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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 25F1FC352A3 for ; Sat, 15 Feb 2020 05:24:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D24DE2082F for ; Sat, 15 Feb 2020 05:24:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=microsoft.com header.i=@microsoft.com header.b="E3oW9mDN" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726445AbgBOFYB (ORCPT ); Sat, 15 Feb 2020 00:24:01 -0500 Received: from mail-eopbgr1300103.outbound.protection.outlook.com ([40.107.130.103]:24928 "EHLO APC01-HK2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725795AbgBOFYA (ORCPT ); Sat, 15 Feb 2020 00:24:00 -0500 ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none; b=WvAHt6cY9m/GgDEl5DMqkDI/UtvNEdSb5xar1Yi5RGQMT5iWa4pYahl0awUARFUh24keLTKLaZvYpEXJOqfw3SJsqr0u3oEXB0mTzn1FQBIgj4oD4rnc3NbdJiLo6tJYZJrpP3XEeSP+EoMuRUnfxfLiI8Ty9RsPwi/SxzTjeD927+2N6jx0guVi6wzwdBmCVom6tFkMdUbBwpBPV20FsHPbsw6j1k0qWsNsyzLlMnJIHhx1gQTO0ZH19+3qbCd+5sPW63zWJmzJCuKAMgZKvuPdy0ak62RpeV+0DVSGDlizjx2A/+DU9Z2TvdMcP6w9Ot/PEXgjlSKaiMAc4IbX6A== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector9901; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=X+TNbtH8Sw3Pdb3j14YmnUVQSiji3nf1c8Pvcdspoyk=; b=B0hr/PYRm7WXgGP4iseUjAap7UdHIdt5TK0sUxV8xyrXTbe45Jhiz8Y4b+3czHWz3h+lpd1g44jwopol0qA2bOF5Z+R2b8to2k0yUc0hlDkeL1loW7jjbDDaq6ypLNqYP9LPnvyG4VNhW04BOAUTZgEOIi39Q5yG4QOroVjcAL5FfwKdB0fOmEW8DxmwVl6PIvYg3GqJCPziLX8Q5WLt0gWqcReWGDBAxHmLj8BYZ7bZxD3uZlqvVQD0IqqZP9G5w8M1QNP7XBepaOobZyQuI76cyi1PTC2DUr+QMp55FmiLR2v3fUNR4P9ae9uTldZoc7U0E/WJCwx4PuTgUNoJvA== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=microsoft.com; dmarc=pass action=none header.from=microsoft.com; dkim=pass header.d=microsoft.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=X+TNbtH8Sw3Pdb3j14YmnUVQSiji3nf1c8Pvcdspoyk=; b=E3oW9mDNsHnMhuo4cRu4lHVTOehAZiAR14euZX3lguPmhFOsZvWv0tXMmdHxeM2ljMNbStP/C4OFxcO/ZxLM3/ZAi2WVgBf1zvOrAtp4oTNWutQZLXpf1jI7qcm82QSvBMi/FxA/W/GS+PKPR89zXQYQNJ0YmJDuDi+UanHgjpA= Received: from HK0P153MB0148.APCP153.PROD.OUTLOOK.COM (52.133.156.139) by HK0P153MB0244.APCP153.PROD.OUTLOOK.COM (10.255.253.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2750.4; Sat, 15 Feb 2020 05:23:55 +0000 Received: from HK0P153MB0148.APCP153.PROD.OUTLOOK.COM ([fe80::58ea:c6ae:4ea3:8432]) by HK0P153MB0148.APCP153.PROD.OUTLOOK.COM ([fe80::58ea:c6ae:4ea3:8432%5]) with mapi id 15.20.2750.014; Sat, 15 Feb 2020 05:23:55 +0000 From: Dexuan Cui To: Haiyang Zhang , Stephen Hemminger , "David S. Miller" , "netdev@vger.kernel.org" , KY Srinivasan , "linux-kernel@vger.kernel.org" CC: "linux-hyperv@vger.kernel.org" Subject: Is it safe for a NIC driver to use all the 48 bytes of skb->cb? Thread-Topic: Is it safe for a NIC driver to use all the 48 bytes of skb->cb? Thread-Index: AdXjukH6tGicVKR8QjWZ+wsQCSffGw== Date: Sat, 15 Feb 2020 05:23:55 +0000 Message-ID: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Enabled=True; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_SiteId=72f988bf-86f1-41af-91ab-2d7cd011db47; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Owner=decui@microsoft.com; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_SetDate=2020-02-15T05:23:53.1818868Z; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Name=General; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Application=Microsoft Azure Information Protection; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_ActionId=0bdb99fb-ade9-4625-91de-e48aae6b21ec; MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Extended_MSFT_Method=Automatic authentication-results: spf=none (sender IP is ) smtp.mailfrom=decui@microsoft.com; x-originating-ip: [2001:4898:80e8:9:9a10:13a:9851:57b7] x-ms-publictraffictype: Email x-ms-office365-filtering-ht: Tenant x-ms-office365-filtering-correlation-id: