reading links in proc - permission denied

From: linda w. (
Date: Wed Jun 04 2003 - 22:12:51 EST

I'm misunderstanding something about links in proc.

I thought 'ps', 'top' et al used /proc to display processes, command lines, etc.

Since neither ps nor top are suid root, they are running with my uid

However, if I do "ls -l" on /proc/<number>/exe, I get a

"ls: cannot read symbolic link /proc/16714/exe: Permission denied"


Now the process is owned by 'named', but the entries in diriectory are
owned by root (is that right/logical?), thus:

# ll /proc/16714
total 0
dr-xr-xr-x 3 named named 0 Jun 4 11:39 ./
dr-xr-xr-x 95 root root 0 May 30 15:38 ../
-r--r--r-- 1 root root 0 Jun 4 11:39 cmdline
-r--r--r-- 1 root root 0 Jun 4 11:39 cpu
lrwxrwxrwx 1 root root 0 Jun 4 11:39 cwd -> /var/named/
-r-------- 1 root root 0 Jun 4 11:39 environ
lrwxrwxrwx 1 root root 0 Jun 4 11:39 exe -> /usr/sbin/named*
dr-x------ 2 root root 0 Jun 4 11:39 fd/
-r--r--r-- 1 root root 0 Jun 4 11:39 maps
-rw------- 1 root root 0 Jun 4 11:39 mem
-r--r--r-- 1 root root 0 Jun 4 11:39 mounts
lrwxrwxrwx 1 root root 0 Jun 4 11:39 root -> //
-r--r--r-- 1 root root 0 Jun 4 11:39 stat
-r--r--r-- 1 root root 0 Jun 4 11:39 statm
-r--r--r-- 1 root root 0 Jun 4 11:39 status

	Purely from a 'cleanliness' standpoint, is the environment owned by
the user-id, or is it a common piece of public, kernel (root) owned data?

From observation of other /proc entries, it appears that 'named' has some unique features in that it is started as root, but then reverts to uid/gid named sometime after startup. Should some (or all) of the UID's in proc change ownership to the new UID or are they still considered to be owned by the old UID? (Would seem a bit inconsistent -- I wonder if it could be security exploitable? -- like if a user process was able to setuid root, would anything be left in the environment owned by the original unpriviledged user that could be changed from another running process, changing things like PATH for the currently running root process....naw...I'm sure that's plugged...and it's only inconsistent with root doing setuid to non-root....hmmmm :-/).

But, here's the part that is bugging me. Running as user 'foo', I can't read that link -- yet the permissions say rxw for group and other. So why am I getting the *permission error*? The binary it is pointing to /usr/sbin/named is also publicly readable, so that can't be the problem.

So why can't I follow the link of 'exe' to see what image the process is executing? Programs like 'ps' and 'top' seem to not have this difficulty.

I'm sure it's some silly misconfiguration on my part, but I guess I want to know how I got here. This isn't my beautiful kernel, it's not my beautiful /proc...(etc...).

I'm running a xfs-patched kernel, V2.4.20/SMP.

Thanks for any insights...I'm trying to write a simple script looking for a running process (by looking at what 'exe' is pointing to). I would find it kludgey to achieve the objective by running 'ps' and doing appropriate filtering.


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This archive was generated by hypermail 2b29 : Sat Jun 07 2003 - 22:00:26 EST