Re: Readable files: a Unix philosophy question for gurus

[Aharon Robbins]

About the only way to do this completely reliably (at least for read and write)
is for test to actually try and open the file.  Exec is another story,
unfortunately.

> Date: Sun, 9 Feb 2003 20:45:08 -0800 (PST)
> From: Paul Eggert <eggert@twinsun.com>
> To: jim@meyering.net
> CC: beebe@math.utah.edu, bug-coreutils@gnu.org, bug-bash@gnu.org,
>    arnold@skeeve.com, SNYDERGA@simon.rochester.edu, 
> evi@anchor.cs.colorado.edu,
>    mckusick@mckusick.com
> Subject: Re: Readable files: a Unix philosophy question for gurus
>
> > From: Jim Meyering <jim@meyering.net>
> > Date: Sun, 09 Feb 2003 10:14:32 +0100
> > 
> > There is indeed a problem with the `test' in the coreutils --
> > and with the nearly identical built-in provided by bash.
> > I've just changed the one in coreutils to use euidaccess
> > (either the one in libc, or the supplied replacement).
> > That solves the problem only in the (albeit common) case
> > in which effective and actual UID and GID are identical.
>
> As I understand it, the euidaccess replacement attempts to guess what
> 'access' would return if the real uid and gid were temporarily set to
> the effective uid and gid.  This guess can be wrong for many reasons
> (e.g., ACLs, read-only filesystems, etc.), andit would be a
> maintenance hassle to improve the guess's quality on all systems.
>
> Wouldn't it be easier to set the real uid and gid temporarily to the
> effective uid and gid, then invoke 'access', and then set the real uid
> and gid back?  This method will handle all that nonportable ACL
> etc. stuff automatically.  The method won't work on all ancient hosts,
> but it'll work on all modern ones and we can fall back on the current
> euidaccess method if we're on an ancient host.  Also, I suppose this
> method would be a pain if you need euidaccess to be reentrant, but
> test(1) shouldn't need that.