From mboxrd@z Thu Jan 1 00:00:00 1970 From: dwilder Subject: [patch] netlink.7: srcfix Change buffer size in example code about reading netlink message. Date: Mon, 14 Nov 2016 14:20:19 -0800 Message-ID: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-man@vger.kernel.org, netdev@vger.kernel.org To: mtk.manpages@gmail.com Return-path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:60446 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933656AbcKNWRK (ORCPT ); Mon, 14 Nov 2016 17:17:10 -0500 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uAEMDgHM083648 for ; Mon, 14 Nov 2016 17:17:10 -0500 Received: from e31.co.us.ibm.com (e31.co.us.ibm.com [32.97.110.149]) by mx0a-001b2d01.pphosted.com with ESMTP id 26qkfp65jn-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 14 Nov 2016 17:17:10 -0500 Received: from localhost by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 14 Nov 2016 15:17:09 -0700 Sender: netdev-owner@vger.kernel.org List-ID: The example code in netlink(7) (for reading netlink message) suggests using a 4k read buffer with recvmsg. This can cause truncated messages on systems using a page size is >4096. Please see: linux/include/linux/netlink.h (in the kernel source) /* * skb should fit one page. This choice is good for headerless malloc. * But we should limit to 8K so that userspace does not have to * use enormous buffer sizes on recvmsg() calls just to avoid * MSG_TRUNC when PAGE_SIZE is very large. */ #if PAGE_SIZE < 8192UL #define NLMSG_GOODSIZE SKB_WITH_OVERHEAD(PAGE_SIZE) #else #define NLMSG_GOODSIZE SKB_WITH_OVERHEAD(8192UL) #endif #define NLMSG_DEFAULT_SIZE (NLMSG_GOODSIZE - NLMSG_HDRLEN) I was troubleshooting some up-stream code on a ppc64le system (page:size of 64k) This code had duplicated the example from netlink(7) and was using a 4k buffer. On x86-64 with a 4k page size this is not a problem, however on the 64k page system some messages were truncated. Using an 8k buffer as implied in netlink.h prevents problems with any page size. Lets change the example so others don't propagate the problem further. Signed-off-by David Wilder --- man7/netlink.7.orig 2016-11-14 13:30:36.522101156 -0800 +++ man7/netlink.7 2016-11-14 13:30:51.002086354 -0800 @@ -511,7 +511,7 @@ .in +4n .nf int len; -char buf[4096]; +char buf[8192]; struct iovec iov = { buf, sizeof(buf) }; struct sockaddr_nl sa; struct msghdr msg;