3cpio
=====
3cpio is a tool to manage initramfs cpio files for the Linux kernel. The Linux
kernel's
[initramfs buffer format](https://www.kernel.org/doc/html/latest/driver-api/early-userspace/buffer-format.html)
is based around the `newc` or `crc` cpio formats. Multiple cpio archives can be
concatenated and the last archive can be compressed. Different compression
algorithms can be used depending on what support was compiled into the Linux
kernel. 3cpio is tailored to initramfs cpio files and will not gain support for
other cpio formats.
As of now, 3cpio supports examining, listing, and extracting the content of the
initramfs cpio.
**Note**: The Rust crate is named threecpio, because package names are not
allowed to start with numbers.
Usage examples
--------------
List the number of cpio archives that an initramfs file contains:
```
$ 3cpio --count /boot/initrd.img
4
```
Examine the content of the initramfs cpio on an Ubuntu 24.04 system:
```
$ 3cpio --examine /boot/initrd.img
0 cpio
77312 cpio
7286272 cpio
85523968 zstd
```
This initramfs cpio consists of three uncompressed cpio archives followed by a
Zstandard-compressed cpio archive.
List the content of the initramfs cpio on an Ubuntu 24.04 system:
```
$ 3cpio --list /boot/initrd.img
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/.enuineIntel.align.0123456789abc
kernel/x86/microcode/GenuineIntel.bin
.
usr
usr/lib
usr/lib/firmware
usr/lib/firmware/3com
usr/lib/firmware/3com/typhoon.bin.zst
[...]
```
The first cpio contains only the AMD microcode. The second cpio contains only
the Intel microcode. The third cpio contains firmware files and kernel modules.
Extract the content of the initramfs cpio to the `initrd` subdirectory on an
Ubuntu 24.04 system:
```
$ 3cpio --extract -C initrd /boot/initrd.img
$ ls initrd
bin cryptroot init lib lib.usr-is-merged run scripts var
conf etc kernel lib64 libx32 sbin usr
```
Benchmark results
-----------------
### Listing the content of the initrd
Runtime comparison measured with `time` over five runs on different initramfs
cpios:
| Ryzen 7 5700G | 6.5.0-27-generic | zstd¹ | 102 MB | 3496 | 0.052s | 14.243s | –³ |
| Ryzen 7 5700G VM | 6.8.0-22-generic | zstd¹ | 63 MB | 1934 | 0.042s | 7.239s | –³ |
| Ryzen 7 5700G VM | 6.8.0-22-generic | zstd² | 53 MB | 1783 | 0.061s | 0.452s | 0.560s |
| RasPi Zero 2W | 6.5.0-1012-raspi | zstd¹ | 24 MB | 1538 | 0.647s | 56.253s | –³ |
| RasPi Zero 2W | 6.5.0-1012-raspi | zstd² | 30 MB | 2028 | 1.141s | 2.286s | 6.118s |
| RasPi Zero 2W | 6.8.0-1002-raspi | zstd¹ | 51 MB | 2532 | 0.713s | 164.575s | –³ |
| RasPi Zero 2W | 6.8.0-1002-raspi | zstd -1² | 47 MB | 2778 | 1.156s | 2.842s | 9.508s |
| RasPi Zero 2W | 6.8.0-1002-raspi | xz² | 41 MB | 2778 | 6.922s | 13.451s | 35.184s |
**Legend**:
1. generated by initramfs-tools
2. generated by `dracut --force --${compression}`. On Raspberry Pi Zero 2W there
is not enough memory for the default `zstd -15`. So using the default from
initramfs-tools there: `dracut --force --compress "zstd -1 -q -T0"`
3. lsinitrd only reads the first two cpio archives of the file, but the
initramfs consists of four cpios.
**Results**:
* 3cpio is 87 to 274 times faster than lsinitramfs for images generated by
initramfs-tools.
* 3cpio is two to eight times faster than lsinitramfs for images generated
by dracut.
* 3cpio five to nine times faster than lsinitrd for images generated by dracut.
Commands used:
```
time lsinitramfs /boot/initrd.img-${version} > /dev/null
time lsinitrd /boot/initrd.img-${version} > /dev/null
```
List the content of single cpio archive that is not compressed (see
[doc/Benchmarks.md](doc/Benchmarks.md) for details) on a Raspberry Pi Zero 2W:
| `3cpio -t initrd.img` | 84.3 ± 1.1 | 82.1 | 87.0 | 1.00 |
| `bsdcpio -itF initrd.img` | 98.4 ± 0.9 | 96.4 | 101.0 | 1.17 ± 0.02 |
| `cpio -t --file initrd.img` | 1321.2 ± 2.8 | 1314.6 | 1327.6 | 15.68 ± 0.20 |
| `3cpio -tv initrd.img` | 109.2 ± 1.1 | 106.9 | 111.7 | 1.00 |
| `bsdcpio -itvF initrd.img` | 114.9 ± 1.1 | 112.6 | 117.4 | 1.05 ± 0.01 |
| `cpio -tv --file initrd.img` | 1423.0 ± 3.5 | 1417.1 | 1440.6 | 13.03 ± 0.13 |
### Extracting the content of the initrd
Benchmarking the time to extraction initrd:
| Ryzen 7 5700G | noble | 6.8.0-35-generic | 70 MB | 2097 | 0.107s | 6.698s |
| Ryzen 7 5700G | jammy | 6.8.0-35-generic | 112 MB | 3789 | 0.455s | 2.217s |
| Ryzen 7 5700G | bookworm | 6.1.0-21-amd64 | 62 MB | 2935 | 0.268s | 1.362s |
| RasPi Zero 2W | noble | 6.8.0-1005-raspi | 53 MB | 2534 | 5.075s | 173.847s |
Raw measurements can be found in [doc/Benchmarks.md](doc/Benchmarks.md).
Naming and alternatives
-----------------------
The tool is named 3cpio because it is the third cpio tool besides
[GNU cpio](https://www.gnu.org/software/cpio/) and `bsdcpio` provided by
[libarchive](https://www.libarchive.org/). 3cpio is also the third tool that can
list the content of initramfs cpio archives besides `lsinitramfs` from
[initramfs-tools](https://tracker.debian.org/pkg/initramfs-tools) and `lsinitrd`
from [dracut](https://github.com/dracut-ng/dracut-ng).