SchOS/docs/plan/ldls.md

# LDLS — the Linux Driver Layer for SchOS

SchOS cannot write a driver for every device that exists. No project can. The
only route to the hardware support an established system has is to run someone
else's drivers, and in practice "someone else's drivers" means Linux's.

**LDLS itself is Apache-2.0**, like everything else in this tree, and stays that
way because it never contains a line of Linux. What the *user* combines it with
on their own machine is a separate question, and
[../licensing.md](../licensing.md) §6 is where that one is answered.

[hybrid.md](hybrid.md) built the mechanisms for this and said so in as many
words — MMIO, DMA, interrupts and config space to a process, with SchOS's own
drivers as the first consumers so the interface would be general rather than a
hole shaped like something foreign. `vgpud` proved it. **LDLS is what those
mechanisms were built for.**

---

## The rules

These are the spine. Everything below is an implementation of them, and
anything that violates one is wrong however well it works.

### 1. Nothing GPL ever enters an image we build

The driver is **the user's**, obtained by them, loaded on their machine. SchOS
ships the layer and never the thing it loads. This is not a new position — it
was decided on 2026-08-08 and is written up in
[../architecture/devices.md](../architecture/devices.md) and
[../licensing.md](../licensing.md) §2:

> Apache-2.0 code and GPL-2.0-only code cannot be combined into one
> distributable work. It is not a matter of taste or of how carefully the files
> are kept apart in the tree.

`RAMDISK_EXCLUDE` and `NOTPORTS` are the existing mechanisms that keep this
true mechanically rather than by intention, and LDLS must stay inside them.

### 2. It runs in a process, never in the kernel

Two independent reasons, and either alone would decide it.

**Legally**, a module linked into our kernel is the strongest derivative-work
argument that exists — it is the case the FSF has always made. A module
combined with our shim *inside a user process, on the user's machine, at their
direction* is a different thing at arm's length, and it is the position
FreeBSD has shipped from for about a decade: a BSD-licensed `linuxkpi` in base,
GPL `drm-kmod` installed by the user from ports.

**Practically**, this is foreign code. A driver we did not write, cannot fully
audit and did not choose the quality of, running with the machine's memory in
reach, is exactly the code that should not be able to take the machine down.
Crash isolation is not a nicety here; it is the reason.

### 3. Facts, not expression

The shim has to present Linux's internal API, and that API is expressed in GPL
headers. The line LDLS holds:

| | |
|---|---|
| A symbol exists, and takes these arguments | **fact** — reproduce it |
| A struct has these fields in this order at these offsets | **fact** — a compiled driver indexes them, so there is no other way to express it. And mostly it need not be in our source at all; see BTF below |
| The comment above it | **expression** — never |
| The macro body, the inline function body, the implementation | **expression** — never |

Every KPI file records where its shape came from. Not decoration: it is the
difference between a clean-room reimplementation and a transcription, and the
only time anyone will ask is the time it matters.

This is the same discipline [../licensing.md](../licensing.md) §5 already
describes — *"the xHCI, USB, PCI, fbdev and ELF specifications were all read
and implemented directly, which is why those drivers have no licence
entanglement at all"* — applied to a case where the specification is thinner.

> **Not legal advice, and it should be checked by someone whose job that is
> before anything ships.** The reasoning rests on interfaces being outside
> copyright (EU Software Directive Art. 1(2)) and on *Google v. Oracle* (2021),
> and neither is a blanket permission.

#### The layouts do not have to be ours to write

The uncomfortable row above is the struct layouts, and it turns out to be
largely avoidable. Kernels built with `CONFIG_DEBUG_INFO_BTF` publish
`/sys/kernel/btf/vmlinux`, which carries the layouts *and* the function
prototypes as generated binary type data about the kernel that is installed:

```
struct pci_dev {
        struct list_head           bus_list;      /*     0    16 */
        struct pci_bus *           bus;           /*    16     8 */

[97814] FUNC 'pci_iomap' type_id=97813 linkage=static
[97815] FUNC_PROTO '(anon)' ret_type_id=108 vlen=4
```

So LDLS reads them at load time, on the user's machine, from the user's own
kernel. Our source contains neither, and there is nothing to have transcribed.
It also mostly dissolves rule 4 below, because BTF describes the kernel that is
actually there rather than one we pinned by guessing.

