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 §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:

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:

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.

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 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. 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 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.