c2a50d69-2c84-4673-5dd9-08d7b1d740bd x-ms-traffictypediagnostic: HK0P153MB0244:|HK0P153MB0244: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:10000; x-forefront-prvs: 03142412E2 x-forefront-antispam-report: SFV:NSPM;SFS:(10019020)(4636009)(366004)(346002)(39860400002)(376002)(396003)(136003)(189003)(199004)(71200400001)(8676002)(7696005)(4326008)(6506007)(86362001)(186003)(33656002)(2906002)(8936002)(81156014)(81166006)(64756008)(66446008)(316002)(66946007)(66556008)(55016002)(76116006)(10290500003)(66476007)(5660300002)(9686003)(478600001)(110136005)(8990500004)(52536014);DIR:OUT;SFP:1102;SCL:1;SRVR:HK0P153MB0244;H:HK0P153MB0148.APCP153.PROD.OUTLOOK.COM;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:1; received-spf: None (protection.outlook.com: microsoft.com does not designate permitted sender hosts) x-ms-exchange-senderadcheck: 1 x-microsoft-antispam: BCL:0; x-microsoft-antispam-message-info: x37SN1arD31duZQnj7GgD4PT8/7K/L6CE6C0vW/xBByIp9y6P4kgd+V5JNk8xdcLjb4NvmcP7w/3dXEbd+vbvMluX/ISQFuEijlcip23iQmb5Fpf2zGHn9J9UO7WvC7hmOJcLILKEoKo3p4kOpCzKZyOPxKEEHn/t+9tNPRBxJMgHAsd/ZVX4mvm0wsX0YYj+jykKiFdO+mSNSyxUrvJuHIyUtF8gzBP57mWwpM/+8d20DPzQe7czZj3HXvd9LMt8MKhRhepP0JWlmFB5w4TaI3o2eu+qYKLJiAiU6tmtufd3sEwTXrOChWLCKKdoVoVx9NI3xpvRnHITLEE3isiqqZuApcMVf7w9n0dFxo/ADC/gUkDsgrYAY9c4ZojndYqPjkDGUrWElO/LCoBZ21xrMKH3X7CWlIs0ZSVNXpK+hfjpBXOPDicpm3iG5OuoWIq x-ms-exchange-antispam-messagedata: NbtndxpGpAXFjUrT0dcL81y/HPrFjJNOsOigRgtvW1gJWa7i3ao6tbha5HcFgu7uvcVV+P3F1gcJ2F5ypIYBnLo9KsiWmRrFsgtz1+AVM7m8iLeY6tBfxtwFY0gUfZ+gulKZ+SeOaV/DKDD8N/lYSPAZkgYLjz8k2iDUt95sWHNkA9KHDEB8zr/9iMvySdgjH690nX+3YyO0nycyK8LF4A== Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: c2a50d69-2c84-4673-5dd9-08d7b1d740bd X-MS-Exchange-CrossTenant-originalarrivaltime: 15 Feb 2020 05:23:55.4443 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: BEshrHY2NbAwqj4OpjbB5ngdtWpR7n147rW2qY7Ui4LOZc3cb2F1gA/7us2kEv2XUI+LtgCfd2PHlBneU6CWAQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: HK0P153MB0244 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, It looks all the layers of drivers among the network stack can use the 48-b= yte skb->cb array. Is there any rule how they should coordinate with each other= ? I noticed the last 16 bytes are used by struct skb_gso_cb: include/linux/skbuff.h: struct skb_gso_cb { union { int mac_offset; int data_offset; }; int encap_level; __wsum csum; __u16 csum_start; }; #define SKB_SGO_CB_OFFSET 32 #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb + SKB_SGO_CB_OFFSE= T)) Does this mean a low level NIC driver (e.g. hv_netvsc) should only use the first 32 bytes? What if the upper layer network stack starts to take up more space in the future?=20 Now hv_netvsc assumes it can use all of the 48-bytes, though it uses only=20 20 bytes, but just in case the struct hv_netvsc_packet grows to >32 bytes i= n the future, should we change the BUILD_BUG_ON() in netvsc_start_xmit() to BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) > SKB_SGO_CB_OFFSET); ? =20 struct hv_netvsc_packet { /* Bookkeeping stuff */ u8 cp_partial; /* partial copy into send buffer */ u8 rmsg_size; /* RNDIS header and PPI size */ u8 rmsg_pgcnt; /* page count of RNDIS header and PPI */ u8 page_buf_cnt; u16 q_idx; u16 total_packets; u32 total_bytes; u32 send_buf_index; u32 total_data_buflen; }; static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net) { ... /* * Place the rndis header in the skb head room and * the skb->cb will be used for hv_netvsc_packet * structure. */ ret =3D skb_cow_head(skb, RNDIS_AND_PPI_SIZE); if (ret) goto no_memory; /* Use the skb control buffer for building up the packet */ BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) > FIELD_SIZEOF(struct sk_buff, cb)); packet =3D (struct hv_netvsc_packet *)skb->cb; Thanks, -- Dexuan