articles/gptar-update.md

@@ -1,110 +0,0 @@
----
-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".