Daniel Samson
65bb04d890
The kernel enabled SSE at boot and both kernel and userspace keep live values in XMM (LLVM emits movdqu/movaps for >=16-byte struct copies, plus floats and SIMD), yet the kernel never saved the SSE/FPU register file anywhere — not across switch_context, not across the syscall boundary, not across interrupts. Any value the compiler parked in an XMM register across a kernel entry could be silently clobbered by kernel code, or by whatever the scheduler ran while the task blocked: a whole-kernel, timing-dependent data-corruption bug. It was the real root cause of the "device_enumerate corruption" Heisenbug: the display service read garbage framebuffer geometry (height=0) because findDisplay held the 16-byte .display field live in xmm0 across the claim/mmio_map syscalls, and a timer preemption to the busy device-manager clobbered it. The tell that it was register-only: memory always read correct, and the bug vanished whenever an added syscall spilled the value to the stack. Fix: fxsave/fxrstor the register file in the asm stubs (isr_common and syscall_entry), right after pushing the GP trap frame — before any Zig kernel code can touch XMM — and right before the pops. rbx bridges the exact rsp across the call to interruptDispatch: it is callee-saved, so it survives even a blocking dispatch that context-switches away and back, and `and $-16,%rsp; sub $512,%rsp` gives fxsave its 16-byte-aligned scratch on the kernel stack. Context-switch-time save/restore alone is not enough — kernel code between the interrupt and switchTo already clobbers XMM. The commit-58927ed Gop.init workaround (copy scalar geometry fields rather than the whole descriptor by value) is now redundant but harmless; left in place. Deferred: a fresh task inherits the previous task's XMM (minor info-leak / nondeterminism), and the fxsave runs on every interrupt including ring0->ring0.
DanOS
Codename: Shodan Version: 1
A small resilient operating system, written from scratch in Zig.
Zen of DanOS:
- Resilient Micro-Kernel Architecture.
- Every process run in an isolated user space not kernel space.
- Processes cannot take down the entire OS with it when they die or is killed
- Stable public runtime library, private OS ABI.
- Keeps a stable runtime for user space processes between OS versions (great for backwards compatibility)
- Allows the underlying OS to be changed without effecting applications
- Provides a boundary to enable compatibility between OS's e.g. POSIX, MUSL etc
- Drivers are just isolated processes in user space.
- Thin binaries that can be restarted like applications.
- Useful during driver development.
- Drivers can claim MMIO / ports
- Driver resources (e.g. IRQ/Port/MMIO) claims are automatically cleaned up if the driver dies or is killed
- Drivers can also hook into the process lifecyle to clean up or reset hardware
- No legacy to deal with
- Zig code uses a clean coding style (Zen of Zig)
- Favor reading code over writing code.
- No magic numbers.
- No shortend names unless its for ABI compatibility or acronyms
- Inter-Process Communication (IPC)
- Publish and subscribe to Asynchronous Messages
- Talk to services and processes synchronously
Prerequisites
- Zig 0.16.x — the build is pinned to this line (
.zig-version); other minor versions are rejected, because Zig makes breaking changes between releases pre-1.0. A toolchain manager such as zvm orzigupwill pick up.zig-versionautomatically. - QEMU (
qemu-system-x86_64) — to run and test the kernel. On macOS,brew install qemualso bundles the OVMF firmware below. - OVMF UEFI firmware — the
edk2-ovmfpackage (Arch),ovmf(Debian/Ubuntu), oredk2-ovmf(Fedora); on macOS it ships inside the Homebrewqemuformula. Both the build and the test harness probe the known Arch/Debian/Fedora/macOS layouts and use the first that exists, so no configuration is normally needed. Override with-Dovmf-code=/-Dovmf-vars=(build) if yours lives elsewhere. - Python 3 — for the QEMU integration test harness.
Build
zig build
Produces a FHS-shaped zig-out/ that is the danos filesystem and the boot volume:
the UEFI bootloader at zig-out/EFI/BOOT/BOOTX64.efi, the kernel at
zig-out/system/kernel, init at zig-out/system/services/init, drivers under
zig-out/system/drivers/, and the initial-ramdisk at zig-out/boot/.
Run
Boot it in QEMU with OVMF (opens a display window):
zig build run-x86-64
# distro with OVMF elsewhere:
zig build run-x86-64 -Dovmf-code=/path/OVMF_CODE.fd -Dovmf-vars=/path/OVMF_VARS.fd
Test
zig build test # host unit tests (the platform-independent shared code)
python3 test/qemu_test.py # QEMU integration tests: boots the kernel and asserts
# on its serial output (see docs/testing.md)
The integration harness builds and boots the kernel once per test case, checking memory, the frame allocator, paging (incl. NX and the null guard), the heap, interrupts, and exception handling. It exits non-zero on any failure, so it drops straight into CI.
Documentation
Design notes explaining why behind the code live in
docs/ — start with docs/README.md.
For the hardware needed to run DanOS — minimum specs plus a plain-language guide
matching Intel/AMD CPU generations by name — see
docs/system-requirements.md.
Logo
San Serif Text "Dan OS" with a black karate belt around it.