From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752752Ab2GHXYo (ORCPT ); Sun, 8 Jul 2012 19:24:44 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:40972 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752390Ab2GHXYn (ORCPT ); Sun, 8 Jul 2012 19:24:43 -0400 Message-ID: <4FFA16B6.9050009@gmail.com> Date: Mon, 09 Jul 2012 09:24:38 +1000 From: Ryan Mallon User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1 MIME-Version: 1.0 To: Ulrich Windl CC: linux-kernel@vger.kernel.org Subject: Re: /sys and access(2): Correctly implemented? References: <4FF6A17B020000A10000A5E8@gwsmtp1.uni-regensburg.de> In-Reply-To: <4FF6A17B020000A10000A5E8@gwsmtp1.uni-regensburg.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/07/12 16:27, Ulrich Windl wrote: > Hi! > > Recently I found a problem with the command (kernel 3.0.34-0.7-default from SLES 11 SP2, run as root): > test -r "$file" && cat "$file" > emitting "Permission denied" > > Investigating, I found that "test" actually uses "access()" to check for permissions. Unfortunately there are some files in /sys that have "write-only" permission bits set (e.g. /sys/devices/system/cpu/probe). > > ~ # ll /sys/devices/system/cpu/probe > --w------- 1 root root 4096 Jun 29 12:43 /sys/devices/system/cpu/probe > ~ # F=/sys/devices/system/cpu/probe > ~ # test "$F" && cat "$F" > cat: /sys/devices/system/cpu/probe: Permission denied Looks like you have a typo here, I think you wanted "test -r $F", not "test $F", the latter will just evaluate "$F" as an expression which will be true, and so you get the permission denied error running cat. Using "test -r $F" on a write-only sysfs file correctly returns false on my machine (Ubuntu 10.04.4 LTS/2.6.32-41-generic). ~Ryan