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. | **done**, except `/dev/pci` — see below |
| **L3a** | **The class bridge.** `/dev/netdev`: a process claims it and becomes an interface. `ping` over a *process*. | **done** — `kernel/dev/netuser.c`, `cmd/netecho` |
| **L3b** | The SchOS-side host: `ldls-run`'s loader and BTF work, as a program on SchOS. | **done** — `cmd/ldlsd`; `/dev/pci` is L3c's half |
| **L3c** | The fifteen symbols, implemented rather than stubbed, against a real rtl8139. | **done** — `ldls/kpi/`; the driver reads the real MAC |
| **L3d** | The packet path: `ndo_open`, `ndo_start_xmit`, receive. | |
| **L3** | **A packet.** The two joined: **`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.

### And what it says about the work queue

Counting *calls* is the wrong metric — one module walking an rbtree called
`rb_first` sixty-four times and told us nothing. Counting **how many modules
need a symbol** is the useful one, and over 110 modules it comes to this:

| symbol | modules |
|---|---|
| `__kmem_cache_create_args` | 25 |
| `crypto_register_template` | 11 |
| `_printk` | 10 |
| `crypto_register_alg` | 10 |
| `__register_nls` | 9 |
| `crypto_register_shash` | 4 |
| `af_alg_register_type` | 4 |
| `register_filesystem`, `misc_register`, … | 2 each |

**Forty-four distinct symbols would carry every one of those 110 modules
through `init_module`.** That is a small enough number to be a plan rather than
an estimate, and it is not a number anybody could have guessed — the slab
allocator at the top, and a long tail of registration calls that mostly want to
record a pointer and return zero.

It says nothing about `probe()`, which is where the real surface is and which
the kernel, not the module, is supposed to call. Reaching that is L2.

---

## L2, and being the PCI core

In Linux, `__pci_register_driver` ends at the PCI core, which matches the
driver's `id_table` against the bus and calls `probe()`. There is no PCI core
here, so LDLS keeps the pointer that call was given and plays the part itself.

To do that it has to know where `pci_driver.probe` is — and **it asks the
kernel rather than being told by us**. `ldls/btf.c` reads
`/sys/kernel/btf/vmlinux`: 122,279 types, every struct layout and every
function prototype, as generated data describing the kernel that is installed.

```
$ ldls-types /sys/kernel/btf/vmlinux --need
  pci_driver             probe                   offset    16  size 8
  pci_driver             id_table                offset     8  size 8
  pci_dev                vendor                  offset    60  size 2
  pci_dev                resource                offset  1000  size 1088
  resource               start                   offset     0  size 8
  ...
  all present
```

Cross-checked against `pahole` on the same file, field for field. **No offset
in this is ours**, which is rule 3's uncomfortable row disappearing rather than
being argued about.

### What the driver then did

```
  pci_driver at 0x41662400, from BTF: probe +16, id_table +8
  driver name "8139too"

  devices it claims:
    10ec:8139  class 000000/000000
    ... and 16 more

  calling probe(pci_dev 10ec:8139, ...)

    1  alloc_etherdev_mqs(0x368, 0x1, 0x1)
    2  pci_enable_device(...)
    3  pci_request_regions(...)
    4  pci_set_master(...)
    5  pci_iomap(pdev, 1, 0)
    6..544   ioread8 / iowrite8   -- 0x88, 0x8a, 0x8c, 0x8e, over and over
  545  dev_addr_mod(...)
  547  netif_napi_add_weight_locked(...)
  549  timer_init_key(...)
  550  register_netdev(...)
  551  netdev_info(...)
      | %s at 0x%p, %pM, IRQ %d

  probe returned 0 after 552 stub call(s)
```

`10ec:8139` is the real RTL8139 id. The driver mapped **BAR 1**, which is the
one we marked `IORESOURCE_MEM`. And those 538 alternating byte accesses with
values stepping `0x88, 0x8a, 0x8c, 0x8e` are **serial EEPROM clocking** — the
driver bit-banging the chip to read its MAC address, then handing the result to
`dev_addr_mod`. It registered a NAPI instance, started a watchdog timer,
registered the interface, printed its "found it" line, and **returned 0**.

That is a complete driver bring-up, on shape alone.

