linux-kernel - Re: /sys and access(2): Correctly implemented?

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Date:	Mon, 09 Jul 2012 09:24:38 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Ulrich Windl <Ulrich.Windl@...uni-regensburg.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: /sys and access(2): Correctly implemented?

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
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