On 09/07/12 16:23, Ulrich Windl wrote: >>>> Ryan Mallon <rmal...@gmail.com> schrieb am 09.07.2012 um 01:24 in Nachricht > <4ffa16b6.9050...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
