+++
title = "Single UEFI executable for kernel+initrd+cmdline"
date = 2015-02-25T15:55:16+00:00
[taxonomies]
tags = ["UEFI", "fedora", "SecureBoot"]
+++

Lately Kay Sievers and David Herrmann created a UEFI
[loader stub](http://cgit.freedesktop.org/gummiboot/tree/src/efi/stub.c), which starts a linux kernel
with an initrd and a kernel command line, which are COFF sections of the executable.
This enables us to create single UEFI executable with a standard distribution kernel,
a custom initrd and our own kernel command line attached.

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Of course booting a linux kernel 
directly from the UEFI has been possible before with the kernel EFI stub. But to add an initrd and 
kernel command line, this had to be specified at kernel compile time.

To demonstrate this feature 
and have a useful product, I created a shell script, which creates a "rescue" image on Fedora with 
the rescue kernel and rescue initrd. The kernel command line "rd.auto" instructs dracut to assemble 
all devices, while waiting 20 seconds for device appearance "rd.retry=20" and drop to a final shell 
because "root=/dev/failme" is specified (which does not exist of course). Now in this shell you can 
fsck your devices, mount them and repair your system.

The shell script can be downloaded and viewed on 
[github](https://github.com/haraldh/mkrescue-uefi).

To run the script, you will need to install gummiboot >= 46 and binutils. 
```console
# yum install gummiboot binutils
```

Run the script: 
```console
# bash mkrescue-uefi.sh BOOTX64.EFI
```
 
Copy `BOOTX64.EFI` to e.g. a USB stick to `EFI/BOOT/BOOTX64.EFI` in the FAT boot partition 
and point your BIOS to boot from the USB stick. Voilà!

A rescue USB stick with just one file! :-)