# The vDSO — the public system-call boundary

> **Status:** design note, not built. The runtime today issues raw `syscall`
> instructions from `library/runtime/system-call.zig` using the numbers in
> `system/abi.zig`. This note designs the layer that replaces that arrangement:
> a **kernel-supplied, C-ABI entry library** mapped into every process — the
> only supported way into the kernel — so the raw numbers can stay private,
> be renumbered at will, and eventually be randomised per boot.

## Why: the ABI danos promises, and the one it doesn't

`system/abi.zig` is the **private** kernel ↔ runtime contract. Its header says
so: the numbers are an implementation detail the runtime hides and may
renumber, the same split as libSystem over the XNU syscalls on macOS or win32
over the NT syscalls on Windows. Linux — with its world-visible, frozen
syscall table — is the outlier, not the norm.

That stance has consequences the moment binaries exist that we don't rebuild
ourselves:

1. **Third-party binaries** (docs/zig-self-hosting.md) must keep working across
   kernel updates. If they contain raw `syscall` instructions with today's
   numbers baked in, every renumbering breaks the world — the ABI would be
   *de facto* public no matter what the header says. Go on macOS made exactly
   this mistake: it issued XNU syscalls directly instead of going through
   libSystem, and macOS updates repeatedly broke every Go binary until Go
   switched to the library like everyone else.
2. **Not everything is Zig.** A Rust or C program can't import the `runtime`
   module. The public boundary has to be expressible in the one calling
   convention every language speaks: the C ABI.
3. **Randomised syscall numbers** — a hardening option we want open — only
   work if no user binary anywhere knows a number at build time. The binding
   must happen at *load time*, from something the kernel controls.

All three point at the same well-known shape: a **vDSO** (virtual dynamic
shared object). The kernel carries a small blob of user-mode code, maps it
into every process at spawn, and that blob — not the application — contains
the `syscall` instructions. Fuchsia works exactly this way: its vDSO is the
*only* kernel entry, version-matched by construction because the kernel itself
injects it. Because the kernel and the blob ship as one artifact, there is
**no version skew, no loader, no search path, and no shared file on disk** —
which is what makes this the resilient way to have a private ABI
(docs/resilience.md), where a conventional `ld.so` + `/lib/libdanos.so`
arrangement would add a loader to every spawn and a single shared point of
failure.

The public danos ABI then has exactly two layers, neither of which is
`abi.zig`:

| Layer | Contract | Spoken by |
|-------|----------|-----------|
| **vDSO** | C-ABI functions, this note | every language's thin shim (`runtime.system` for Zig, a `-sys` crate for Rust, a header for C) |
| **IPC wire protocols** | byte layouts over `ipc_call` ([vfs-protocol.md](vfs-protocol.md) is the first one documented) | any client that can lay out bytes |

Everything above those — the heap, `runtime.fs`, the service harness — is
per-language convenience, compiled into each binary from source, exactly as
today. Nothing about the Zig runtime's shape changes; it just stops being the
*only* door.

## The blob

A single copy of the vDSO code lives in the kernel image (built by
`build.zig` as a tiny freestanding object, embedded like the AP trampoline).
At boot the kernel finalises it once — this is where randomised numbers would
be patched in — and thereafter maps the **same physical pages** read-execute
into every process's address space. The blob is:

- **Position-independent.** It is mapped at a per-process randomised base, so
  it must be PIC (rip-relative addressing only — no relocations to process).
- **Stateless and re-entrant.** No writable data. Anything stateful belongs to
  the process, not the vDSO.
- **Architecture-specific.** The x86-64 blob wraps `syscall`; an aarch64 blob
  wraps `svc #0`. It lives beside the other per-architecture kernel sources
  (`system/kernel/architecture/<arch>/`), selected the same way the
  `architecture` module is (docs/arch.md).

### Shape: a function table, not an ELF

A real `.so` with a dynamic symbol table is the conventional vDSO shape, but
linking against one at load time needs a dynamic linker in every binary —
machinery danos deliberately doesn't have. Instead the v1 shape is the
simplest thing that is still a stable contract — a **function-pointer table**
at the vDSO base:

```
offset 0   u64  magic        'danosVDS' — a mapped-the-wrong-thing guard
offset 8   u64  api_level    incremented when the table grows
offset 16  u64  count        number of table entries that follow
offset 24  u64  table[count] function pointers into the vDSO's own code
```

Table *indices* are the public constants (published in a C header,
`danos.h`), assigned once and append-only — the same discipline the IPC
protocols use for operation values. The pointers point at stubs inside the
blob; what those stubs put in `rax` is nobody's business but the kernel's.
A language shim binds in one step: read the base from the init block, check
the magic, keep the table pointer. Feature detection for a binary built
against older headers is `count`/`api_level` — a kernel never removes or
reorders entries.

(If danos ever grows a real dynamic linker, the same blob can additionally
present an ELF `dynsym` without breaking the table — Fuchsia's vDSO is
likewise both a mappable blob and a linkable `.so`. That is a later
convenience, not a requirement.)

### Delivery: the auxiliary vector

The kernel already builds a System V entry block — argc, argv, envp
terminator, **auxiliary vector** — on every new process's stack
(`buildEntryStack`, read by `runtime.start`). The vDSO base rides in a new
auxv entry, exactly Linux's `AT_SYSINFO_EHDR` move. No new syscall, no magic
address, and a language shim finds it the same portable way on every
architecture.

