d0f7acf9eb
An x86-64 operating system built from xv6, now largely rewritten: SMP, a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF rasteriser, and a Linux-numbered syscall ABI. No history before this point -- the tree was never under version control. This commit exists so the rename that follows has something to undo to.
241 lines
10 KiB
Markdown
241 lines
10 KiB
Markdown
# Licensing
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The one page to check before copying anyone else's code into Schunix.
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Referenced from `LICENSE`, `LICENSE.MIT` and `NOTICE`, all three of which
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pointed here long before this file existed.
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---
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## 1. Schunix is Apache-2.0
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Copyright Nexoniarz. See `LICENSE`.
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Apache-2.0 was chosen over MIT/BSD for one reason that actually matters at OS
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scale: it contains an **express patent grant** and a patent-retaliation clause.
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MIT and BSD are silent on patents, which means a contributor can hand you code
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and later assert a patent over it. For a kernel that is intended to accept
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outside contributions, that silence is the expensive kind.
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What that grant means in practice: Apache-2.0 §2 gives everyone a perpetual,
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worldwide, irrevocable licence to reproduce, modify, sublicense and distribute
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Schunix. Any additional restriction stated elsewhere — in a README, a website,
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or a policy — does not narrow it. The licence text is what holds up.
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If restricted distribution is ever genuinely wanted, that is a licence change
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(a bespoke proprietary licence, or something like BUSL-1.1 with a conversion
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date), not a wording change, and it means giving up the Apache grant.
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---
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## 2. What can be copied in
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| Licence | Verdict | Obligation |
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| Public domain, 0BSD, CC0 | **Yes** | None |
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| MIT, ISC | **Yes** | Keep the notice |
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| BSD 2-clause | **Yes** | Keep the notice |
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| BSD 3-clause | **Yes** | Keep the notice; no endorsement claims |
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| BSD **4-clause** (advertising) | **Avoid** — see §3 | Notice *and* an acknowledgement in advertising materials |
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| Apache-2.0 | **Yes** | Keep NOTICE contents |
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| MPL-2.0 | Case by case | File-level copyleft; keep those files separate |
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| **GPL-2.0-only** | **No** | — |
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| **GPL-3.0 / LGPL-3.0** | **No** for linked code | — |
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**Every "yes" above requires an entry in `NOTICE`.** That file is the project's
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attribution record and the thing that makes the permissive licences actually
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satisfied rather than merely tolerated.
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### Why GPL-2.0-only is a hard no
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Apache-2.0 imposes conditions GPLv2 treats as "further restrictions" under its
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§6 — principally the patent-retaliation term. The FSF and the ASF agree on
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this: **Apache-2.0 code and GPL-2.0-only code cannot be combined into one
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distributable work.** It is not a matter of taste or of how carefully the files
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are kept apart in the tree; it is that the combined binary cannot satisfy both
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licences at once.
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GPL-3.0 fixed this in the other direction (Apache-2.0 is compatible *into*
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GPLv3), but that does not help: the result would have to ship under GPLv3, and
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Schunix does not.
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**Practical consequence:** BSD drivers can be adopted. Linux drivers cannot,
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ever, no matter how convenient. This is why the USB stack was written from the
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specification rather than ported.
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---
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## 3. NetBSD in particular
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`src-trunk/` is a NetBSD checkout kept as reference material and a parts bin
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(see [architecture/schapi.md](architecture/schapi.md) §5). NetBSD is **not
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uniformly licensed**, and the difference matters per file:
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| Template | Status |
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| 2-clause BSD — modern NetBSD Foundation files | **Fine** |
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| 3-clause, "Regents of the University of California" | **Fine.** Berkeley formally rescinded the advertising clause in 1999 |
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| 4-clause with the advertising clause, **non-UC** copyright holder | **Avoid.** Berkeley's rescission covers Berkeley's copyright, not other people's |
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The last row is real, not hypothetical. Measured on this checkout:
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```
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sys/kern 26 of 220 files carry the advertising clause
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sys/dev 500 of 3565
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lib/libc 101 of 1575
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sys/net 5 of 180 ← the TCP/IP stack is nearly clean
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```
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4-clause is still permissive and still redistributable; the burden is an
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advertising obligation that accumulates quietly and is tedious to honour
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properly. Prefer a 2-clause or UC-Regents file. If a 4-clause file is genuinely
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the right answer, it goes into `NOTICE` with the full clause quoted.
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Check before copying, not after:
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```sh
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grep -l "All advertising materials" <file>
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```
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### And check *whose* clause it is
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A file can carry several licence blocks from several copyright holders. Finding
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the advertising clause is only half the check — Berkeley's rescission covers
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Berkeley's copyright and nobody else's, so the question is always *whose*
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clause you found.
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Measured on `src-trunk`, the networking tree:
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```
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sys/net 106 files, 5 with the advertising clause
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sys/netinet 121 files, 25 " " "
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sys/netinet6 51 files, 5 " " "
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```
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Encouraging at first glance — the 25 in `sys/netinet` are almost all SCTP,
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DCCP and multicast routing, none of which Schunix needs. `ip_input.c`,
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`ip_output.c`, `udp_usrreq.c`, `if_arp.c`, `ip_icmp.c`, `if.c` and `route.c`
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are all UC 3-clause, i.e. fine.