### Fifteen symbols

The whole of that probe touches **15 distinct symbols**, and 538 of the 552
calls are two of them:

| | |
|---|---|
| `ioread8` / `iowrite8` / `ioread32` | 540 calls, 3 symbols |
| PCI setup | `pci_enable_device`, `pci_request_regions`, `pci_set_master`, `pci_iomap` |
| net | `alloc_etherdev_mqs`, `dev_addr_mod`, `netif_napi_add_weight_locked`, `register_netdev`, `netdev_info` |
| other | `mutex_lock`, `mutex_unlock`, `timer_init_key` |

L1 said 44 symbols carry 110 modules through init. L2 says 15 carry this one
through probe. **Neither is a number anyone would have guessed**, and both came
out of running the thing rather than reading about it.

### What is not done

`probe()` receives a `pci_dev` whose *shape* is right and whose *contents are
invented*. The BARs are numbers this harness made up; `pci_iomap` returns a
zeroed page, not a mapping; nothing has touched hardware and nothing could,
because this runs on the build machine.

**The half of L2 that says "all through `/dev/pci`" belongs to the SchOS
side**, where those calls have somewhere real to land. What is settled is the
part that had to come first: that probe can be reached at all, with a struct
the driver accepts, built from offsets nobody wrote down.

---

## L3a: a network, in a process

L3 is one sentence in the milestone table and three pieces of work. This is the
first, and it is the one the rest plugs into: **`/dev/netdev`, an interface
whose backend is a process.**

The kernel side is `netdev_ops` and nothing else — `transmit`, `receive`,
`mac`, `link_up`, the same four entries `e1000` publishes. A process claims the
device, supplies a MAC, and from then on `read()` gives it frames the stack
wants sent and `write()` hands it frames that arrived. It is `/dev/pci`'s shape
with the arrows reversed: that hands a device *to* a process, this lets a
process *be* one.

`cmd/netecho` is the proof, and it is deliberately **not** a driver — it has no
hardware and claims none. It answers every frame itself: DHCP so the interface
gets an address at all, ARP so the stack can find it, ICMP so it can be pinged.

```
# netecho &
netuser: ldls0 is a process now, 2:4c:44:4c:53:1
net: ldls0 attached as interface 1, asking for an address
netecho: this process is now an interface. I am 10.9.0.1 and will offer 10.9.0.2
netecho: answered DHCP, offering an address

# cat /proc/netif
interface   ldls0
driver      netuser
address     10.9.0.2
gateway     10.9.0.1

# ping 10.9.0.1 3
netecho: answered an ARP
netecho: answered a ping
64 bytes from 10.9.0.1: icmp_seq=1 time=30 ms
64 bytes from 10.9.0.1: icmp_seq=2 time=10 ms
64 bytes from 10.9.0.1: icmp_seq=3 time=10 ms
3 packets transmitted, 3 received, 0% packet loss
```

**The interface got its address by DHCP from a process**, and then answered
ping from one. lwIP never learns that the thing on the other side of
`netdev_ops` is not a driver — which is the entire property the bridge exists
to have.

### It dies the way it should

The same test, continued:

```
# kill 8
# schdbg netuser
  interface ldls0  DOWN, nobody holds it
# ping 10.9.0.1 1
ping: sendto: -5
0 packets transmitted, 0 received

# netecho &
# ping 10.9.0.1 2
2 packets transmitted, 2 received, 0% packet loss
```

`link_up()` answers false the moment nobody holds the descriptor, so the stack
refuses to send rather than swallowing packets into a driver that is not there
— and when a new process claims it, the *same* interface comes back, keeping
its address. That is H2's property applied to a network interface, and it came
free: `netuser_release_all()` sits beside `pciuser_release_all()` in `exit()`
for exactly the same reason.

### One new thing the kernel needed

`netinit()` starts DHCP on the devices that exist when it runs. An interface
created later — which is how an LDLS-hosted driver arrives — missed that
entirely and would have come up with no address and no way to get one. So
`net_attach_late()` exists, in `kernel/net/net.c` because that is the only file
allowed to include lwIP's headers.

