Subject: Re: [RFC] archive: behavior of --prefix with absolute or parent path components

From: "brian m. carlson" <[email-masked]>
Date: Tue, 7 Apr 2026 22:24:14 +0000

On 2026-04-07 at 19:24:54, Jeff King wrote:
> Yes, but note that with "-P" tar will happily allow those paths. They
> _can_ be useful, if you know what you are doing, but they aren't
> necessarily safe when coming from untrusted sources.
> 
> We can also generate zip files, but I think most unzip implementations
> have similar restrictions (info-zip does, with "-:" to override).

I suspect there are people using this with `/` because they want to
deploy files to places like `/etc`.  We've actually had requests for the
ability to have multiple roots in a repository so that people can do
this kind of thing, so I'm certain there are people finding _some_ way
to do it, even if not with this exact approach.

In conjunction with a tool like mtree(1) to adjust ownership and
permissions, this could be useful.

> In theory we could support other formats, but after 20 years I don't
> think anybody has bothered to do so. Cpio, anyone? :)

cpio doesn't have the long filename support that our pax (tar) archives
have, so I wouldn't recommend adding it.  The only place I still see
people use it is initramfs images for Linux.

> I don't recall it ever being discussed. Of the three you mentioned,
> "../" and leading "/" are potentially useful, so I don't think we'd want
> to disallow them entirely. At least some tar implementations require
> "-P" on the generating side to avoid mistakes, so we could follow that
> path.  It may be considered a regression by anybody who is using the
> feature currently, though.
> 
> The "////" is meaningless AFAICT, and could be replaced with a single
> slash. But I think it's also mostly harmless, as the reading side (well,
> the kernel) will equate "foo/////file" and "foo/file". I don't know if
> there are systems where that would not be the case.

Technically, POSIX allows `//` to be different than `/`, I believe,
although I'm not aware of anyone outside of Windows (and maybe Interix)
where that has any special meaning.  If you have such a system, it could
be useful to provide that as well as `/`.

I agree that it's more likely a typo, though.
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA