From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753333AbbAKA1K (ORCPT ); Sat, 10 Jan 2015 19:27:10 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:38114 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750787AbbAKA1J (ORCPT ); Sat, 10 Jan 2015 19:27:09 -0500 Date: Sun, 11 Jan 2015 00:27:06 +0000 From: Al Viro To: "Carlos R. Mafra" Cc: LKML , Hauke Mehrtens , "John W. Linville" , netdev@vger.kernel.org Subject: Re: /proc/net/dev regression Message-ID: <20150111002706.GC22149@ZenIV.linux.org.uk> References: <20150110232518.GA3212@linux-g29b.site> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150110232518.GA3212@linux-g29b.site> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 10, 2015 at 11:25:18PM +0000, Carlos R. Mafra wrote: > [mafra@linux-g29b:wmnet]$ cat net_dev_bad.txt > Inter-| Receive | Transmit > face |bytes packets errs drop fifo frame compressed multicast|bytes packets errs drop fifo colls carrier compressed > lo: 600 8 0 0 0 0 0 0 600 8 0 0 0 0 0 0 > wlp3s0b1: 9266848 7298 0 0 0 0 0 0 372229 4030 0 0 0 0 0 0 > > And for some reason this change confuses 'wmnet'. Reading the source code > of 'wmnet' I found that it reads the packets as follows, > > totalpackets_in = strtoul(&buffer[15], NULL, 10); > > I am not sure if 'wmnet' could do this better (any suggestions?), *snort* well, yes - it's called scanf(). And if one is really, really nervous about the overhead of parsing a bunch of integers (as if fopen/ fgets/fclose alone won't cost enough to make constantly calling that sucker a bad idea), just use ptr + - 6 instead of &buffer[] in there. That thing has just found where the colon was (and replaced it with NUL), so dealing with "the first field turned out to be too long and shifted everything past it" isn't hard. > but the fact is that it was working before and now it is not. True. Mind you, the real issue is that this code expects the interface names to be never longer than 6 characters, but then /proc/net/dev layout strongly suggests that. Hell knows; it is a regression and it does break real-world userland code. The only way to avoid that, AFAICS, is to prohibit interface names longer than 6 chars ;-/ Lovely combination of crappy ABI (procfs file layout), crappy userland code relying on details of said ABI out of sheer laziness and triggering kernel change producing bloody long interface names... Incidentally, sufficiently long interface name will produce other fun issues for a docked app - it simply won't fit into 64x64 square on screen ;-)