Merge pull request 'Add "GPTar update" article' (#22) from gptar-update into main

Reviewed-on: #22

articles/gptar-update.md

@@ -0,0 +1,110 @@
+---
+title: GPTar (update)
+date: 2024-10-28
+description: libarchive vs hybrid GUID partition table and GNU tar volume header
+tags:
+  - OCaml
+  - gpt
+  - tar
+  - mbr
+  - persistent storage
+author:
+  name: Reynir Björnsson
+  email: reynir@reynir.dk
+  link: https://reyn.ir/
+---
+
+In a [previous post][gptar-post] 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 *block* that are important to tar headers and *protective* master boot records used in GPT respectively.
+I recommend reading it first if you haven't already for context.
+
+After writing the above post I read an excellent and fun *and totally normal* article by Emily on how [she created **executable** tar archives][tar-executable].
+Therein I learned a clever hack:
+GNU tar has a tar extension for *volume headers*.
+These are essentially labels for your tape archives when you're forced to split an archive across multiple tapes.
+They can (seemingly) hold any text as label including shell scripts.
+What's more is GNU tar and bsdtar **does not** extract these as files!
+This is excellent, because I don't actually want to extract or list the GPT header when using GNU tar or bsdtar.
+This prompted me to [use a different link indicator](https://github.com/reynir/gptar/pull/1).
+
+This worked pretty great.
+Listing the archive using GNU tar I still get `GPTAR`, but with verbose listing it's displayed as a `--Volume Header--`:
+
+```shell
+$ tar -tvf disk.img
+Vr-------- 0/0           16896 1970-01-01 01:00 GPTAR--Volume Header--
+-rw-r--r-- 0/0              14 1970-01-01 01:00 test.txt
+```
+
+And more importantly the `GPTAR` entry is ignored when extracting:
+
+```shell
+$ mkdir tmp
+$ cd tmp/
+$ tar -xf ../disk.img
+$ ls
+test.txt
+```
+
+## BSD tar / libarchive
+
+Unfortunately, this broke bsdtar!
+
+```shell
+$ bsdtar -tf disk.img
+bsdtar: Damaged tar archive
+bsdtar: Error exit delayed from previous errors.
+```
+
+This is annoying because we run FreeBSD on the host for [opam.robur.coop](https://opam.robur.coop), our instance of [opam-mirror][opam-mirror].
+This Autumn we updated [opam-mirror][opam-mirror] to use the hybrid GPT+tar GPTar *tartition table*[^tartition] instead of hard coded or boot parameter specified disk offsets for the different partitions - which was extremely brittle!
+So we were no longer able to inspect the contents of the tar partition from the host!
+Unacceptable!
+So I started to dig into libarchive where bsdtar comes from.
+To my surprise, after building bsdtar from the git clone of the source code it ran perfectly fine!
+
+```shell
+$ ./bsdtar -tf ../gptar/disk.img
+test.txt
+```
+
+I eventually figure out [this change][libarchive-pr] fixed it for me.
+I got in touch with Emily to let her know that bsdtar recently fixed this (ab)use of GNU volume headers.
+Her reply was basically "as of when I wrote the article, I was pretty sure bsdtar ignored it."
+And indeed it did.
+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:
+
+```diff
+ /*
+  * Interpret 'V' GNU tar volume header.
+  */
+ static int
+ header_volume(struct archive_read *a, struct tar *tar,
+     struct archive_entry *entry, const void *h, size_t *unconsumed)
+ {
+-       (void)h;
++       const struct archive_entry_header_ustar *header;
++       int64_t size, to_consume;
++
++       (void)a; /* UNUSED */
++       (void)tar; /* UNUSED */
++       (void)entry; /* UNUSED */
+
+-       /* Just skip this and read the next header. */
+-       return (tar_read_header(a, tar, entry, unconsumed));
++       header = (const struct archive_entry_header_ustar *)h;
++       size = tar_atol(header->size, sizeof(header->size));
++       to_consume = ((size + 511) & ~511);
++       *unconsumed += to_consume;
++       return (ARCHIVE_OK);
+ }
+```
+
+So thanks to the above change we can expect a release of libarchive supporting further flavors of abuse of GNU volume headers!
+🥳
+
+[gptar-post]: gptar.html
+[tar-executable]: https://uni.horse/executable-tarballs.html
+[opam-mirror]: https://git.robur.coop/robur/opam-mirror/
+[libarchive-pr]: https://github.com/libarchive/libarchive/pull/2127
+
+[^tartition]: Emily came up with the much better term "tartition table" than what I had come up with - "GPTar".