B2 — Linux AArch64 syscalls vs ctos SVC

Track B child issue #42 (parent #40). Frame: ADR-031. Freestanding ABI: ADR-021 / syscall.md.

This page is a gap map. It does not add Linux syscall numbers to src/. It does not decide compat layer vs reimplement vs never (B6 / #46). Not claiming Linux userspace. File presence of this note is not a Linux ABI.

Probe (what Verified means here)

Claim	Probe	Status
Linux AArch64 numbers in the table	Read `include/uapi/asm-generic/unistd.h` from Linux v6.10 (`torvalds/linux`). AArch64 uses that generic table (`svc #0`, number in `x8`).	Document inspection
ctos public / reserved SVC surface	Read `src/syscall.rs` (`SYS_*` 16–23) + ADR-021 + ADR-027 + reserved 0–2 in ADR-013 on this tree	Document inspection
Linux userspace / musl / Alpine on the guest	No such serial marker	Not claimed. Ledger “Guest runs host apps” stays Planned.

A later kernel qemu-smoke is a different row. This mile did not edit src/.

Status words

Status	Means
present	Same Linux AArch64 number and comparable semantics, callable by a Linux `svc #0` / `x8` binary with no translation.
partial	A Track A SVC exists with a related effect. Number, calling convention, args, or meaning still differ.
absent	No ctos analog. B3–B6 may still discuss a named subset. Not a promise to implement.
never-per-ADR-031	Would imply a full Linux host / distro / container promise that ADR-031 already ruled out (guest OCI also ADR-029).

No Linux AArch64 syscall is present. The convention and the number space both miss.

Calling convention (the first gap)

Item	Linux AArch64	ctos (ADR-021)
Instruction	`svc #0` (immediate is always 0)	`SVC #<n>` — `n` is the number
Number register	`x8`	Immediate (not `x8`)
Args	`x0`–`x5`	`x0`, `x1`, `x2` only
Return	`x0`; negative Linux errno	`x0`; `0` or `u64::MAX` reject (not errno)
Unknown number	`-ENOSYS`	Unhandled immediate parks (fail-closed)

Linux svc #0 collides with the ADR-013 first-mile probe (SVC #0). ADR-021 rejected adopting the Linux convention for that reason. A translation shim would be a later epic after B6 — not this page.

Public ctos numbers 16–26 are also not the Linux meaning of those integers:

Immediate / `x8`	Linux AArch64 (v6.10 generic)	ctos
0	`io_setup`	ADR-013 first-mile return (reserved)
1	`io_destroy`	ADR-013 standing announce (reserved)
2	`io_cancel`	ADR-013 standing restore (reserved)
16	`fremovexattr`	`exit`
17	`getcwd`	`uart_write`
18	`lookup_dcookie`	`yield`
19	`eventfd2`	`fs_create`
20	`epoll_create1`	`fs_open`
21	`epoll_ctl`	`fs_read`
22	`epoll_pwait`	`fs_write`
23	`dup`	`fs_close`
24	`dup3`	`net_mac`
25	`fcntl`	`net_ping`
26	`inotify_init1`	`net_udp_dns`

You cannot “just accept Linux numbers” in the SVC immediate, and you cannot treat today’s 16–26 as Linux.

ctos surface (inspection)

Reserved 0–2 are probe-only. Public:

ctos #	Name	Related Linux name(s)	Why not `present`
16	`exit`	`exit` (93), `exit_group` (94)	Ends the standing EL0 trip; no process table; not `exit_group`.
17	`uart_write`	`write` (64) to a TTY	PL011 only; no fd; cap 64; `\n` → `\r\n`.
18	`yield`	`sched_yield` (124)	Dispatch-only hint; does not run EL1 `sched::yield_now` (ADR-010).
19	`fs_create`	`openat` (56) + `O_CREAT`	Flat memfs name; no `dirfd`, flags, or mode.
20	`fs_open`	`openat` (56)	Existing name only; FAT `/probe` and `/hello` are the known on-disk files.
21	`fs_read`	`read` (63)	Handle 1–4; cap 64; no stdin. UART RX is kernel-side, not this SVC.
22	`fs_write`	`write` (64)	memfs only (FAT write is `ReadOnly`); cap 64; not fd 1/2.
23	`fs_close`	`close` (57)	Drops the handle; the file stays. Four handles max.
24	`net_mac`	—	Guest MAC copy; not Linux `dup3` (24). Kernel owns NIC (ADR-068).
25	`net_ping`	—	Quiet ICMP echo; not Linux `fcntl` (25).
26	`net_udp_dns`	—	Quiet UDP DNS probe; not Linux `inotify_init1` (26). DNS bait only (ADR-071).

