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

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Message-Id: <4FF6A17B020000A10000A5E8@gwsmtp1.uni-regensburg.de>
Date:	Fri, 06 Jul 2012 08:27:39 +0200
From:	"Ulrich Windl" <Ulrich.Windl@...uni-regensburg.de>
To:	<linux-kernel@...r.kernel.org>
Cc:	"Ulrich Windl" <Ulrich.Windl@...uni-regensburg.de>
Subject: /sys and access(2): Correctly implemented?

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

In times of capabilities and ACLs, shouldn't there be a test for permissions that actually works?

Regards,
Ulrich
P.S. Please make sure my CC: gets your replies, too -- thanks

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