| Filename | Latest commit message | Latest commit date |
|---|---|---|
1e52cb591b
Boot died straight after "exiting boot services" with a KVM internal error,
RIP=0x801a5dc5 and RBX=0x80000000 -- the framebuffer base. The bytes at RIP were
0x0C0E13 and 0xD1D5DC: SCH_BG and a grey from the splash's metal gradient. The
machine was executing the picture of itself starting up.
The hand-off to the kernel writes rbx and did not say so:
"mov %1, %%rbx\n\t" <- writes rbx
"jmp *%2\n\t"
: : "r"(PML4_ADDR), "r"(fb), "r"(entry)
: "memory"); <- rbx not declared clobbered
With rbx absent from the clobber list the compiler may allocate an input there,
and clang 21 allocated `entry`:
movq %rax, %cr3
movq %rcx, %rbx <- the framebuffer, into rbx
jmpq *%rbx <- %2 was rbx, and now holds the framebuffer
Declaring "rbx" forbids the allocation. Verified in the generated assembly
rather than by reasoning: jmpq *%rbx became jmpq *%r14, and the machine now
boots through to schinit: ready.
This had been latent since the loader was written. It worked only because every
compiler that had built it happened to choose a different register, and it
surfaced the first time the binary was relinked with a different clang -- which
only happened because the previous commit fixed the Makefile dependency that had
been leaving the loader stale. A stale artifact hides the bug the fresh one
reveals.
It is also the third time this project has been broken by a register the code
depends on but never asks for: docs/history/kernel.md section 4 (the BIOS
loader passing the framebuffer in EAX by luck) and section 8 (%fs:0 used as
scratch when the ABI reserves it). Written up as section 7b, with the one-line
recipe that settles all three -- read the generated assembly.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
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||
| .. | ||
| common | ||
| uefi | ||
| README.md | ||
Booting SchOS
One firmware path:
boot/
uefi/ the loader: a 64-bit PE application
common/ the user interface, shared with nothing else now
There were two. boot/bios/ held an MBR, an El Torito CD stub and a real-mode
stage 2, and it was a separate directory rather than #ifdefs because the two
shared almost nothing: a BIOS loader lives in 16-bit real mode and talks to the
firmware through software interrupts, while UEFI hands a loader long mode with
paging already on and a table of function pointers to call.
That asymmetry is what removed it. Everything since roughly 2012 boots UEFI,
CSM — the firmware-side BIOS emulation that let legacy loaders run at all — has
been going away since Intel dropped support for it in 2020, and the cost of the
second path was not the code but the tax on everything that touched it: stage 2
had a 65-sector ceiling that it silently overflowed, and the resolution menu
could not be shared because VBE mode setting is INT 10h and needs real mode.
The contract below survives it. It was written to let a second loader exist, and it is worth keeping in that shape: it is what the kernel expects, stated once, rather than a description of whoever happens to be calling.
What the loader must hand the kernel
Whichever path runs, the kernel expects the same things. This is the contract; a second loader is correct exactly when it satisfies it.
| Where | Notes | |
|---|---|---|
| Kernel image | loaded per its ELF program headers | entry at entry64 |
| CPU state | see below -- the two paths differ here | |
| Memory map | E820_MAP_ADDR, 24-byte entries, count at E820_COUNT_ADDR |
kernel/arch/x86_64/e820.c reads it |
| Framebuffer | VBE_INFO_ADDR |
address, pitch, width, height, depth |
| Verbose flag | BOOT_VERBOSE_FLAG_ADDR |
set by the F8 menu |
The addresses are defined once, in include/memlayout.h and include/schboot.h.
The kernel does not care which firmware produced them.
The entry point
UEFI hands a loader long mode with paging already on. Dropping back to
protected mode only to climb out again is a mode switch whose failure mode is a
triple fault with no console, so the kernel publishes an entry that takes the
machine as it already is.
kernel/arch/x86_64/entry.S carries a sch64_header — magic SCH64BOO,
8-byte aligned within the first 16 KB, followed by a 64-bit physical address —
found by scanning. include/schboot.h holds the magic so loader and kernel
cannot disagree.
The 32-bit multiboot entry is still in entry.S and now has no caller: the
BIOS loader was the only thing that used it. It is left in place rather than
removed in the same change that removed the loader, because deleting kernel
entry code is its own piece of work with its own way of failing silently.
What that entry expects:
- long mode, interrupts off
- CR3 pointing at tables that identity-map the low 4 GB and map the same
4 GB at
0xffff800000000000.entry64low's first instruction is amovabsto a high address, so identity alone triple faults immediately. - RBX = the framebuffer physical address, and never 1 — that value marks an
application processor in
entry64high.
The kernel loads its own GDT at that entry (entry64uefi), rather than the
loader doing it. A loader arriving in long mode is still on the firmware's
descriptors, and the first thing the kernel does that depends on its own
selectors takes a #GP against a GDT the firmware no longer owns — it
surfaced as unexpected trap 13 ... cs=38, a long way from the cause.
e820.c reads a BIOS memory map. UEFI hands out a different structure
(EFI_MEMORY_DESCRIPTOR, from GetMemoryMap), so the UEFI loader has to
translate it into the same 24-byte entries rather than inventing a second
format for the kernel to learn.
What booting looks like now
make esp stages EFI/BOOT/BOOTX64.EFI and schos/kernel, and QEMU is
pointed at OVMF with that directory as a virtual FAT volume — make qemu.
make iso builds a disc with a single El Torito entry pointing at a FAT image
holding the same two files.
Neither is an installable disk. A real GPT image with a real FAT partition is the installer's job, and until it exists there is nothing here that writes to a disk a machine could boot from unaided.
Secure Boot is not addressed either: the loader is unsigned, so a machine enforcing it will refuse to run this.