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Re: /proc filesystem allows bypassing directory permissions on Linux


Proc does not need to be fixed, because /proc is referring to a file inode. And due to the fact that it's being presented in /proc as an fd, you treat it
as an fd, therefor your expectations do not match.

Your assumptions is; because you accessed the world writable file via
a read-only fd, you also expect the file to appear as read-only in / proc.
There is the mis-match.

The idea of /proc is to refer to files being opened by the process,
while RETAINING the original INODE attributes, regardless of the fact that
this particular process accessed the file via read-only access.

You are expecting transactional behavior in /proc, where /proc only
registers object information.

I think this discussion had never existed if they used another name for
the 'fd' placeholder in /proc... Because then you wouldn't linked the
/proc fd to the fd being used within the actual process space.

And I think you would agree about your mis-interpretation earlier if not so many people had the same mis-interpretation of the virtual /proc mechanism (I hope you don't get me wrong; I'm not taking this personal or being rude to you,
because that's definitely NOT my intention).

Best regards,

On 30 okt 2009, at 18:38, Pavel Machek wrote:


Your assumption that the same file descriptor is being re-opened is
The file descriptor retrieved via /proc is a new one. It is not the
same as the
initial read-only.

True, we were just being sloppy with the words. But it does not matter
one iota.

As Martin Rex already explained yesterday, /proc is all virtual.
The item referred as fd in /proc is not a real file descriptor and as
of that, that 'not-tfor-real file descriptor' is also not re-opend
and so does
not become read-write.


- a house surrounded with a fence with all doors unlocked (file with
perm 0666)
- a drive-way leads to the gate in the fence and the gate is
unlocked (dir with perms 777)
- next we put a lock on the gate and don't give guest the key (dir
with perms 700)
- guest cannot access the house because he can't pass the gate
- now we take an airplane and parachute guest straight into the
perimeter of the fence (/proc access)
- guest can access the house (write the file), because the house has
all doors unlocked

Exactly, and I'm saying that airplanes should not exist (fix the
/proc).  Martin Rex had another solution -- allow fcntl() to remove
read-only and append-only limitations, so that behaviour is at least

(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html