From: Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups: fa.linux.kernel
Subject: Re: [bug] /etc/profile: line 30: /dev/null: Permission denied (Was:
Date: Fri, 18 Sep 2009 00:18:56 UTC
Message-ID: <fa.wL8LkdpbFLnGWP1g1GLljsizOPc@ifi.uio.no>

On Fri, 18 Sep 2009, Kay Sievers wrote:
>
> > So I suspect /dev/null and /dev/zero should be special - just make them
> > have 0666 permissions. Because they really _are_ special, and no other
> > permissions ever make sense for them.
>
> That's true. I guess there are a few more devices that need special
> permissions.

/dev/tty is probably the only remaining one - I don't think there should
be any other devices that are so special that normal programs expect them
to be there, and expect to be able to open them.

/dev/null (and to a lesser degree /dev/zero) really are special, and they
are special not so much because they are special devices, but because they
are part of the unix environment in rather deep ways. For example, mmap()
on /dev/zero is deeply special, and really is about shm rather than any
devices, so it's a VM thing with an odd special case.

And /dev/tty is special in that you'd expected to be able to open it even
if you can't open the device that it points to - you may have inherited a
tty from a program that _used_ to have permission to the underlying
/dev/ttyxyz thing, but even if you no longer can open that device,
/dev/tty still works.

The rest of /dev really should be rather esoteric, or it should be about
real devices. So I do think that with just null, zero and tty having 0666
permissions, a "normal UNIX" program is supposed to work. That should be
the minimal set, but also the maximal set of devices that people should
_expect_ to work.

(Yeah, there are things like /dev/stderr etc, but they are symlinks to
/proc/self/fd anyway, so permissions don't matter)

			Linus