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: M5 of 10 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

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
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

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