Unknown immediates park. Not POSIX. Not Linux VFS.

Gap table (representative Linux AArch64 set)

Numbers from Linux v6.10 asm-generic/unistd.h. AArch64 has no fork, open, creat, dup2, pipe, poll, or select syscalls — those are clone / openat / dup3 / pipe2 / ppoll / pselect6. __NR_syscalls in that header is 463. This table is a slice, not the full list.

Process and lifetime (B3)

Linux #	Name	ctos status	Notes
93	`exit`	partial	Analog `SYS_EXIT` 16. Halts the EL0 trip and returns to the EL1 caller. Status is recorded. Not a thread/process exit.
94	`exit_group`	absent	No process group. musl/glibc CRT usually ends here.
—	`fork`	absent	Not in the Linux AArch64 table. Userspace `fork()` is `clone`. Stance: ADR-035 / B3 / #43.
220	`clone`	absent	No child address space, no `CLONE_`. Standing EL0 (ADR-024) is one loaded trip, not `clone` (ADR-035). `CLONE_NEW` is never-per-ADR-031 (see namespace rows).
435	`clone3`	absent	Same class as `clone`.
221	`execve`	absent	A3 maps freestanding `PT_LOAD` and rejects `PT_INTERP`. A9 FAT `/hello` is an EL1 load path, not `execve` (ADR-035). ELF / auxv / interpreter gap: ADR-033 (B4).
281	`execveat`	absent	Same class as `execve`.
260	`wait4`	absent	No child to wait for. ADR-035.
95	`waitid`	absent	ADR-035.
129	`kill`	absent	No pid / signal delivery.
172	`getpid`	absent	No process table.
178	`gettid`	absent	No tid.
96	`set_tid_address`	absent	Typical musl/glibc `_start` requirement.

File descriptors and I/O (B5 concept compare: ADR-034)

Linux #	Name	ctos status	Notes
56	`openat`	partial	`fs_open` 20 / `fs_create` 19. Path is `/` + `[a-z0-9_-]` (1–15). No `dirfd`, `O_*`, or mode. At most 8 files. ADR-027.
437	`openat2`	absent	No `open_how`.
57	`close`	partial	`fs_close` 23. File stays after close.
63	`read`	partial	`fs_read` 21. Cap 64. No fd 0 / line discipline.
64	`write`	partial	`uart_write` 17 (console, no fd) or `fs_write` 22 (memfs). Neither is POSIX `write(1, …)`.
65 / 66	`readv` / `writev`	absent	No iovec.
67	`pread64`	absent	Handle offset exists inside VFS; no positioned SVC.
62	`lseek`	absent	No seek SVC.
29	`ioctl`	absent	No TTY (`TCGETS` / `TIOCGWINSZ`), no block `ioctl`. PL011 is kernel-owned.
25	`fcntl`	absent	No `F_GETFL` / `F_DUPFD`.
23 / 24	`dup` / `dup3`	absent	Handles 1–4 only; no dup. Linux 23 is `dup`; ctos 23 is `fs_close`.
59	`pipe2`	absent	No pipes.
61	`getdents64`	absent	No directory listing. Flat namespace.
79 / 80	`newfstatat` / `fstat`	absent	No `stat`.
291	`statx`	absent	No `stat`.
48	`faccessat`	absent
34	`mkdirat`	absent	No directories.
35	`unlinkat`	absent
38	`renameat`	absent
49	`chdir`	absent	No cwd.
17	`getcwd`	absent	Linux 17 is `getcwd`; ctos 17 is `uart_write`.

