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<!doctype html>
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<html lang="en">
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<meta name="viewport" content="width=device-width, initial-scale=1">
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Robur's blog - GPTar (update)
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<meta name="description" content="libarchive vs hybrid GUID partition table and GNU tar volume header">
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<link type="text/css" rel="stylesheet" href="/css/hl.css">
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<link type="text/css" rel="stylesheet" href="/css/style.css">
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<script src="/js/hl.js"></script>
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<link rel="alternate" type="application/rss+xml" href="/feed.xml" title="blog.robur.coop">
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<header>
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<h1>blog.robur.coop</h1>
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<blockquote>
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The <strong>Robur</strong> cooperative blog.
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</blockquote>
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<main><a href="/index.html">Back to index</a>
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<article>
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<h1>GPTar (update)</h1>
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<ul class="tags-list"><li><a href="/tags.html#tag-OCaml">OCaml</a></li><li><a href="/tags.html#tag-gpt">gpt</a></li><li><a href="/tags.html#tag-tar">tar</a></li><li><a href="/tags.html#tag-mbr">mbr</a></li><li><a href="/tags.html#tag-persistent storage">persistent storage</a></li></ul><p>In a <a href="gptar.html">previous post</a> I describe how I craft a hybrid GUID partition table (GPT) and tar archive by exploiting that there are disjoint areas of a 512 byte <em>block</em> that are important to tar headers and <em>protective</em> master boot records used in GPT respectively.
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I recommend reading it first if you haven't already for context.</p>
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<p>After writing the above post I read an excellent and fun <em>and totally normal</em> article by Emily on how <a href="https://uni.horse/executable-tarballs.html">she created <strong>executable</strong> tar archives</a>.
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Therein I learned a clever hack:
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GNU tar has a tar extension for <em>volume headers</em>.
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These are essentially labels for your tape archives when you're forced to split an archive across multiple tapes.
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They can (seemingly) hold any text as label including shell scripts.
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What's more is GNU tar and bsdtar <strong>does not</strong> extract these as files!
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This is excellent, because I don't actually want to extract or list the GPT header when using GNU tar or bsdtar.
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This prompted me to <a href="https://github.com/reynir/gptar/pull/1">use a different link indicator</a>.</p>
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<p>This worked pretty great.
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Listing the archive using GNU tar I still get <code>GPTAR</code>, but with verbose listing it's displayed as a <code>--Volume Header--</code>:</p>
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<pre><code class="language-shell">$ tar -tvf disk.img
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Vr-------- 0/0 16896 1970-01-01 01:00 GPTAR--Volume Header--
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-rw-r--r-- 0/0 14 1970-01-01 01:00 test.txt
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</code></pre>
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<p>And more importantly the <code>GPTAR</code> entry is ignored when extracting:</p>
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<pre><code class="language-shell">$ mkdir tmp
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$ cd tmp/
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$ tar -xf ../disk.img
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$ ls
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test.txt
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</code></pre>
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<h2 id="bsd-tar--libarchive"><a class="anchor" aria-hidden="true" href="#bsd-tar--libarchive"></a>BSD tar / libarchive</h2>
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<p>Unfortunately, this broke bsdtar!</p>
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<pre><code class="language-shell">$ bsdtar -tf disk.img
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bsdtar: Damaged tar archive
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bsdtar: Error exit delayed from previous errors.
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</code></pre>
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<p>This is annoying because we run FreeBSD on the host for <a href="https://opam.robur.coop">opam.robur.coop</a>, our instance of <a href="https://git.robur.coop/robur/opam-mirror/">opam-mirror</a>.
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This Autumn we updated <a href="https://git.robur.coop/robur/opam-mirror/">opam-mirror</a> to use the hybrid GPT+tar GPTar <em>tartition table</em><sup><a href="#fn-tartition" id="ref-1-fn-tartition" role="doc-noteref" class="fn-label">[1]</a></sup> instead of hard coded or boot parameter specified disk offsets for the different partitions - which was extremely brittle!
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So we were no longer able to inspect the contents of the tar partition from the host!
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Unacceptable!
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So I started to dig into libarchive where bsdtar comes from.
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To my surprise, after building bsdtar from the git clone of the source code it ran perfectly fine!</p>
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<pre><code class="language-shell">$ ./bsdtar -tf ../gptar/disk.img
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test.txt
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</code></pre>
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<p>I eventually figure out <a href="https://github.com/libarchive/libarchive/pull/2127">this change</a> fixed it for me.
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I got in touch with Emily to let her know that bsdtar recently fixed this (ab)use of GNU volume headers.
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Her reply was basically "as of when I wrote the article, I was pretty sure bsdtar ignored it."
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And indeed it did.
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Examining the diff further revealed that it ignored the GNU volume header - just not "correctly" when the GNU volume header was abused to carry file content as I did:</p>
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<pre><code class="language-diff"> /*
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* Interpret 'V' GNU tar volume header.
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*/
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static int
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header_volume(struct archive_read *a, struct tar *tar,
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struct archive_entry *entry, const void *h, size_t *unconsumed)
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{
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- (void)h;
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+ const struct archive_entry_header_ustar *header;
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+ int64_t size, to_consume;
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+
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+ (void)a; /* UNUSED */
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+ (void)tar; /* UNUSED */
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+ (void)entry; /* UNUSED */
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- /* Just skip this and read the next header. */
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- return (tar_read_header(a, tar, entry, unconsumed));
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+ header = (const struct archive_entry_header_ustar *)h;
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+ size = tar_atol(header->size, sizeof(header->size));
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+ to_consume = ((size + 511) & ~511);
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+ *unconsumed += to_consume;
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+ return (ARCHIVE_OK);
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}
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</code></pre>
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<p>So thanks to the above change we can expect a release of libarchive supporting further flavors of abuse of GNU volume headers!
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🥳</p>
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<section role="doc-endnotes"><ol>
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<li id="fn-tartition">
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<p>Emily came up with the much better term "tartition table" than what I had come up with - "GPTar".</p>
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<span><a href="#ref-1-fn-tartition" role="doc-backlink" class="fn-label">↩︎︎</a></span></li></ol></section>
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</article>
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