---

## L3b: the same loader, on SchOS

`ldls/ko.c`, `ldls/btf.c` and `ldls/stub.c` were written freestanding so they
would build unchanged on both sides. `cmd/ldlsd` is the collection of that
promise: the same three files, linked against `lib/` instead of a libc, in an
ordinary SchOS process.

```
# ldlsd /lib/modules/8139too.ko /lib/modules/btf
/lib/modules/8139too.ko
  module  8139too
  license GPL
  wants   94 symbols
  defused 6 %gs stack-guard reference(s)
  loaded  33988 bytes at 0x000000004001f000

  calling init_module
  1  _printk(...)
      | 8139too: 8139too Fast Ethernet driver 0.9.28
  2  __pci_register_driver(...)
  init_module returned 0 after 2 stub call(s)

  btf     122279 types
  layout  pci_driver.probe +16  pci_dev 2680 bytes
  driver  "8139too"
  devices it claims:
    10ec:8139
    ... and 18 more

  calling probe
  1  alloc_etherdev_mqs(0x368, 0x1)
  probe returned -12 after 1 stub call(s)
```

Byte for byte the behaviour of the host harness, which is what the shared
source was for. Two predictions from earlier milestones came good without any
work: a process's mmap area starts at `0x40000000`, so the module lands inside
the window `R_X86_64_32S` needs, and the kernel never sets `EFER.NXE`, so every
mapping is executable.

That second one is **a gap, not a feature**. A process being able to execute
its own heap is normally a thing an operating system prevents, and LDLS is
quietly relying on its absence. It belongs on Gate 0's list.

### The one thing that did not just work

The first run on SchOS faulted before probe's first call:

```
pid 8 ldlsd: trap 14 err 5 on cpu 0 rip 0x4002138e addr 0xffff80007de5fc60
```

`err 5` is present, read, user mode, and the address is a *kernel* one. The rip
resolved to `.text+0x206e`, which is:

```
mov %gs:0x0(%rip),%rax      # rtl8139_init_one+0x26
```

Linux builds modules with `-mstack-protector-guard-reg=gs`, so the stack canary
is fetched through the GS segment. That is an ABI between the kernel and its
own per-CPU area, and **a host that is not that kernel cannot provide it** — on
SchOS the GS base belongs to the kernel, so a user-mode read of it faults on a
kernel address.

`ko_defuse_gs()` rewrites those references to `xor %reg, %reg`. There are two
forms and the second is easy to miss: the prologue *loads* the guard with `mov`
(`8b`), and the epilogue *compares* it with `sub` (`2b`) followed by a `jne` to
`__stack_chk_fail`. `xor` is correct for both — it makes the saved canary zero,
and it sets ZF so the epilogue's branch falls through exactly as a passing
check does. Six sites in `8139too`.

It checks every site before patching any, and refuses the whole operation on
anything it does not recognise. A loader that half-understands an instruction
should not be editing it — and that refusal is what found the `sub` form, by
declining to guess.

### Getting the files there

`make ldls-stage KO=… BTF=…` copies a module and its kernel's BTF into the
local sysroot. It is deliberately not part of `all`, `.gitignore` refuses
`*.ko`, and `make ldls-unstage` removes them again. An image built between
those two commands has somebody else's GPL code in it and must not be
redistributed — which is exactly what [../licensing.md](../licensing.md) §6
says about the combination, and why it is a thing you type on purpose.

---

## L3c: a Linux driver, driving real hardware

`ldls/kpi/` is the shim: 41 of `8139too`'s 94 symbols, implemented rather than
logged. Register access goes to a mapped BAR; `pci_enable_device`,
`pci_iomap`, `pci_request_regions` and `pci_set_master` go to `/dev/pci`;
allocation goes to `mmap` and `PCIIOC_DMA`; `register_netdev` goes to
`/dev/netdev`. Every file records its provenance, per rule 3.

```
# ldlsd /lib/modules/8139too.ko /lib/modules/btf
  found   10ec:8139 at 00:05.0
  bar 0    0x000000000000c000  256 io
  bar 1    0x0000000081060000  256 mem
  netdev  2688 bytes

  calling probe, with 41 of 94 symbols implemented

  kpi: net_device 2688 bytes + 872 private
  kpi: mapped BAR 1 -- phys 0x0000000081060000, 256 bytes
  kpi: the driver read a MAC off the hardware: 52:54:00:ab:cd:ef
  net: ldls0 attached as interface 1, asking for an address
  netuser: ldls0 is a process now, 52:54:0:ab:cd:ef
  kpi: registered as an interface, with the hardware's own MAC

  probe returned 0 after 0 stub call(s)
```

**`52:54:00:ab:cd:ef` is what the machine was configured with.** Nothing here
invented it: the driver bit-banged the card's EEPROM, serially, through a BAR
mapped by `/dev/pci`, and handed back the address it found. That it matches
means the loader, the relocations, the BTF offsets, the BAR mapping and the
register accessors are all right at once — which is why it was chosen as the
thing to aim at.

