SchOS/docs/licensing.md

# Licensing

The one page to check before copying anyone else's code into SchOS.

Referenced from `LICENSE`, `LICENSE.MIT` and `NOTICE`, all three of which
pointed here long before this file existed.

---

## 1. SchOS is GPL-2.0-or-later

Copyright Nexoniarz. See `LICENSE`. Until 2026-10 SchOS was Apache-2.0; it was
never published under that licence, and as the sole author Nexoniarz relicensed
it outright.

**Why the GPL.** Two reasons, one about others and one about us:

- **Copyleft.** Anyone may use, study, modify and redistribute SchOS — but
  whoever ships it, or something built from it, must ship the source of that
  too, under the same terms. Under Apache-2.0 a fork could be closed and sold
  with no changes given back. The GPL does not forbid commercial use; it forbids
  taking the work private.
- **Linux.** Linux is GPL-2.0-only, and Apache-2.0 could not be combined with
  it. Under the GPL, Linux drivers and code can be adopted (§2).

**Why "or later".** It keeps one door open that "only" would close: GPLv3 and
Apache-2.0 code can still be combined with SchOS code, with that combination
shipping under GPLv3. See §2 for why the kernel should not walk through it.

**What was given up.** Apache-2.0 had an express patent grant; GPLv2 has only
an implied one. That mattered most for outside contributions, and there are
none yet.

New first-party files carry `SPDX-License-Identifier: GPL-2.0-or-later`.

---

## 2. What can be copied in

| Licence | Verdict | Obligation |
|---|---|---|
| Public domain, 0BSD, CC0 | **Yes** | None |
| MIT, ISC | **Yes** | Keep the notice |
| BSD 2-clause | **Yes** | Keep the notice |
| BSD 3-clause | **Yes** | Keep the notice; no endorsement claims |
| BSD **4-clause** (advertising) | **No** — see §3 | GPL-incompatible |
| GPL-2.0-only, GPL-2.0-or-later, LGPL-2.1 | **Yes** | Source for the combined work; keep the notices |
| MPL-2.0 | Case by case | Compatible through its secondary-licence clause, unless marked otherwise |
| Apache-2.0, GPL-3.0, LGPL-3.0 | **Not in the kernel**; separate programs and files fine | See below |

**Every "yes" above requires an entry in `NOTICE`.** That file is the project's
attribution record and the thing that makes the licences actually satisfied
rather than merely tolerated.

### One binary, one GPL version

"Or later" means SchOS code can go out under GPLv2 *or* GPLv3. A single linked
binary still has to pick one:

- Add **Linux code** (GPL-2.0-only) and that binary is GPLv2.
- Add **Apache-2.0 or GPLv3 code** and that binary is GPLv3, because
  Apache-2.0's patent terms are "further restrictions" under GPLv2 §6.

Those two cannot meet in one binary. Linux is the main reason for the GPL, so
**the kernel stays GPLv2-compatible**: no Apache-2.0 or GPLv3 code is linked
into it. Userland programs are separate works, so one may be GPLv3 or
Apache-2.0 on its own without affecting anything else.

Data files such as fonts are not linked code. Roboto (Apache-2.0) sits beside
the system as an aggregate, and that holds inside the ramdisk too: the image is
a filesystem carried by the kernel binary, not code linked with it.

---

## 3. NetBSD in particular

`src-trunk/` is a NetBSD checkout kept as reference material and a parts bin
(see [plan/schapi.md](plan/schapi.md) §5). NetBSD is **not
uniformly licensed**, and the difference matters per file:

| Template | Status |
|---|---|
| 2-clause BSD — modern NetBSD Foundation files | **Fine** |
| 3-clause, "Regents of the University of California" | **Fine.** Berkeley formally rescinded the advertising clause in 1999 |
| 4-clause with the advertising clause, **non-UC** copyright holder | **No.** Berkeley's rescission covers Berkeley's copyright, not other people's, and the clause is GPL-incompatible |

The last row is real, not hypothetical. Measured on this checkout:

```
sys/kern      26 of  220 files carry the advertising clause
sys/dev      500 of 3565
lib/libc     101 of 1575
sys/net        5 of  180      ← the TCP/IP stack is nearly clean
```

4-clause is still permissive on its own, but the advertising clause is a
further restriction the GPL does not allow, so **a 4-clause file cannot be
combined with SchOS at all**. Use a 2-clause or UC-Regents file, or write it.

Check before copying, not after:

```sh
grep -l "All advertising materials" <file>
```

### And check *whose* clause it is

A file can carry several licence blocks from several copyright holders. Finding
the advertising clause is only half the check — Berkeley's rescission covers
Berkeley's copyright and nobody else's, so the question is always *whose*
clause you found.

Measured on `src-trunk`, the networking tree:

```
sys/net        106 files,  5 with the advertising clause
sys/netinet    121 files, 25   "        "         "
sys/netinet6    51 files,  5   "        "         "
```

Encouraging at first glance — the 25 in `sys/netinet` are almost all SCTP,
DCCP and multicast routing, none of which SchOS needs. `ip_input.c`,
`ip_output.c`, `udp_usrreq.c`, `if_arp.c`, `ip_icmp.c`, `if.c` and `route.c`
are all UC 3-clause, i.e. fine.