**BTF gives shape, not behaviour.** What `napi_complete_done` *does* is still
ours to implement from documentation, and that is the whole of the real work.
A kernel without BTF needs hand-written layouts for a named version, which is
why this section stays rather than being replaced.

### 4. One kernel version at a time, pinned and named

*(Mostly superseded by BTF, above — but it still governs any kernel that does
not carry it.)*

Linux states in writing that it has no stable in-kernel ABI —
`Documentation/process/stable-api-nonsense.rst` exists to say so. Believing
them is cheaper than being surprised by them.

So LDLS targets **one version**, says which, and is re-pinned deliberately.
FreeBSD's LinuxKPI does exactly this per drm-kmod release. A layer that claims
to support "Linux drivers" without a version is claiming something nobody has
ever delivered.

### 5. The unresolved-symbol list is the specification

Do not guess the API surface. **Load a real module and let the loader say what
is missing.** That list is the work queue, it is exact, and it is available
before a single line of shim is written.

---

## What it is made of

Four pieces, of which three are ordinary and one is the project.

### The loader — ours, clean, and small

A `.ko` is a relocatable ELF64 object. Loading one is: lay out the allocatable
sections, apply the relocations, resolve the undefined symbols against a table,
and find the entry point. SchOS already parses ELF in `kernel/fs/exec.c`, for a
different object type but with the same plumbing.

There is no licence question here at all — ELF is a published specification and
so is the relocation set.

### The KPI — the shim, and the actual work

Our Apache-2.0 reimplementation of the Linux symbols a given driver needs,
organised under Linux's own header names so it is navigable: `kpi/slab.h`,
`kpi/pci.h`, `kpi/netdevice.h`. Rule 3 governs every file.

### The back end — already built

`/dev/pci`: `PCIIOC_INFO`, `PCIIOC_CLAIM`, `PCIIOC_CFG`, `mmap` of a BAR,
`PCIIOC_DMA`, `PCIIOC_IRQ`. A Linux driver's `pci_iomap`, `dma_alloc_coherent`
and `request_irq` land on these, which is the whole reason they were specified
in terms of PCI devices rather than in terms of drivers.

### The class bridge — the one new kernel interface

This is the piece that does not exist, and it is the interesting one.

A hosted driver produces packets in a process. SchOS's network stack consumes
them through `struct netdev_ops` in the kernel — `transmit`, `receive`, `mac`,
`link_up`, four entries and nothing else. So what is missing is a **netdev
whose backend is a process**: the kernel publishes the vtable, the four calls
are forwarded over a descriptor, and lwIP never learns that the driver is not
in the kernel.

It is the exact analogue of what `display_ops` was for the GPU, and
`netdev.h`'s own opening comment already anticipates the shape — *"the stack
should ask for the network interfaces and get whatever is bound … exactly as a
compositor asks for the display and does not care which driver answers."*

One device, one process, claimed and released, on the `/dev/pci` discipline.

---

## Milestones

| | | |
|---|---|---|
| **L0** | **The loader.** Parse a real `.ko`, lay it out, relocate it, and **print every unresolved symbol**. Nothing executes. The output is the measured API surface. | **done** — `ldls/ko.c`, `make ldls-check` |
| **L1** | **Stubs.** Every symbol resolves, to a function that logs its name. `init_module` runs, which yields the *call order* — the second thing you need and the other thing you cannot guess. | **done** — `ldls/stub.c`, `ldls-run` |
| **L2** | **The hardware.** `probe()` receives a working `struct pci_dev`, maps its BARs, allocates coherent DMA and registers an interrupt, all through `/dev/pci`. The driver reaches its device. | |
| **L3** | **A packet.** The process-backed netdev, wired to the driver's transmit and receive paths. **`ping` over a Linux driver.** This is the milestone that proves the whole idea and the one that cannot be skipped. | |
| **L4** | **Kill it.** The host process dies, the kernel clears bus mastering and reclaims the DMA, a new host starts and the interface comes back. | free — H2 built all of this |

L4 is listed for completeness rather than as work. `pciuser_stop()`,
`pciuser_release_all()` and `KeepAlive` already do it, and that they apply
unchanged to a foreign driver is the point of having built them against our
own.

### The first target: `8139too`, not `r8169`

The obvious choice is a Realtek `r8169`, because that silicon is everywhere.
It is the wrong first choice for one decisive reason: **QEMU cannot emulate
it**, so L0 through L3 would have no way to be tested, and a driver layer
developed against hardware you cannot run is a layer nobody can debug.

