From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 47CC2C43461 for ; Mon, 14 Sep 2020 20:35:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0A9D3206DC for ; Mon, 14 Sep 2020 20:35:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726135AbgINUfa (ORCPT ); Mon, 14 Sep 2020 16:35:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38176 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726201AbgINUfC (ORCPT ); Mon, 14 Sep 2020 16:35:02 -0400 Received: from shards.monkeyblade.net (shards.monkeyblade.net [IPv6:2620:137:e000::1:9]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7A0BCC06174A; Mon, 14 Sep 2020 13:34:58 -0700 (PDT) Received: from localhost (unknown [IPv6:2601:601:9f00:477::3d5]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) (Authenticated sender: davem-davemloft) by shards.monkeyblade.net (Postfix) with ESMTPSA id 5A67A12782CD9; Mon, 14 Sep 2020 13:18:10 -0700 (PDT) Date: Mon, 14 Sep 2020 13:34:56 -0700 (PDT) Message-Id: <20200914.133456.855640911695260789.davem@davemloft.net> To: xie.he.0141@gmail.com Cc: kuba@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, willemdebruijn.kernel@gmail.com, eric.dumazet@gmail.com, briannorris@chromium.org, xiyou.wangcong@gmail.com Subject: Re: [PATCH net-next v2] net/packet: Fix a comment about hard_header_len and headroom allocation From: David Miller In-Reply-To: <20200914074154.1255716-1-xie.he.0141@gmail.com> References: <20200914074154.1255716-1-xie.he.0141@gmail.com> X-Mailer: Mew version 6.8 on Emacs 27.1 Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [2620:137:e000::1:9]); Mon, 14 Sep 2020 13:18:10 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Xie He Date: Mon, 14 Sep 2020 00:41:54 -0700 > This comment is outdated and no longer reflects the actual implementation > of af_packet.c. > > Reasons for the new comment: > > 1. > > In af_packet.c, the function packet_snd first reserves a headroom of > length (dev->hard_header_len + dev->needed_headroom). > Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header, > which calls dev->header_ops->create, to create the link layer header. > If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of > length (dev->hard_header_len), and checks if the user has provided a > header sized between (dev->min_header_len) and (dev->hard_header_len) > (in dev_validate_header). > This shows the developers of af_packet.c expect hard_header_len to > be consistent with header_ops. > > 2. > > In af_packet.c, the function packet_sendmsg_spkt has a FIXME comment. > That comment states that prepending an LL header internally in a driver > is considered a bug. I believe this bug can be fixed by setting > hard_header_len to 0, making the internal header completely invisible > to af_packet.c (and requesting the headroom in needed_headroom instead). > > 3. > > There is a commit for a WiFi driver: > commit 9454f7a895b8 ("mwifiex: set needed_headroom, not hard_header_len") > According to the discussion about it at: > https://patchwork.kernel.org/patch/11407493/ > The author tried to set the WiFi driver's hard_header_len to the Ethernet > header length, and request additional header space internally needed by > setting needed_headroom. > This means this usage is already adopted by driver developers. > > Cc: Willem de Bruijn > Cc: Eric Dumazet > Cc: Brian Norris > Cc: Cong Wang > Signed-off-by: Xie He Applied, thank you.