**But `tcp_input.c` and `tcp_output.c` are not.** Their advertising clause
belongs to the **WIDE Project** (the KAME IPv6 people), whose copyright sits at
the top of the file; the UC block is a separate licence further down. Berkeley
never spoke for WIDE, so the rescission does not reach it. The two most
valuable files in the stack are the two encumbered ones.

Under the GPL that rules them out, and it is exactly the case
where a **second BSD tree earns its disk space**: FreeBSD's TCP has been
through a relicensing effort NetBSD's has not, so the same file there may well
be 2-clause. Check it the same way before assuming either:

```sh
grep -n "Copyright" <file> | head        # who claims it
grep -n -B6 "All advertising" <file>     # whose clause is it
```

### FreeBSD's TCP is clean where NetBSD's is not

Checked, 2026-08-08, exactly that way. Against FreeBSD `main`:
`sys/netinet/tcp_input.c` contains **no advertising clause at all**. Its
copyright holders are the UC Regents, Swinburne University, Lawrence Stewart,
the FreeBSD Foundation and Juniper Networks -- all BSD-2 or BSD-3.

So if BSD networking code is taken, it must come from **FreeBSD, not NetBSD**,
and the specific reason is TCP. This reverses the earlier assumption in
`docs/plan/roadmap.md` that NetBSD was the intended starting point; NetBSD remains
the better-written reference to *read*, but FreeBSD is the one that can be
copied from.


### The third option: lwIP

Before porting either BSD stack, weigh **lwIP** against both. Verified
2026-08-08 from its `COPYING`: 3-clause BSD, Swedish Institute of Computer
Science, **no advertising clause anywhere in the file**. Compatible, one NOTICE
entry, done.

The licence is the smaller half of the argument. The larger half is that
FreeBSD's `sys/netinet` is not a library -- `tcp_input.c` alone is 122 KB and
reaches into mbufs, `epoch(9)`, netisr, VNET, `uma(9)`, sockbufs, inpcb hash
tables and FreeBSD's locking. Taking it means porting FreeBSD's kernel
infrastructure first and finding the TCP somewhere underneath, which is the same
conclusion `docs/guides/adding-a-driver.md` already reached about `sys/dev/usb`.

lwIP was written for the case SchOS is in: a small operating system with a raw
Ethernet driver and no infrastructure to speak of. Its port layer is a handful
of files, it has no dependency on the host kernel's allocator or locking, and it
brings ARP, IPv4, ICMP, UDP, TCP, DHCP and DNS.

### uACPI

The AML interpreter, `ports/uacpi`, compiled into the kernel like lwIP.
Verified 2026-10-22 from its `LICENSE`: MIT, Daniil Tatianin. Compatible, one
NOTICE entry. The choice over ACPICA was not about licensing but size: uACPI
asks a host for far less, and its whole host side is `kernel/acpi/host.c`.

### Fonts

Chosen 2026-08-09, both licences checked at source rather than assumed:

| Font | Where | Licence |
|---|---|---|
| Roboto | desktop, window manager, compositor | **Apache-2.0** |
| JetBrains Mono | terminal, TTY | **SIL OFL 1.1** |

Roboto is Apache-2.0, which is fine for a font: it is a data file beside the
system, not code linked into it (§2). A NOTICE entry and nothing else.

OFL is permissive and bundling is precisely what it is for, with two conditions
worth knowing before anyone subsets or converts a font:

- **The font stays under OFL** even inside a GPL-licensed system. It does not
  infect anything else; it just cannot be relicensed.
- **It may not be sold on its own.** Shipping it as part of an OS is fine.
- **Reserved Font Names.** JetBrains Mono declares *none* -- its copyright line
  names no reserved name -- so a modified version may keep the name. Most OFL
  fonts do reserve theirs, and then a derivative must be renamed.
  Pre-rasterising a font into bitmaps at build time **is** a modification, so
  this is not a theoretical point for this project.

---

## 4. Things that are deliberately not shipped

Some code is fine to *build and run locally* and impossible to *distribute*
with SchOS. Keeping the distinction sharp is what stops it eroding:

- **`Refrence/`** — a FreeBSD checkout (`Refrence/freebsd-src`), kept as
  reference material and a parts bin. Never built, never in an image. The
  directory name is spelled as it is on disk; renaming it is a one-line change
  if it ever grates.
- **`src-trunk/`** — a NetBSD checkout, same rules. Currently absent; it was
  deleted for disk space and can be re-fetched sparsely if needed.
- **`dist/`** — upstream tarballs and other downloaded blobs. Cached so a
  rebuild never needs the network; not part of any image, and not all of it is
  redistributable.

### The userland, and why it is permissive anyway

While SchOS was Apache-2.0, **BusyBox** (GPL-2.0-only) and **nano** (GPL-3.0)
were ruled out, and the userland was built from toybox (0BSD), mksh and kilo
(BSD-2-Clause) instead. Both would be possible now — nano as a separate program
— but nothing needs replacing: toybox gives 268 applets, kilo is installed as
`/bin/edit`, and `cmd/httpd` is first-party.