QEMU emulates `rtl8139`. `8139too` is small, self-contained, needs no firmware
blob, and uses the thin `mii` helpers rather than dragging in phylib — which
`r8169` does, and which is a substantial surface on its own.

So: **`8139too` to build the layer, `r8169` to prove it was general.** The
second is the interesting one and it is much cheaper once the first works.

This is the `/dev/pci` lesson applied backwards. There, a native driver was the
first consumer so the interface would not be shaped like something foreign.
Here the interface is fixed by Linux, and what is chosen instead is the
*smallest real consumer that can be tested* — because the risk is no longer
designing the wrong interface, it is building a hundred stubs nobody has ever
run.

---

## L0, and what it measured

`ldls/ko.c` is the loader: index the sections, place the allocatable ones,
resolve the undefined symbols against a table, apply the relocations. It is
freestanding — no libc, no allocation, no I/O — so the same source builds as a
host tool today and as the SchOS-side host later, which is the arrangement
`mkschfs` and the kernel already share.

```
$ ldls-inspect 8139too.ko
  119288 bytes, 55 sections, 363 symbols
  module    8139too
  license   GPL
  describes RealTek RTL-8139 Fast Ethernet driver
  loads to  33988 bytes

  relocations: 1980
    R_X86_64_64      363
    R_X86_64_PC32    991
    R_X86_64_PLT32   428
    R_X86_64_32S     169
    R_X86_64_PC64     29

  undefined symbols: 94
    ...
  stops at: unresolved: __fentry__
```

**The licence is printed first, before anything is loaded**, because it is the
one property that decides whether the object may be on this machine at all and
it is a plain string in `.modinfo`. Rule 1 is easier to keep when the answer is
on screen.

### What the measurement says

**94 symbols** is the API surface of `8139too` on Linux 6.18.46. Not an
estimate — the list, exactly, and it is L1's work queue. It is also smaller
than it looks, because a good fraction of it is not API at all: `__fentry__`,
`__stack_chk_fail`, `__ref_stack_chk_guard`, `__x86_return_thunk` and
`__fortify_panic` are compiler and instrumentation artefacts that stub out in a
line each.

**Five relocation types**, and a corpus run says that is the whole set: over
**2,440 real modules** from a stock 6.18.46 tree, every one lays out and
relocates to completion with nothing unhandled. That includes `nvidia-modeset`
at 7,069 sections and 74,305 relocations, which is what retired the loader's
fixed section table — the caller supplies it now, so there is no limit to run
into rather than a generous one.

Relocating against stubs is a separate question from listing what is missing,
and it is the one that says the loader is finished: with nothing resolved, the
first unknown name stops everything and no relocation type past it is ever
reached.

### Nothing foreign is in this tree

`make ldls-check` runs the loader against `ldls/test/refmod.c` — ours, compiled
with `-c`, written to contain one of each thing the loader has to get right:
undefined functions *and* undefined data, `.text` with calls, `.data` holding a
pointer into `.rodata`, a `.bss` that must arrive zeroed, and a `.modinfo`
shaped the way a module's is. The modules above were read from the host's own
`/nix/store` and never copied here; `.gitignore` refuses `*.ko` outright so
that cannot happen by accident.

---

## L1, and the driver's own voice

Every undefined symbol resolves to a trampoline — twenty generated bytes that
load the symbol's index and jump to a shared thunk, which saves the argument
registers and reports the call. `init_module` is then simply called.

```
$ ldls-run 8139too.ko
  module  8139too
  license GPL
  wants   94 symbols
  loaded  33988 bytes at 0x416f8000, 94 stubs at 0x40377000

  calling init_module at 0x416fc020

    1  _printk(0x416fb6c0, 0, 0)
      | 8139too: 8139too Fast Ethernet driver 0.9.28
    2  __pci_register_driver(0x416fb400, 0x416fff80, 0x416fbdbb)

  init_module returned 0 after 2 stub call(s)
```

**That second line is the driver's own banner**, printed through a `_printk`
that is ours, from a string in the module's own `.rodata` that a relocation put
there. There is no stronger evidence that a load worked than the loaded thing
speaking.