## The function surface

One table entry per kernel call, C ABI (System V AMD64), names prefixed
`danos_`. The current `SystemCall` set maps directly; integer arguments and
returns are `u64`, errors return as negative values exactly as today.

The calls that return two values in `rax:rdx` today — `dma_alloc`
(virtual_address + physical_address), `msi_bind` (address + data), `shared_memory_create` (virtual_address + handle) —
become functions returning a two-`u64` struct. The System V ABI returns a
16-byte struct in `rax:rdx`, so the stub is a plain `syscall; ret` — the
C-ABI spelling of the existing convention, at zero cost.

Grouped as `abi.zig` groups them:

| Group | Functions |
|-------|-----------|
| process | `danos_exit`, `danos_yield`, `danos_sleep`, `danos_spawn`, `danos_process_enumerate`, `danos_process_kill`, `danos_process_exit_reason`, `danos_process_subscribe`, `danos_process_signal`, `danos_signal_bind` |
| memory | `danos_mmap`, `danos_munmap`, `danos_dma_alloc`, `danos_dma_free`, `danos_shared_memory_create`, `danos_shared_memory_map`, `danos_shared_memory_physical` |
| ipc | `danos_endpoint_create`, `danos_ipc_register`, `danos_ipc_lookup`, `danos_ipc_call`, `danos_ipc_reply_wait`, `danos_ipc_send` |
| devices | `danos_device_enumerate`, `danos_device_claim`, `danos_device_register`, `danos_mmio_map`, `danos_irq_bind`, `danos_irq_ack`, `danos_msi_bind`, `danos_io_read`, `danos_io_write` |
| time | `danos_clock`, `danos_wall_clock`, `danos_timer_bind` |
| diagnostics | `danos_debug_write` (leveled, kernel-stamped records), `danos_klog_read`, `danos_klog_status` |
| filesystem naming | `danos_fs_resolve`, `danos_fs_node`, `danos_fs_mount`, `danos_fs_unmount` (naming only — file DATA still crosses the vfs-protocol IPC, see below) |

The constants that ride alongside the calls — mmap protection bits, DMA
flags, notification badge bits, `ExitReason`, `Signal`, well-known service
ids, `page_size`, the IPC message maximum — move to the public header too:
they are wire values a Rust program needs verbatim. What stays private in
`abi.zig` is exactly the thing the vDSO exists to hide: the `SystemCall`
numbers and the trap convention.

## Enforcement, and an honest threat model

Renumbering only has teeth if the kernel **refuses syscalls that don't come
from the vDSO**. The check is cheap: on kernel entry, the saved user `rip`
must lie inside the calling process's vDSO mapping; otherwise the process is
killed with a fault-class exit reason (its supervisor restarts or gives up,
docs/process-lifecycle.md — a foreign-syscall attempt is a bug or an attack,
never something to limp past). Fuchsia enforces exactly this.

What this buys, precisely:

- **ABI freedom** — the real prize. The numbers can change per release or per
  boot and nothing outside the kernel image cares. The private ABI stays
  actually private, permanently.
- **A single audited chokepoint** for kernel entry, per process, at a
  randomised address.
- **Raised bar for exploits**: shellcode can't issue a hard-coded `syscall`;
  it must first discover the per-process vDSO base (ASLR) and call through
  it.

What it does *not* buy: an attacker with arbitrary code execution in a
process can still *call* the vDSO functions — they are mapped executable in
that process, and return-oriented chains reach them. Syscall randomisation is
hardening, not a security boundary; the security boundary remains the
capability model (what the process's endpoints and device claims let it do).
It is worth building anyway — for the ABI freedom first and the hardening
second — but the design should never be sold as more than that.

## Migration

Phased so every step ships alone (the M-milestone discipline):

1. **The blob + the table.** Build the vDSO, map it at spawn, deliver the
   base via auxv. `runtime.system-call.zig` binds through the table when the
   auxv entry is present, falls back to raw `syscall` when absent — the whole
   tree keeps booting during the transition.
2. **Cut the runtime over.** Delete the raw stubs; `runtime` no longer
   imports the `SystemCall` numbers at all (`abi.zig`'s enum becomes
   kernel-internal). The QEMU suite passing proves the table carries the
   whole system.
3. **Enforce + randomise.** Add the `rip`-range check, then per-boot number
   randomisation patched into the blob at kernel init. A test boots with
   randomisation on and runs the full suite.
4. **The other languages.** Publish `danos.h`; a Rust `danos-sys` crate wraps
   the table. This is also the seam `std.os.danos` calls through when the Zig
   self-hosting fork lands (docs/zig-self-hosting.md) — the vDSO is what
   makes that seam stable across kernel versions.

## What deliberately stays out

- **No dynamic linker, no `/lib/*.so`.** The vDSO is kernel-injected precisely
  so danos binaries can stay fully static above it. Sharing *library code*
  across processes stays what it is today: a service behind IPC, or source
  compiled into each binary.
- **No file/device I/O in the vDSO.** The microkernel line doesn't move: the
  vDSO wraps the same deliberately tiny table (docs/syscall.md). The kernel
  resolves file NAMES (`fs_resolve` — the mount table moved in-kernel), but
  file data is still the filesystem server's business over the vfs-protocol
  IPC; the kernel never blocks on a userspace filesystem.
- **No fast-path user-mode implementations yet.** Linux's vDSO exists mostly
  to answer `gettimeofday` without a kernel entry. `danos_clock` could one
  day read the calibrated TSC in user mode the same way — the blob is where
  such an optimisation would live — but that is an optimisation, not part of
  this design's contract.
