Merge pull request 'Add "GPTar update" article' (#22) from gptar-update into main
Reviewed-on: #22
This commit is contained in:
commit
e993307d83
1 changed files with 110 additions and 0 deletions
110
articles/gptar-update.md
Normal file
110
articles/gptar-update.md
Normal file
|
@ -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".
|
Loading…
Reference in a new issue