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Path: utzoo!utgpu!water!watmath!clyde!rutgers!umnd-cs!umn-cs!ems!mark
From: mark@ems.UUCP
Newsgroups: comp.unix.questions
Subject: Re: access(2) question
Message-ID: <318@ems.UUCP>
Date: Fri, 19-Jun-87 17:49:11 EDT
Article-I.D.: ems.318
Posted: Fri Jun 19 17:49:11 1987
Date-Received: Sat, 20-Jun-87 10:07:12 EDT
References: <530@applix.UUCP> <1341@xanth.UUCP>
Reply-To: mark@ems.UUCP (Mark H. Colburn)
Organization: EMS/McGraw-Hill, Eden Pairie, MN
Lines: 29
Keywords: access permissions

In article <1341@xanth.UUCP> john@xanth.UUCP (John Owens) writes:
>In article <530@applix.UUCP>, mark@applix.UUCP (Mark Fox) writes:
>> Does anybody know why access checks file accessibility using the real [ug]id
>> in place of the effective [ug]id? It seems to me that access should agree
>> with open(2) as far as whether a program has write or read access to a file.
>> Inside a set-uid program that assumption obviously doesn't work while
>> uid != euid.
>
>Well, the reason that access was added was to make it possible for a
>set[ug]id program to be able to discern whether or not the real user
>running it would have access to a certain file.

The problem with this is that some of the standard System V subroutine
libraries use the access call to determine the accessability for suid/sgid
programs.  An example that comes to mine here is a situation that bit me
a while back.  The tmpnam call apparently uses the access call to determine
whether a temporary file can be created.  This can lead to problems if you
want to direct temporary files to a directory that is local to a suid/guid
program that is executed via uux.

Given this scenario, the id=uucp, but the euid=demo (in my case).  The 
temporary directory is owned by demo.  However when tmpnam attempts to
create a file in /demo/tmp it fails, becase the id(uucp) does not have access
the /demo/tmp.  Anyways this can be gotten around in a number of ways,
including (what we did), writing a new version of access that uses the 
euid to check for access permissions.

As somebody suggested, this could be included in a library and named something
like eaccess().