**Zero stub calls.** Every symbol that probe reaches is implemented.

### The bug a foreign driver found in its first minute

The first run against real hardware wrote to address `0x4e` and faulted.
`0x4e` is 13 below where the driver meant to write, and 13 is `EACCES`:
`pci_iomap` had returned `-13` as a pointer, because the check for a failed
`mmap` looked only for `-1` while SchOS returns `-errno`.

Underneath that was the real one. `sys_mmap` rounds a length up to a page —
the only granularity an MMU has — and `pciuser_mmio_ok()` compared that against
the BAR's own size. rtl8139's registers are **256 bytes**, the request became
4096, and the answer was `-EACCES` for a device the process legitimately held.

**A BAR smaller than a page could never be mapped at all**, and nothing had
noticed because the only consumer until now was virtio-gpu, whose BARs are
large. The check is now against the pages the BAR lies in. The widening is
real and named in the code: a sub-page BAR hands the process the whole page it
sits in, which is forced by the hardware rather than chosen.

That is precisely what [hybrid.md](hybrid.md) said a foreign driver would be
good for — *"what it asks for is what a foreign driver would ask for too"* —
arriving as a concrete bug rather than as a prediction.

### Register access is checked

`kpi_io_ok()` bounds every `ioread`/`iowrite` against the mapping `pci_iomap`
returned, reports the first few that miss, and returns without touching
memory. Two reasons: a stray access would otherwise be a page fault with no
context, and **there is no IOMMU** — a hosted driver's wild writes are exactly
the class of thing that has to be caught early or not at all.

It is also what turned the bug above from a crash into two lines naming the
offsets, which is how the `-13` was spotted.

### What is not done

`probe()` succeeds, so the driver is up and the interface exists with the
hardware's own MAC. **No frame has moved.** `ndo_open` is never called, there
is no `ndo_start_xmit`, and nothing services the receive side — so `ldlsd`
releases the interface rather than leaving it up:

```
  the driver is up on real hardware and has an interface.
  carrying frames is the half of L3 that is not built, so the
  interface is released rather than left up and silent.
```

A link that reports itself up with nothing draining its queue is a black hole:
the stack would send DHCP into it for ever and every layer above would believe
the network was merely quiet. Down is the true answer until packets move, and
that is L3d.

---

## 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.
- **The stack protector is gone from a hosted driver.** `ko_defuse_gs()`
  removes it because the ABI it depends on cannot be supplied here. That is a
  hardening measure lost, on foreign code, in a host with no IOMMU — worth
  saying plainly rather than filing as a detail.
- **LDLS relies on SchOS having no NX.** Every mapping is executable because
  `EFER.NXE` is never set. If that is ever fixed — and it should be — `ldlsd`
  needs a way to ask for executable memory, and nothing in `mmap` currently
  offers one.
- **BTF has to come from somewhere.** On Linux it is `/sys/kernel/btf/vmlinux`.
  SchOS has no such file and never will — so the user brings the BTF along with
  the module, from the kernel the module was built for. Both are theirs,
  neither is shipped, and a kernel built without `CONFIG_DEBUG_INFO_BTF` has
  neither.
- **L3a proves the bridge, not the driver.** `ping` works over a *process*;
  nothing in that test loads a module, and `netecho` is a fake network rather
  than a NIC. That is the point of doing it first — if `ping` had not worked
  here the bridge would be wrong, and now a failure with a real driver can only
  be the driver's side.
- **One interface, one process.** `/dev/netdev` is a single device and a second
  claimant gets `-EBUSY`. Two hosted NICs would need it to be indexed like
  `/dev/input/eventN`, which is a change of shape rather than of size.
- **The interface is never removed.** Releasing it leaves the device registered
  with the link down, because nothing in lwIP supports taking an interface out
  of its list. From the stack's point of view that is the same thing; from
  `/proc/netif`'s it is a `ldls0` that stays listed after its process is gone.
- **No frame has moved.** L3c ends at a successful `probe()`. The packet path
  — `ndo_open`, `ndo_start_xmit`, receive, and the skb handling all three need
  — is L3d and is the larger half.
- **41 of 94 symbols.** The rest are still logging stubs, which is deliberate:
  an unimplemented symbol that reports itself keeps the work queue writing
  itself, exactly as it did at L1.
- **A lock is a no-op.** In one process with no preemption inside the driver
  there is nobody to race with, which is correct right up until the driver has
  an interrupt thread. That is the sleeping-context assumption named above and
  it is the deepest unfixed thing in the layer.
- **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.