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SubjectRe: Antw: Re: /sys and access(2): Correctly implemented?
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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