---
name: schunix-rewrite-priorities
description: "The ordered list of what actually blocks Schunix, established by a full read of the kernel on 2026-08-01"
metadata: 
  node_type: memory
  type: project
  originSessionId: d1a1ea76-f919-411c-b0ef-0e1bc5064563
  modified: 2026-08-01T19:01:25.126Z
---

Assessed the whole Schunix kernel (~12k hand-written lines) on 2026-08-01.
Code quality is good; the problem is that xv6 lacks the *concepts* a real UNIX
needs, so the fix is missing struct fields and subsystems, not bug fixing.

Order to work in (each item unblocks the next):

1. ~~**`struct proc` fields.**~~ **DONE 2026-08-01.** `xstate`, `killsig`,
   `pgid`, `sid`, and uid/euid/suid + gid/egid/sgid added; `exit()` takes a
   wait(2)-encoded status; waitpid() reports it. Verified: `sh -c "exit 42"`
   yields `rc=42`, `id` prints `uid=0(root)`. Also required coupling
   TIOCGPGRP to the real pgid — see [[schunix-tty-fg-pgrp-coupling]].
2. **Signal delivery on the trap return path.** Currently only fires on
   syscall return (`kernel/sys/syscall.c`), so a process spinning in userland
   never receives a signal.
3. **Fault-recoverable `copy_from_user`/`copy_to_user` with an exception
   table.** `uva2ka()` returns a P2V alias valid for exactly one page; two
   separate historical crashes trace to that single design flaw.
4. **A page-fault handler.** `trap.c` has no `T_PGFLT` case at all — hence no
   demand paging, no COW, no lazy stack growth.
5. **SCHFS directory format.** Every directory is ≥113KB on disk, capped at
   119KB; `DIRSIZ` is 14; `dirent.inum` is a `ushort` (65535-inode ceiling).
   Disk-format break, so decide before accumulating images worth keeping.
6. **Then** the Ring 0A/0B split. Its own Phase 1 is "just file moves" — doing
   it before 1-5 means moving the same files twice.

Separately, a real confirmed bug: `kernel/arch/x86_64/trapasm.S` saves
`rax/rcx/rdx` to three **global** scratch words (`_sc_rax` etc. in `vm.c`) with
a comment claiming "IF is off, no race" — but the Makefile runs `-smp 2`, and
`cli` only masks the local CPU. Two CPUs in `syscall_entry` clobber each
other's saved user `rip`. Use per-CPU storage.

Also: `KSTACKSIZE` is 4096 (one page) on x86-64; Linux uses 16K. The stack
canary exists because it really did overflow.

See [[schunix-linux-abi-decision]], [[schunix-layout-migration]].