The `wget` failure that started that discussion was not a userland problem:
SchOS had no DNS reachable from a process, which BusyBox would have hit
identically. See [architecture/networking.md](architecture/networking.md).

The mechanisms that keep this from eroding, since good intentions have already
failed once here:

- `NOTPORTS` in `ports/Makefile` — a port that lives in the tree but is never
  built by `make`.
- `RAMDISK_EXCLUDE` in the root `Makefile` — paths kept out of the ramdisk root.
  This one matters more than it looks: the ramdisk image is linked *into the
  kernel binary*, so anything sitting in `sysroot/` when that image is built
  becomes part of a distributable artifact. A GPL-2.0 or non-redistributable
  file staged there does not merely ride along, it makes the kernel itself
  undistributable.

A previous acceptance test — a ported 1993 shooter, GPL-2.0 engine on
non-redistributable shareware data — was carried under exactly these rules and
has since been removed. It is worth recording *why* it was safe: it was never in
`PORTS`, and its 33 MB of data was excluded from the ramdisk. It is equally
worth recording that the safety depended on two separate opt-outs staying
correct, which is a lot of standing risk for a test that had stopped finding
bugs. See `docs/guides/testing.md`.

---

## 5. Specifications are not code

A published specification carries no copyright over an independent
implementation of it. 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 despite equivalents existing in both Linux and BSD.

When a subsystem is small enough to write from the spec, that is usually the
cheapest option in the long run — not because copying is hard, but because a
copied file is a file to be understood, attributed and maintained forever.

---

## 6. LDLS, and shipping Linux's drivers

[architecture/ldls.md](architecture/ldls.md) runs Linux drivers. This section used to be about a
distribution boundary: written when SchOS was Apache-2.0, it said a GPL-2.0-only `.ko` could not be
shipped and so SchOS would ship the layer and never the modules. **That no longer holds.** SchOS is
GPL-2.0-or-later, GPL-2.0-only code combines with it, and the owner has decided that Linux code and
firmware blobs are shipped. The ISO carries Linux's **r8169 set** — `mdio-bus`, `libphy`,
`mdio_devres`, the Realtek PHY driver and `r8169`, unmodified — and the RTL8168 PHY firmware.

### What shipping them asks of us

- **The licence stays with the code.** The modules are GPL-2.0-only, the firmware is Realtek's, redistributed
  under linux-firmware's terms. `NOTICE` lists both, with where they come from.
- **The source goes with the binary** (GPLv2 §3). The modules are stock builds of the Linux kernel version
  that `make ldls-r8169` took them from, so the corresponding source is that release of Linux, unmodified.
- **Nothing combines at link time.** The modules are loaded into an ordinary process by our loader; our code
  is GPL-2.0-or-later, theirs GPL-2.0-only, and both are GPLv2-compatible whichever way one reads "combined".
- **No binary in git.** The repository stays source-only: `.gitignore` refuses `*.ko`, and `make ldls-r8169`
  stages the set from a Linux build into the image.

### LDLS is ours

Every file under `ldls/` is ours, GPL-2.0-or-later. The loader was written from the ELF-64 gABI and the
x86-64 psABI; the shim is written under the rule in [architecture/ldls.md](architecture/ldls.md). Copying
Linux code into it would no longer be a licence problem, but the shim's value is that it is small and
understood, so the existing rule stands.

### `EXPORT_SYMBOL_GPL` does not apply

Linux marks much of its internal API `EXPORT_SYMBOL_GPL`, and modules carry
`MODULE_LICENSE`. Supplying those symbols while asserting a licence one does
not hold would be circumvention, and it is worth saying why LDLS is not doing
that.

**LDLS has no Linux symbols to gate.** `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 GPL-2.0-or-later. There is no
gate being bypassed, because there is no Linux symbol being exposed.

So LDLS implements no licence check. It **reads** the module's declared licence
and prints it before loading anything, because that is the one property that
decides whether an object should be on a machine at all and it costs nothing to
show.

### BTF: the layouts are not in our source at all

Rule 3 of [architecture/ldls.md](architecture/ldls.md) has one uncomfortable row — struct layouts. A compiled driver
indexes fields by offset, so the shim has to reproduce them exactly, and a
field-for-field match *looks* like transcription even when it is not.

It does not have to be in our source. Kernels built with `CONFIG_DEBUG_INFO_BTF`
— which is most distribution kernels — publish `/sys/kernel/btf/vmlinux`, and
that file carries both halves of what the shim needs:

```
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
```

Layouts **and** prototypes, as generated binary type data describing the kernel
the user already has installed. LDLS can read them at load time, on the user's
machine, from the user's own kernel — so our source contains neither, and there
is nothing to have transcribed.

Three things improve together: the provenance question largely disappears, the
version-pinning problem (plan §4) mostly dissolves because BTF describes the
kernel that is actually there, and it is less work than writing hundreds of
struct definitions by hand.

**What BTF does not give is behaviour.** It says `napi_complete_done` takes
these arguments and returns that type. What it *does* is still ours to
implement from documentation, and that is the real work — BTF does not touch
it.