The stubs return zero rather than stopping, which is a deliberate departure
from what this milestone originally said. Stopping at the first call tells you
one name; carrying on tells you the *sequence*, and the sequence is the thing
that cannot be read off a symbol table. A driver that then dereferences the
zero faults, and that is a result too — it says exactly how far it got before
it needed the kernel to have really done something.

### The constraint nobody would guess

The first run failed on a relocation, not on anything to do with stubs:

```
ldls-run: relocate: a signed 32-bit relocation does not fit
```

A module is compiled `-mcmodel=kernel`, so the compiler emits `R_X86_64_32S` —
a **signed 32-bit absolute address** — for anything it believes lives in the
top 2 GB. Load the object at an ordinary `mmap` address around `0x7f…` and
those relocations have nowhere to go. Linux avoids this by giving modules a
region within 2 GB of the kernel text; the harness uses `MAP_32BIT` to reach
the same place from the other end.

**On SchOS this costs nothing** — a process's mmap area starts at `0x40000000`
and is already inside the window — but it is a real constraint on where the
LDLS host may put a module, and it would have been found much later and much
more confusingly without a loader that checks its own arithmetic.

### What running 500 of them says

| | |
|---|---|
| `init_module` ran and returned | **449** |
| faulted on a stub's zero | 22 |
| no `init_module` at all | 29 |
| **loader errors** | **0** |

The 22 are not failures of the loader; they are drivers that needed an
allocation to come back non-null. The 29 without an entry point are mostly
library modules that exist to be depended on.

---

## What it will not do

- **It is not a security boundary, and here that is worse than usual.** There
  is no IOMMU, so a hosted driver can point its device at any physical address.
  For SchOS's own drivers that isolates bugs and not intent, which
  [hybrid.md](hybrid.md) says at length. For *foreign* drivers the distinction
  between bug and intent is one we are in no position to make at all. This is
  the single strongest argument for an IOMMU that this project has produced,
  and it should be recorded as such rather than waved past.
- **It implements no licence check, and that is not circumvention.**
  `EXPORT_SYMBOL_GPL` is Linux marking *Linux's own* symbols, and the check it
  drives protects those. Every symbol LDLS provides is ours, reimplemented from
  facts under Apache-2.0, so there is no gate being bypassed because there is
  no Linux symbol being exposed. LDLS **reads** the declared licence and prints
  it before loading anything, because that is the property that decides whether
  an object belongs on a machine at all. See
  [../licensing.md](../licensing.md) §6.
- **No firmware loading.** `request_firmware` is a stub that fails. Many
  drivers need a blob, most blobs are redistributable only under their own
  terms, and that is a second licensing problem which is not this one.
- **No module-to-module dependencies.** One object, no `depmod`, no symbol
  exports between hosted modules.
- **No sysfs, procfs or debugfs.** Drivers that only *report* through them will
  work; drivers that are *configured* through them will not.
- **No unload.** `module_exit` is never called; the way to stop a driver is to
  kill the process, which H2 made a supported operation rather than a failure.
- **No sleeping-context fidelity.** Linux drivers assume a kernel's rules about
  what may sleep, what runs in interrupt context and what `spin_lock_irqsave`
  costs. A process has different rules, and where they differ the shim will be
  wrong in ways that show up as rare races rather than as failures to build.
  This is the deepest technical risk in the whole plan.
- **Running init is not working.** L0 proves 2,440 modules lay out and
  relocate; L1 proves 449 of 500 run their init to completion. Both say the
  *mechanism* is right and neither says a driver would function: every symbol
  is a stub that logs and returns zero, so what has been demonstrated is that
  drivers ask for things in a readable order, not that anything answers them.
  The distance from here to a packet is the rest of this plan.
- **The stubs cannot tell code from data.** Every undefined symbol in a module
  is `NOTYPE` in the ELF symbol table, so `jiffies` and `pci_iomap` are
  indistinguishable and both get a trampoline. A driver that *reads* `jiffies`
  reads trampoline bytes. BTF distinguishes them — `VAR` against `FUNC` — and
  that is where the fix belongs rather than in a guess.
- **`ldls-run` is a development harness, not the deliverable.** It runs on the
  build machine so that loader failures cost a second instead of a boot. The
  thing that ships is the SchOS-side host.
- **It will not support "Linux drivers".** It will support *one driver, on one
  kernel version, that has been tried*. Anything beyond that is a claim, and
  the list of what has actually been run belongs in the architecture document
  when there is one.