Re: Supporting arbitrary file attributes

[Joerg Schilling]

>From: Mike Gerdts <address@hidden>

>A common problem with each vendor's version of tar is that it is unable
>to support things like ACL's, extended attributes, etc. effectively. 
>Furthermore, if one vendor comes up with a solution for the problem and
>an analagous structure exists on another vendor's platform, there is no
>guarantee that the information can be transported via a tar, cpio, or
>any other known cross-platform utility.

>I propose the following change as a potential change to a new GNU tar
>extension that could address this problem.


>1) Assign a new value to typeflag in posix_header that specifies GNU tar
>extension type 2.  I suggest the value of the typeflag to be 'E',
>mnemomic for "extensible".

>2) 1 or more headers MUST follow the POSIX header.  Each header will
>have the format:

>    struct extensible_header
>    {
>        char magic[20];                /* descriptive name */
>        char size[12];         /* size in bytes of this header */

Does not look standard compliant while now there is a decent standard:
POSIX.1-2001 extended headers.

The first implementation is found in 'star' at

        ftp://ftp.fokus.gmd.de/pub/unix/star/alpha/

Star started to support POSIX.1-2001 extended headers in August 2001, the time
when the standard editor granted that no major change could brake star in 
future.

The implementation in star is fairly clean and adding a new attibute usually
takes less than a day. I am not sure if it would make sense to try to support
extended headers in GNUtar before a major rewrite has been done.

ACLs are supported in star for about half a year. 
Star supports ACLs fomk Solaris and from system that implemented the
last POSIX ACL draft in a way that allows archive exchange between different
systems. It may be that Cygwin ACLs could be supported too as it seems that
Cygwin did implements Solaris ACLs (which are using a faster system interface
than the POSIX draft).

BSD and Linux extended flags are also supported with star.

People who like to add support for DOS file flags to the current extended flag
implementation are welcome.


>A slightly more complicated example would be to store Solaris Access
>Control Lists (ACLs).  Since these are quite similar (the same?) as
>those implemented on HP-UX, and those that are emerging on various Linux
>file systems, this could be a real boon to cross-platform portability of
>files.  An Access Control List (ACL) is made up of one or more Access
>Control Entries (ACE).

If you know how to deal with HP-UX ACLs, you are welcome to extend the
ACL interface in star! I already added some autoconf tests for HP-UX....


Jörg

 EMail:address@hidden (home) Jörg Schilling D-13353 Berlin
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