BIOS stage 2 subtract overflows

[hawkw]

I'm attempting to migrate my kernel from v0.10.x of bootloader, and after switching to v0.11.x and running in QEMU, I see a panic in the BIOS bootloader's stage 2 code:

The subtract that overflows appears to be on this line:

bootloader/bios/stage-2/src/fat.rs

Lines 96 to 99 in fa87d01

	let data_sectors = total_sectors
	- (self.reserved_sector_count as u32
	+ (self.num_fats as u32 * self.fat_size_in_sectors())
	+ root_dir_sectors);

I'm creating the image using BiosBoot::create_disk_image, and passing it to QEMU with the arguments -drive format=raw,file=/path/to/bios.img as suggested in the documentation on creating a disk image, so I don't think anything about my configuration is particularly out of the ordinary. I can provide additional information if there's anything that would help, though.

Bootloader version: 0.11.2
QEMU version:

$ qemu-system-x86_64 --version
QEMU emulator version 7.2.0
Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers

[phil-opp]

Thanks for reporting! I'm not sure what's going wrong here yet. Do you have your code online somewhere so that I can do some debugging?

[hawkw]

@phil-opp The branch updating my OS to use bootloader v0.11.2 is here: hawkw/mycelium#406

Unfortunately, it's definitely not a minimal reproduction of the crash. I did try running the tests in the bootloader repo on my machine with the same version of QEMU, and they all passed for me. So, it seems like something about my code is required to reproduce this...I'm just not sure what. Happy to keep debugging if there's anything I can do to help.