Re: git archive generates tar with malformed pax extended attribute

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: May 25, 2019, 20:46 UTC
Message-ID: <878suufejr.fsf@evledraar.gmail.com>
In-Reply-To: <20190524070644.GF25694@sigill.intra.peff.net>

On Fri, May 24 2019, Jeff King wrote:

Show 33 quoted lines

> On Fri, May 24, 2019 at 08:45:23AM +0200, Keegan Carruthers-Smith wrote:
>
>> git archive can generate a malformed tar archive. bsdtar reports the
>> error "tar: Ignoring malformed pax extended attribute" when reading
>> the archive. Go's "tar/archive" package also reports the error
>> "archive/tar: invalid tar header". However, BusyBox's tar does not
>> report the error (unsure if it just has less error logging).
>>
>> I can reproduce this when generating the tar on linux and mac. I
>> tested this with "git version 2.21.0" and a build of next I did today:
>> "git version 2.22.0.rc1.257.g3120a18244"
>>
>> Reproduction:
>>
>>   $ git clone https://github.com/SSW-SCIENTIFIC/NNDD.git
>>   $ cd NNDD
>>   $ git archive --format tar c21b98da2ca7f007230e696b2eda5da6589fe137
>> | tar tf - > /dev/null
>>   tar: Ignoring malformed pax extended attribute
>>   tar: Error exit delayed from previous errors.
>
> I can't reproduce on Linux, using GNU tar (1.30) nor with bsdtar 3.3.3
> (from Debian's bsdtar package). What does your "tar --version" say?
>
> Git does write a pax header with the commit id in it as a comment.
> Presumably that's what it's complaining about (but it is not malformed
> according to any tar I've tried). If you feed git-archive a tree rather
> than a commit, that is omitted. What does:
>
>   git archive --format tar c21b98da2^{tree} | tar tf - >/dev/null
>
> say? If it doesn't complain, then we know it's indeed the pax comment
> field.

Solaris tar also complains about this. I've seen that for ages, but never thought to report it, I figured it was well-known.

When you "tar xf" an archive git-archive it complains:

    tar: pax_global_header: typeflag 'g' not recognized, converting to regular file

It will then extract the "pax_global_header" as if it were a file at the root of the archive. That file will look like this:

    $ wc -c x/pax_global_header
      52 x/pax_global_header
    $ cat x/pax_global_header
    52 comment=$40_CHAR_SHA_1

Where $40_CHAR_SHA_1 is whatever commit this archive was produced from.