SchOS/docs/architecture/ldls.md

# LDLS — running somebody else's drivers

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 Linux's
drivers, and LDLS is what does that.

It works. `8139too` — an unmodified GPL kernel module — loads into a SchOS
process, drives QEMU's rtl8139 through `/dev/pci`, and carries packets:

```
# ldlsd /lib/modules/8139too.ko /lib/modules/btf &
  kpi: the driver read a MAC off the hardware: 52:54:00:ab:cd:ef
  ndo_open returned 0
  carrying frames. the interface is ldls0.

# ping 10.9.0.2 4
64 bytes from 10.9.0.2: icmp_seq=1 time=30 ms
...
4 packets transmitted, 4 received, 0% packet loss
```

`10.9.0.0/24` is reachable only through `ldls0`. The address on it came from
DHCP, over the same driver.

---

## The rules it is built on

Five, and everything else is an implementation of them.

### 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. `.gitignore` refuses `*.ko`,
`make ldls-stage` is the one command that puts a module into a local image and
is deliberately not part of `all`, and `make ldls-unstage` takes it out again.

[../licensing.md](../licensing.md) §6 is the whole argument. The short form is
that GPL obligations attach to *distribution*, we distribute Apache-2.0 code
containing no GPL, and the combination happens in a user process on the user's
machine — so no combined work is distributed by anyone.

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

A module linked into our kernel is the strongest derivative-work argument that
exists. A module combined with our shim inside a user process is a different
thing at arm's length — the position FreeBSD has shipped from for a decade.

And practically: this is foreign code. A driver we did not write, cannot fully
audit and did not choose the quality of should not be able to take the machine
down.

### 3. Facts, not expression

A symbol's signature and a struct's field offsets are facts a compiled driver
depends on. Comments, macro bodies and implementations are expression and are
never carried over. Every file in `ldls/kpi/` records where its shape came
from.

Mostly the question does not arise, because of rule 3½:

### 3½. The layouts are not in our source at all

A kernel built with `CONFIG_DEBUG_INFO_BTF` publishes
`/sys/kernel/btf/vmlinux`: every struct layout and every function prototype, as
generated type data describing the kernel that is installed. `ldls/btf.c` reads
it at load time, on the user's machine, from the user's own kernel.

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

Nothing there is ours to have transcribed, and it is right for the kernel
actually present rather than one pinned by guessing.

### 4. One kernel version at a time

Linux states in writing that it has no stable in-kernel ABI. BTF mostly
dissolves this, because it describes the kernel that is there — but a kernel
without BTF needs hand-written layouts for a named version, and that is when
this rule bites.

### 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. Every symbol LDLS does not implement still *resolves*, to a stub that
reports itself and returns zero, so the driver carries on and the next thing it
wants becomes visible. The work queue keeps writing itself.

---

## The four pieces

### The loader — `ldls/ko.c`

A `.ko` is an ELF64 relocatable object. Index the sections, place the
allocatable ones, resolve the undefined symbols, apply the relocations. Written
from the ELF-64 gABI and the x86-64 psABI, both published specifications, so
there is no licence question in it at all.

Measured over a stock 6.18.46 tree: **2,440 modules, every one lays out and
relocates to completion**, five relocation types, nothing unhandled. That sweep
is also what retired the fixed section table — `nvidia-modeset` has 7,069
sections against a median of 50 — so the caller supplies it and there is no
limit to run into.

It is **freestanding**: no libc, no allocation, no I/O. The same source builds
as a host tool, where a format failure costs a second, and as the SchOS-side
host. That is the arrangement `mkschfs` and the kernel already share.

#### Two things it has to fix in the object

**The `%gs` stack guard.** Modules are built `-mstack-protector-guard-reg=gs`,
so the canary is fetched through the GS segment — an ABI between the kernel and
its own per-CPU area that a host which is not that kernel cannot provide.
`ko_defuse_gs()` rewrites those references to `xor %reg, %reg`. There are two
forms: the prologue *loads* the guard with `mov`, the epilogue *compares* it
with `sub` before a `jne`. `xor` is right for both — it zeroes the canary and
sets ZF, so the branch falls through as a passing check does. It checks every
site before patching any and refuses the whole operation on anything it does
not recognise.

**Where it is loaded.** `-mcmodel=kernel` means `R_X86_64_32S` relocations, so
the object has to live where a signed 32-bit absolute address reaches. A
process's mmap area starts at `0x40000000`, which is inside that window; on the
host harness `MAP_32BIT` reaches it from the other end.

