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From: davy@ea.ecn.purdue.edu (Dave Curry)
Newsgroups: comp.unix.wizards
Subject: Re: Strange reaction to wierd file mode
Message-ID: <366@ea.ecn.purdue.edu>
Date: Thu, 20-Aug-87 09:41:17 EDT
Article-I.D.: ea.366
Posted: Thu Aug 20 09:41:17 1987
Date-Received: Sat, 22-Aug-87 11:35:27 EDT
References: <719@csun.UUCP>
Reply-To: davy@ea.ecn.purdue.edu.UUCP (Dave Curry)
Organization: Purdue University Engineering Computer Network
Lines: 45

In article <719@csun.UUCP> aeusesef@csun.UUCP (Sean Eric Fagan) writes:
>While playing with an AT&T SystemV 2.0 Unix, I made a little file
>with access mode 0446 (r--r--rw-), and tried to write into it.
>Surprise!  The kernal said I couldn't.

No surprise.  The three sets of permissions on a file are:

	- permissions for the owner of the file,
	- permissions for other members of the file's group,
	  EXCLUDING the owner, and
	- permissions for other users of the system EXCLUDING
	  the owner and members of the file's group.

These permissions are applied by the kernel in a left to right order
until they match.  I.e., if you own the file, the first set of
permissions apply.  If you do not own the file, but are a member of
the group, the second set applies.  If you are not the owner and not
in the group, only then does the third set apply.

Since you own the file, the set of permissions applied to you are the
owner permissions, which do not include permission to write.

>(A friend suggested putting code in the kernal for creat, chmod, etc. so
>that this type of mode wouldn't happen;  I disagree about this type of
>kludge.)

Please ask your friend to stay away from the kernel until he knows what
he's doing.  There are perfectly valid uses for permisssions like this,
such as preventing a given group of people from doing things to a file,
and so on.

>(*I* think it's a bug, but then I'm only a lowly student who isn't allowed
>to look at source code 8-))

It most certainly is NOT a bug.  If the kernel had allowed you to write
the file when world-write was on and owner-write was off, then what
would be the point of having owner permissions at all?  As a more
useful example, consider the group permissions - if the world
permissions allowed write to everyone as under your scheme, there
would be no way to PREVENT a given group of people from writing the
file (since group permissions would be meaningless).  Under the
current scheme, you can do this.

--Dave Curry
Purdue University