linux-kernel - procfs doesnt honor cap_dac

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Message-ID: <op.vlpyahbg25uvhe@wwwwww-701sd>
Date:	Fri, 05 Nov 2010 18:33:45 -0400
From:	abirvalg@...abit.com
To:	linux-kernel@...r.kernel.org
Subject: procfs doesnt honor cap_dac_override

We have a folder /proc/1/fd which contains symlinks.
A superuser can do readlink() on those symlinks.
I expected that a non-root process with cap_dac_override capability set to  
EP could also readlink() those symlinks in /proc/1/fd, but alas  
"Permission denied".
Just to make sure that my process isn't the culprit, I ran it as a regular  
user with UID=1000, I set cap_dac_override=ep and navigated to /root  
folder (which is readable only by root) and did readlink() there. It  
worked. (/root folder is on ext4.)
Bottom line: procfs doesnt allow readlink() on certain symlinks owned by  
root even with cap_dac_override set.

Is this an expected behaviour or is this a bug?
Cheers.

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