Hacker News Reader

Comment by distcs | Hacker News Reader

Thanks for the answer! I've got some more questions now. Sorry, but if anyone is willing to take a stab at these questions, it'd be helpful to me.

1. IIUC PE executables are Windows executables. So a Linux system that targets UEFI ends up writing a PE executable to the EFI System Partition?

2. I know that some UEFIs (or is it all?) support BIOS boot sector as backward compatibility feature? How does that work? If I write a "hello world" program in pure machine code in the 1st sector of the boot disk, would UEFI read that and execute that? How would it even know whether what's in the first sector is valid code or garbage? By checking the magic 0x55 0xaa at the end of the boot sector?

View on HN

> 1. IIUC PE executables are Windows executables. So a Linux system that targets UEFI ends up writing a PE executable to the EFI System Partition?

Short answer: Yes.

Longer answer: Yes, but it is not exactly a classic Windows desktop application. A PE file itself is at its core just a way to store data in a somewhat structured way. It has metadata (like imports/exports and the target architecture) and actual contents (like data and code). I think it is a good idea to reuse an already existing, common executable file format for this use case. That the choice was PE instead of ELF or Mach-O is, IMO, the result of Microsoft being in the UEFI consortium. They are a big player and likely pushed for this. Whether or not ELF would have been a better choice, I can not say, but at this point I don't think it matters.

More info on that topic at OSDev Wiki: https://wiki.osdev.org/UEFI#Binary_Format

File output on my GRUB install:

  /boot/EFI/GRUB/grubx64.efi: PE32+ executable (EFI application) x86-64 (stripped to external PDB), for MS Windows, 4 sections

"I think it is a good idea to reuse an already existing, common executable file format for this use case."

Similarly how many file formats are actually just pkzip files with standardzied names and contents.

1. Yes, that's exactly right. It installs a GRUB version compiled into a PE executable, typically.

2. Boot sector support requires UEFI CSM support, which most PCs have (but ARM based devices and Intel Macs don't). With the deprecation of 32-bit architectures in a lot of operating systems, some vendors also started dropping CSM support in some devices, but most devices still ship with CSM either enabled by default or as an option in the settings.

Reply on news.ycombinator.com