microkernel

A small L4-style microkernel for x86_64, written in C. Drivers, file systems and networking run as isolated user-space servers that talk over IPC (inter-process communication) and hold capabilities. Goal: a driver can crash and be restarted without taking the system down.

See docs/DESIGN.md for the architecture and milestones.

Status: M7 of 10 done, M8 half done.

• M0: boots via Limine on QEMU, serial console, memory map
• M1: own GDT (Global Descriptor Table), TSS (Task State Segment) and IDT (Interrupt Descriptor Table); CPU exceptions reported with registers and a backtrace; double faults caught on a separate stack
• M2: physical frame allocator, the kernel's own 4-level page tables with W^X (write xor execute) section permissions, separate address spaces, slab caches for kernel objects
• M3: LAPIC (local APIC) timer, kernel threads with guard-paged stacks, context switching, preemptive priority scheduler
• M4: user mode: ELF loader, syscall entry, a root task loaded as a boot module; a faulting user process is killed and the kernel keeps running
• M5: capability tables, synchronous IPC endpoints with a direct-switch fast path, notifications, capability transfer, and object creation limited by factory capabilities
• M6: the root task reads a manifest and starts a console server and its clients, handing each exactly the capabilities its manifest line lists
• M7: the console server is a user-space serial driver: it holds COM1's I/O ports and IRQ 4 as capabilities, gets interrupts as notifications, and is the only way the programs it serves can print
• M8 (first part): memory capabilities (frames a process maps into its own address space, with per-program quotas) and a user-space PCI server that hands each driver only its own device's ports and interrupt line; a virtio disk driver finds the disk and reads its size

Build and run

Requirements: gcc, binutils, make, git, xorriso, qemu-system-x86_64 (on Ubuntu/WSL2: sudo apt install build-essential xorriso qemu-system-x86).

make run     # build ISO and boot in QEMU, serial output in the terminal (Ctrl+C to quit)
make test    # boot once per test case in scripts/test.sh and check the output
make debug   # QEMU waits for GDB: gdb build/kernel.elf -ex 'target remote :1234'

make run CMDLINE="selftest=pagefault"     # watch a page-fault report
make run CMDLINE="selftest=doublefault"   # watch a double fault caught on its own stack
make run CMDLINE="selftest=write-text"    # writing to kernel code faults (W^X)
make run CMDLINE="selftest=stack-overflow" # a thread overflows into its guard page
make run CMDLINE="selftest=user-pagefault" # a user process crashes; the kernel survives
make run CMDLINE="selftest=ipc-server-dies" # a server dies mid-call; its client gets -ERR_DEAD
make run CMDLINE="selftest=app-crash"       # one application crashes; the system carries on
make run CMDLINE="selftest=console-input"   # type a line and press Enter: it arrives by IRQ 4

The first build clones the Limine bootloader into limine/.

To turn a backtrace address into a source line:

addr2line -e build/kernel.elf -f -p 0xffffffff800020ec