From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-176.mta0.migadu.com (out-176.mta0.migadu.com [91.218.175.176]) (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 7B9728BFF for ; Sat, 8 Mar 2025 06:18:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.176 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741414683; cv=none; b=YicnsO7qjprkFKZrfq+tzfeh+rYY0ywUD3jUfqsSq55Y5fZC46eLHG2ApU5XkfTFPOLloAvuawNJ7DV0hTyMw/p0zwFR/EBFg+lfg8VOl1TzmQ/NtQJbqseHwlJX2HI0bFWxFtlxxotEc7378jljL3vZC8htkE9KdUxul/6YjT4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741414683; c=relaxed/simple; bh=W9Zpsk4va65XjOKJQ4BZsvpT+YEceRpv1MtKhh31Shk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WDTGkR9clTZHR/S+tkUAYxy3wD8CjpgnKRECGXzKpznJQRHJjetCy7c/8FzDfhN/1Z23v8tkzFVedzXSV2YUfi1a6U+1x3dhUZLS+uHe6Z7fLUBGym7vyrchJVIxx4ySCm6xJ5Dt9d2W37bwxgmbsa4q1TMjaYOxs1VMnOSoD00= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=i+h3hCZ3; arc=none smtp.client-ip=91.218.175.176 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="i+h3hCZ3" Message-ID: <908c6a63-3049-4dd2-859a-215b31e5d1ea@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1741414669; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0GmX1LESwc5JByLp69e4e/sfrIhlf3MfMRPUw8cGdoA=; b=i+h3hCZ32v6BKcv0iDEvAWb5Z7bbh34KxRYTPxLWbfCHYu7GRpsAe2D7Hw0bBepOMDjBVY HfEzzjGXgDjY234aywVaHYFfmF/HCC+z/emQVbSs0iLwE5eKtljxOnU3B86YyZAqr7nKkj jwyHvjwRuhJilO6plQYb7ozVp9W8qAw= Date: Fri, 7 Mar 2025 22:17:44 -0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next 1/2] bpf: add get_netns_cookie helper to tracing programs Content-Language: en-GB To: Martin KaFai Lau , Mahe Tardy Cc: daniel@iogearbox.net, john.fastabend@gmail.com, ast@kernel.org, andrii@kernel.org, jolsa@kernel.org, bpf@vger.kernel.org, Network Development References: <20250227182830.90863-1-mahe.tardy@gmail.com> <96dbd7df-1fa7-4caa-a52c-372d696e0f38@linux.dev> <36637c9d-b6bc-4b8c-a2fd-9800c5a7a6dc@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 3/7/25 3:06 PM, Martin KaFai Lau wrote: > On 3/6/25 9:03 AM, Mahe Tardy wrote: >>>>> The immediate question is whether sock_net(sk) must be non-NULL >>>>> for tracing. >>>> We discussed this offline with Daniel Borkmann and we think that it >>>> might not be the question. The get_netns_cookie(NULL) call allows >>>> us to >>>> compare against get_netns_cookie(sock) to see whether the sock's netns >>>> is equal to the init netns and thus dispatch different logic. >>> bpf_get_netns_cookie(NULL) should be fine. >>> >>> I meant to ask if sock_net(sk) may return NULL for a non NULL sk. >>> Please check. >> Oh sorry for the confusion, I investigated with my humble kernel >> knowledge: essentially sock_net(sk) is doing sk->sk_net->net, retrieving >> the net struct representing the network namespace, to later extract the >> cookie, and thus dereference the returned pointer (here is the concern). >> The sk_net intermediary (in reality __sk_common.skc_net) is here because >> of the possibility of switching on/off network namespaces via >> CONFIG_NET_NS. It's a possible_net_t type containing (or not) the struct >> net pointer, explaining why we use write/read_pnet to no-op or return >> the global net ns. >> >> Now by adding this helper to tracing progs, it allows to call this >> function in any function entry or function exit, but unlike kprobes, >> it's not possible to just hook at an obvious arbitrary point in the code >> where the net ns would be NULL in the sock struct. With that in mind, I >> failed to crash the kernel tracing a function (some candidates were >> inlined). I mostly grepped for sock_net_set, but I lack the knowledge to > > Thanks for checking. > > I took a quick look at the callers of sock_net_set. I suspect > "fentry/sk_prot_alloc" and "lsm/sk_alloc" could have a NULL? > >> guarantee that this could not happen right now or in the future. Maybe >> that would be just safer to add a check and return 0 in that case if >> that's ok? Not sure since the helper returns an 8-byte long opaque >> number which thus includes 0 as a valid value. > > I assume net_cookie 0 is invalid, but then it leaks the implementation > details of what is a valid cookie in a uapi helper > >  * u64 bpf_get_netns_cookie(void *ctx) >  * ... >  *      Return >  *              A 8-byte long opaque number > > Note that, the tracing program can already read most fields of the sk, > including sk->sk_net.net->net_cookie. Therefore, what this patch aims > to achieve has already been supported in tracing. It can also save a > helper call. > > The only thing that may be missing in your use case is determining the > init_net. I don't think reading a global kernel variable has been > supported yet. Not sure if init_net must have net_cookie 1. Otherwise, > we could consider to add a kfunc to return &init_net, which could be > used to compare with sk->sk_net.net. Having a pointer to &init_net > might be more useful for other tracing use cases in general. There is the workaround for this tracing use case. 1. Declare a global variable in the bpf program, e.g. struct net *init_net; 2. After skel_open and before skel_load, find init_net address (from /proc/kallsyms) and assign the address to skel->bss->init_net. 3. In the prog, do struct net *netns = bpf_rdonly_cast(init_net, bpf_core_type_id_kernel(struct net)); bpf_printk("%u\n", netns->net_cookie); There is an effort to add global variables to BTF. See https://lore.kernel.org/bpf/20250207012045.2129841-1-stephen.s.brennan@oracle.com/ The recommended way is to put these global variables in a module to avoid consume too much kernel memory unconditionally.