSwUNab2jjlgC3fe58ddhjH/1/LJjHNXemoMmRqemwll0OnvnoXn5aGxfIYTk9XRwWdqxJlas1RaZ6eXiuI7xjRq2IDX9a1PqnpPxLqtL1eterlh/u/GWcivhWphI1Lck= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742295283; c=relaxed/simple; bh=Dy2y4rjWztF5wfx840waetEMwtCqD5L73zeiOfZPqdQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oXA7U18NUFz4UTMRRj7i0Gixzt5z+cuyuvMYat5EyAkvvqZqNSsahmj1LXMIp+88KzS2/CIouXrVIFvC2KIl2Pibe4MylHM6X1exx6hO+BmKv5QNdBeudJxa6DEMQptiMwXGkD/YiL+aZGQNcFso/a6Vh2Z4dAPwrwaCsSWUud4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G+wWdbp0; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="G+wWdbp0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2424DC4CEDD; Tue, 18 Mar 2025 10:54:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1742295283; bh=Dy2y4rjWztF5wfx840waetEMwtCqD5L73zeiOfZPqdQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=G+wWdbp0rfMBtlgT1WpBLAWTadK+mDeWVS3WD+y1T9lpCcoPyOQpMCxp7sYauTlBM MRymH28hXmWSQ+KEjSjnwKlx0FpQopUzkjCnqpaTDJ9+Bg2sViUp41YblAkAN9mN9k vghUjhS8neRdOzdSLYh7M59oUgah0ruUxI80Jnjad8CWsE9k4LGzumYERTnVzZoOq2 4XoyqCXbe9iw1vWljEGB6uHXfbglwepsoReZotOptv76bk6eTr4g/Y6a9nm4IvPH7l p9q8AYttaGK/N7H7TuqJV7VPZ196JIYAESgDSLuC1iMD2Wy1Gg/ozLZkPgdju9Vl0I eUZJ0F75TZVLQ== Message-ID: <2816385e-d07e-4180-ab1c-599069630ab2@kernel.org> Date: Tue, 18 Mar 2025 11:54:40 +0100 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH mptcp-next v3 1/2] mptcp: add bpf_iter_task for mptcp_sock Content-Language: en-GB To: Mat Martineau Cc: Geliang Tang , Geliang Tang , mptcp@lists.linux.dev References: <2b680d20eb5873f14f35d9d23aa78b2f5a9d5bfd.camel@kernel.org> <459fd93c-d99d-4733-9194-3f62467854c6@kernel.org> <2dacce8d-95a4-a5d3-acc5-201a09b7a39c@kernel.org> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Mat, On 18/03/2025 02:25, Mat Martineau wrote: > On Mon, 17 Mar 2025, Matthieu Baerts wrote: > >> Hi Geliang, >> >> On 17/03/2025 11:59, Geliang Tang wrote: >>> Hi Matt, >>> >>> On Mon, 2025-03-17 at 11:29 +0100, Matthieu Baerts wrote: >>>> Hi Geliang, Mat, >>>> >>>> On 17/03/2025 10:41, Geliang Tang wrote: >>>>> On Mon, 2025-03-10 at 11:30 +0800, Geliang Tang wrote: >>>>>> From: Geliang Tang >>>>>> >>>>>> To make sure the mptcp_subflow bpf_iter is running in the >>>>>> MPTCP context. This patch adds a simplified version of tracking >>>>>> for it: >>>>>> >>>>>> 1. Add a 'struct task_struct *bpf_iter_task' field to struct >>>>>> mptcp_sock. >>>>>> >>>>>> 2. Do a WRITE_ONCE(msk->bpf_iter_task, current) before calling >>>>>> a MPTCP BPF hook, and WRITE_ONCE(msk->bpf_iter_task, NULL) after >>>>>> the hook returns. >>>>>> >>>>>> 3. In bpf_iter_mptcp_subflow_new(), check >>>>>> >>>>>>     "READ_ONCE(msk->bpf_scheduler_task) == current" >>>>>> >>>>>> to confirm the correct task, return -EINVAL if it doesn't match. >>>>>> >>>>>> Also creates helpers for setting, clearing and checking that >>>>>> value. >> >> (...) >> >>>>>> +static inline bool mptcp_check_bpf_iter_task(struct mptcp_sock >>>>>> *msk) >>>>>> +{ >>>>>> +    struct task_struct *task = READ_ONCE(msk- >>>>>>> bpf_iter_task); >>>>>> + >>>>>> +    if (task && task == current) >>>>>> +        return true; >>>>>> +    return false; >>>>>> +} >>>>> >>>>> This v3 has a bug. When I was testing MPTCP BPF selftests in a >>>>> loop, I >>>>> found that the test would break in some cases. After debugging, I >>>>> found >>>>> that "task" and "current" were not equal: >>>>> >>>>> [  520.209749][T11984] MPTCP: bpf_iter_mptcp_subflow_new >>>>> msk=00000000fc8f7370 in_interrupt=0 task=00000000ef28139f >>>>> current=0000000024db2987 >>>>> >>>>> I will try to fix it, but haven't found a solution yet. >>>> >>>> (sorry for the delay, I need a bit of time to catch up) >>>> >>>> I talked a bit to Alexei Starovoitov last week. He told me that with >>>> the >>>> BPF struct_ops, it is possible to tell the verifier that some locks >>>> are >>>> taken either by some struct_ops types, or even per callbacks of some >>>> specific struct_ops (WIP on sched_ext side). It is also possible to >>>> get >>>> some locks automatically (polymorphism), and there are examples on >>>> VFS side. >>> >>> Thanks for your reminder, I will look at these BPF codes. Our goal is >>> to make mptcp_subflow bpf_iter only used by struct_ops defined by MPTCP >>> BPF (bpf_mptcp_sched_ops and bpf_mptcp_pm_ops), right? Other struct_ops >>> are not allowed to use mptcp_subflow bpf_iter. >> >> Yes, that's correct. I didn't check, but **maybe** some mptcp_sched_ops >> and mptcp_pm_ops callback might not be allowed to use bpf_iter. In this >> case, it might be needed to allow only some of them to use bpf_iter. >> >> Note that it sounds like all mptcp_{pm,sched}_ops callbacks should be >> done while holding the msk lock, e.g. being called from the worker and >> not from a subflow event, etc. but maybe there are some exceptions >> needed. > > Hi Matthieu - > > Is this last paragraph also feedback from Alexei? Can you elaborate a > little - is the lock needed for BPF-specific reasons or to avoid having > the callback modify msk state? No, it is not from Alexei, but from me. Currently, some actions on a subflow will call some code in pm.c that will simply schedule the worker to be called from a different context to do other actions. To convert this to an mptcp_pm_ops, I think we should not have two callbacks -- one to schedule the worker, and one to react from the worker context -- but only one called from the worker context if such callback is set. This should also simplify the BPF PMs if all/most callbacks are done from the same context while owning the msk lock, no? > I'll try to look around at the verifier and automatic locking techniques > you mentioned above, it would be great to have a better way to handle > socket locks w/ BPF. Alexei suggested to look at the VFS code. If I understood correctly, the idea is to have some locks automatically taken for some actions, but without having to dedicate new kfunc, checking if they are already taken, or requiring the BPF code to handle them (auto-releasing them then). But I don't have more details about that. Cheers, Matt -- Sponsored by the NGI0 Core fund.