Memory (loader / libc heap)

Linux #	Name	ctos status	Notes
214	`brk`	absent	Kernel first-fit heap is EL1 (ADR-009). No user program break.
222	`mmap`	absent	A3 maps `PT_LOAD` at load time into the user window. No mmap SVC. Anonymous / file maps are out.
215	`munmap`	absent
226	`mprotect`	absent	Image W^X is a kernel policy (ADR-015), not an EL0 syscall.
233	`madvise`	absent

Scheduling, time, sync, identity

Linux #	Name	ctos status	Notes
124	`sched_yield`	partial	Analog `yield` 18 is a recorded hint and `ERET` to EL0. It does not switch EL1 tasks.
101	`nanosleep`	absent	CNTP tick is EL1.
113	`clock_gettime`	absent	No guest clock SVC. CNTPCT probes are kernel/NFR-07.
98	`futex`	absent	No user futex.
99	`set_robust_list`	absent	Typical libc `_start`.
134 / 135 / 139	`rt_sigaction` / `rt_sigprocmask` / `rt_sigreturn`	absent	Lower-EL IRQ still parks. No POSIX signals.
73	`ppoll`	absent
160	`uname`	absent
167	`prctl`	absent
163 / 261	`getrlimit` / `prlimit64`	absent
278	`getrandom`	absent

Sockets (not a Track B promise)

Linux #	Name	ctos status	Notes
198	`socket`	absent	No virtio-net, no stack. Same class as the host-apps “sockets” gap. Not a B2 implementation item.

Namespace / mount — never on this track

Linux #	Name	ctos status	Notes
97	`unshare`	never-per-ADR-031	Linux namespaces. Guest OCI is a non-goal (ADR-029).
268	`setns`	never-per-ADR-031	Same.
40	`mount`	never-per-ADR-031	Linux mount/overlay class. A7 FAT is not `mount(2)`. Concept compare: ADR-034 (B5). Must not grow POSIX mount.
39	`umount2`	never-per-ADR-031	Same.
41	`pivot_root`	never-per-ADR-031	Same.

clone with CLONE_NEWNS / CLONE_NEWPID / CLONE_NEWNET is the same never-per-ADR-031 class. A later clone subset, if B6 ever names one, would still exclude those flags.

If a subset is ever useful (not a decision)

Issue #42 asked to prioritize a minimal subset if any. This is a research hint for B6, not an implementation list.

In-tree libctos apps need no Linux numbers. A1–A9 already have exit / uart_write / yield / memfs / FAT /hello. Keep that path.
A static musl “hello” is not a small subset. Besides convention translation, typical CRT wants set_tid_address, exit_group, brk or mmap, often write on fd 1, uname / geteuid-class probes, and signal/prctl stubs. That is already a libc-shaped surface. ADR-031 rejected “runs musl / Alpine” as a Track B promise.
The smallest named research slice (only if B6 chooses a compat path, and only with a probe) would still be: convention translation + exit/exit_group + write to a console fd. openat/read/close are the next file slice. brk/mmap are the next heap slice. clone/execve/wait4 are the process gap (ADR-035); they are not a small add-on.
Do not start with ioctl, sockets, or mount. ioctl is a device encyclopedia. Sockets need a stack. mount/unshare stay never-per-ADR-031.

B3 is Documented (ADR-035). B4 is Documented (ADR-033). B5 is Documented (ADR-034). They must not add Linux numbers to src/. B6 may choose never.

Honesty

Analysis Verified means: this file was read against src/syscall.rs and Linux v6.10 unistd.h. It is not ABI Verified and not Linux userspace.
Do not say “ctos implements write(2)” because uart_write or fs_write exists.
Do not mint FR-16+ / NFR-15+. Frozen set: fr-nfr.md.
Guest containers stay non-goal. Host docker-smoke.sh is unrelated.

Roadmap: track-b.md. Extra stance: host-apps.md.

Keyboard shortcuts

ctos (ctsOS)

File descriptors and I/O (B5 concept compare: ADR-034)