### The KPI — `ldls/kpi/`

The Linux symbols, implemented. Register access to a mapped BAR; the PCI calls
to `/dev/pci`; allocation to `mmap` and `PCIIOC_DMA`; `register_netdev` to
`/dev/netdev`. Organised under Linux's own header names so a reader who knows
where something lives in Linux knows where it is here.

`kpi_io_ok()` bounds every `ioread`/`iowrite` against the mapping `pci_iomap`
returned. A stray access would otherwise be a page fault with no context, and
with no IOMMU a hosted driver's wild writes have to be caught early or not at
all.

### The back end — `/dev/pci`

Already built, for `vgpud`, before any of this existed.
[hybrid.md](../plan/hybrid.md) insisted the interface be about *PCI devices*
rather than about a driver model, on the argument that an interface whose first
consumers are native drivers is a general mechanism rather than a hole shaped
like something foreign. A Linux driver's `pci_iomap`, `dma_alloc_coherent` and
`request_irq` land on it unaltered, which is that argument being collected.

### The class bridge — `/dev/netdev`

The one piece the kernel had to grow. `netdev_ops` is four entries —
`transmit`, `receive`, `mac`, `link_up` — so a process claims the device,
supplies a MAC, and `read()`/`write()` carry the frames. `/dev/pci` with the
arrows reversed: that hands a device *to* a process, this lets a process *be*
one. See [devices.md](devices.md).

lwIP never learns that the thing behind the vtable is not a driver.

---

## What it costs

**It polls.** A Linux driver expects its interrupt handler to be called from an
interrupt. `/dev/pci` does deliver them — by blocking `read()`, which needs a
thread `ldlsd` does not have — so the driver's NAPI poll is called from a loop
instead. The same work, in a context LDLS can provide, at the cost of latency
and a spinning process.

**The stack protector is gone**, per `ko_defuse_gs()` above. Foreign code, no
IOMMU, one hardening measure fewer.

**A lock is a no-op.** In one process with no preemption inside the driver
there is nobody to race with. That is correct right up until a driver has an
interrupt thread, and it is the deepest unfixed assumption in the layer.

---

## It can be killed

The whole H2 machinery applies to a foreign driver unchanged, which was the
point of having built it against our own:

| | |
|---|---|
| `ldlsd` running | `ping` works; it holds the device and two DMA blocks |
| killed | interface **down**, device released, bus mastering cleared, **DMA reclaimed** |
| `ping` | fails honestly — `sendto: -5` |
| a second `ldlsd` | `ping` works again, and the arena hands back the same addresses |

`pciuser_release_all()` and `netuser_release_all()` sit beside each other in
`exit()` for the same reason: a process holding a device is one that may crash.

---

## What it does not do

- **One driver, one kernel version, tried once.** `8139too` on 6.18.46 against
  QEMU's rtl8139. Nothing here has been tried on a second driver, a second
  kernel, or real silicon, and until it has, "LDLS runs Linux drivers" is a
  claim rather than a description.
- **No interrupts**, per the polling note above.
- **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; for *foreign*
  drivers the distinction between bug and intent is one we are in no position
  to make. This is the strongest argument for an IOMMU the project has
  produced.
- **It 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 `mmap` does not offer one.
- **Nothing is ever freed.** A hosted driver's allocations last as long as the
  process, which is a leak in the strict sense and the right trade for a
  program that will be killed to restart it.
- **BTF has to come from somewhere.** SchOS has no `/sys/kernel/btf/vmlinux`,
  so the user brings it with the module, from the kernel the module was built
  for. A kernel built without `CONFIG_DEBUG_INFO_BTF` has neither.
- **No config-space writes.** `/dev/pci` has none, deliberately — a write is
  how a BAR gets moved out from under the kernel — so a driver that needs one
  is refused loudly rather than quietly succeeded.
- **No module-to-module dependencies, no firmware loading, no sysfs, no
  unload.** `request_firmware` fails, `module_exit` is never called, and the
  way to stop a driver is to kill the process.
- **Some symbols are still logging stubs.** Deliberate, per rule 5 — but a
  driver reaching one gets zero and carries on, and `memcpy` proved how quiet
  that can be: it was a stub while the receive path was being brought up, so
  every frame arrived as the right *length* of zeroes and nothing reported an
  error. **A stub that returns zero is not safe for a function whose whole job
  is a side effect.**