Options for creating a UEFI application image


This post is about some of the ways in which an operating system / kernel can be made to boot as a UEFI application image (through UEFI firmware).

To be clear, let’s make sure we have our definitions straight:

  • UEFI firmware: The vendor firmware itself, which may or may not support legacy BIOS options. On the OSDev side, we won’t really have control over this. For emulation, I’ll be using TianoCore’s OVMF (OvmfPkgX64) specifically. More setup instructions here.
  • UEFI application image: A relocatable PE/COFF executable file which the UEFI firmware loads through a FAT partition, usually the file /EFI/BOOT/BOOTX64.EFI on the filesystem.

With RTEMS, we use a cross-compiler toolchain (for eg. x86_64-rtems5-gcc, instead of the host computer’s gcc). The toolchain is built automatically by the RTEMS Source Builder (RSB), which does cool things like pull in a release version of the various tools, backports patches that the RTEMS community may need (that are not in a release version yet), and compiles the entire toolchain. These tools (gcc, binutils, etc.) are then used to compile all of the RTEMS kernel, link the kernel to user applications, the testsuite, etc.


Simplest: Use gnu-efi, then use objcopy to convert ELF to PEI

Likely the simplest option, and the one used by FreeBSD as well, and documented on the OSDev Wiki, is to:

  • Build an ELF64 shared library including an elf_main function
  • Link it with gnu-efi’s libraries and crt0 runtime
  • Use objcopy with the --target=efi-app-x86_64 / --target=pei-x86-64
    • x86_64-rtems5-objcopy doesn’t support the pei-x86-64 target at the moment, but this is easy to add with this patch to bfd
  • Put the PE image file on a FAT partition. Here’s a quick script I used.
  • Boot the PE image with UEFI firmware now

I’ve tested this process with a “hello world” UEFI image using the x86_64-rtems5-* toolchain and confirmed it works in QEMU.

(Reproduction steps:

  • Use the RSB with my bfd patch to add the pei-x86-64 target to x86_64-rtems5-objcopy. (I’ll try to have this upstreamed based on discussions on the RTEMS developers mailing list.)
  • Download gnu-efi
  • Run make CC=x86_64-rtems5-gcc OBJCOPY=x86_64-rtems5-objcopy ARCH=x86_64 within the project
  • Run the make-img.sh script with the the path to the t.efi test file (within gnu-efi-3.0.8/x86_64/apps/ at the moment)
  • Run qemu-system-x86_64 --bios /path/to/OVMF.fd -net none -cpu qemu64 -drive file=/path/to/uefi.img,if=ide)

Note: This will involve making gnu-efi a part of the RTEMS build process. I’m not sure where this will fit in yet and if there might be license incompatibility issues here.

Use a tool such as iPXE’s elf2efi utility

The iPXE bootloader includes a utility that can convert an existing ELF to an EFI bootable image. I haven’t explored this option much since the one using objcopy seemed more standardized and one that would need less maintenance overall to me. There would also be licensing considerations, and we would have to confirm that the tool runs on all hosts that RTEMS intends to support for development.


Use an assembly-generated PE header for a “chimera” ELF?

The wimboot project associated with iPXE includes a prefix.S file which seems to generate the PE header table automagically in assembly, allowing us to possibly create a chimeric file which can act like a PE - I’m not sure about the differences between ELF and PE and how compatible they are, so I can’t speak to how viable this method is, but it seems like something that has worked for some in the past; namely the Linux kernel uses a technique called the EFI boot stub (or EFISTUB for short), using a header.S file to masquerade itself as a PE/COFF image. This is documented here and on the Arch Wiki here as well.

Given the possible license incompatibilities with the RTEMS project and the need to shave a PE file-format yak, I’m leaning away from this method.

P.S. - I’d like to say thanks to Chris Johns for helping me discover that the last 2 options were even a possibility. Thank you! :D

Conclusion: I’m leaning towards the first method, but I’d love to hear what you think about the options I’ve laid out, and for any others I may have missed. Hit me up! The RTEMS mailing list discussion thread for this blog post is here.

Important resources: