Messages in this thread | | | Date | Mon, 09 Jul 2012 10:11:56 +0200 | From | "Ulrich Windl" <> | Subject | Re: Antw: Re: /sys and access(2): Correctly implemented? |
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Hi!
Still the problem seems to be related to the sysfs: # cd /tmp # touch testfile # chmod u=w,go= testfile # F=/tmp/testfile # test -r "$F" && cat "$F"
So it seems access(2) works correctly for root and "normal" filesystems. That's why I came up with the issue here.
Regards, Ulrich
>>> Ryan Mallon <rmallon@gmail.com> schrieb am 09.07.2012 um 09:22 in Nachricht <4FFA86C5.7090601@gmail.com>: > On 09/07/12 16:23, Ulrich Windl wrote: >>>>> Ryan Mallon <rmallon@gmail.com> schrieb am 09.07.2012 um 01:24 in Nachricht > > <4FFA16B6.9050009@gmail.com>: > >> 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. > > > > Hi! > > > > You are right: It's a typo, but only in the message; the actual test was > done correctly, and the outcome is quite the same. > > > >> > >> 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). > > > > Not here, unfortunately: > > Oops, I missed the bit about you running as root. I get the same results > running as root on my machine as you, both for sysfs and regular files. > > It appears that access(2) as the super-user is might be implementation > defined, see: > > http://pubs.opengroup.org/onlinepubs/000095399/functions/access.html > http://lists.gnu.org/archive/html/bug-bash/2010-07/msg00071.html > > However, I can't find any concrete information on it for Linux, and the > manpage doesn't mention anything other the the X_OK bit. > > ~Ryan >
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