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**But `tcp_input.c` and `tcp_output.c` are not.** Their advertising clause
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belongs to the **WIDE Project** (the KAME IPv6 people), whose copyright sits at
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the top of the file; the UC block is a separate licence further down. Berkeley
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never spoke for WIDE, so the rescission does not reach it. The two most
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valuable files in the stack are the two encumbered ones.
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That is not fatal — 4-clause is still permissive — but it is exactly the case
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where a **second BSD tree earns its disk space**: FreeBSD's TCP has been
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through a relicensing effort NetBSD's has not, so the same file there may well
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be 2-clause. Check it the same way before assuming either:
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```sh
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grep -n "Copyright" <file> | head # who claims it
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grep -n -B6 "All advertising" <file> # whose clause is it
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```
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### FreeBSD's TCP is clean where NetBSD's is not
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Checked, 2026-08-08, exactly that way. Against FreeBSD `main`:
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`sys/netinet/tcp_input.c` contains **no advertising clause at all**. Its
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copyright holders are the UC Regents, Swinburne University, Lawrence Stewart,
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the FreeBSD Foundation and Juniper Networks -- all BSD-2 or BSD-3.
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So if BSD networking code is taken, it must come from **FreeBSD, not NetBSD**,
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and the specific reason is TCP. This reverses the earlier assumption in
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`docs/roadmap.md` that NetBSD was the intended starting point; NetBSD remains
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the better-written reference to *read*, but FreeBSD is the one that can be
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copied from.
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### The third option: lwIP
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Before porting either BSD stack, weigh **lwIP** against both. Verified
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2026-08-08 from its `COPYING`: 3-clause BSD, Swedish Institute of Computer
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Science, **no advertising clause anywhere in the file**. Compatible, one NOTICE
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entry, done.
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The licence is the smaller half of the argument. The larger half is that
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FreeBSD's `sys/netinet` is not a library -- `tcp_input.c` alone is 122 KB and
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reaches into mbufs, `epoch(9)`, netisr, VNET, `uma(9)`, sockbufs, inpcb hash
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tables and FreeBSD's locking. Taking it means porting FreeBSD's kernel
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infrastructure first and finding the TCP somewhere underneath, which is the same
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conclusion `docs/roadmap.md` §1 already reached about `sys/dev/usb`.
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lwIP was written for the case Schunix is in: a small operating system with a raw
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Ethernet driver and no infrastructure to speak of. Its port layer is a handful
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of files, it has no dependency on the host kernel's allocator or locking, and it
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brings ARP, IPv4, ICMP, UDP, TCP, DHCP and DNS.
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### Fonts
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Chosen 2026-08-09, both licences checked at source rather than assumed:
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| Font | Where | Licence |
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| Roboto | desktop, window manager, compositor | **Apache-2.0** |
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| JetBrains Mono | terminal, TTY | **SIL OFL 1.1** |
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Roboto is the same licence as Schunix, so it is the easy case: a NOTICE entry
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and nothing else.
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OFL is permissive and bundling is precisely what it is for, with two conditions
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worth knowing before anyone subsets or converts a font:
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- **The font stays under OFL** even inside an Apache-2.0 system. It does not
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infect anything else; it just cannot be relicensed.
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- **It may not be sold on its own.** Shipping it as part of an OS is fine.
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- **Reserved Font Names.** JetBrains Mono declares *none* -- its copyright line
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names no reserved name -- so a modified version may keep the name. Most OFL
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fonts do reserve theirs, and then a derivative must be renamed.
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Pre-rasterising a font into bitmaps at build time **is** a modification, so
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this is not a theoretical point for this project.
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---
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## 4. Things that are deliberately not shipped
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Some code is fine to *build and run locally* and impossible to *distribute*
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with Schunix. Keeping the distinction sharp is what stops it eroding:
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- **`Refrence/`** — a FreeBSD checkout (`Refrence/freebsd-src`), kept as
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reference material and a parts bin. Never built, never in an image. The
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directory name is spelled as it is on disk; renaming it is a one-line change
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if it ever grates.
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- **`src-trunk/`** — a NetBSD checkout, same rules. Currently absent; it was
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deleted for disk space and can be re-fetched sparsely if needed.
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- **`dist/`** — upstream tarballs and other downloaded blobs. Cached so a
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rebuild never needs the network; not part of any image, and not all of it is
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redistributable.
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The mechanisms that keep this from eroding, since good intentions have already
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failed once here:
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- `NOTPORTS` in `ports/Makefile` — a port that lives in the tree but is never
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built by `make`.
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- `RAMDISK_EXCLUDE` in the root `Makefile` — paths kept out of the ramdisk root.
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This one matters more than it looks: the ramdisk image is linked *into the
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kernel binary*, so anything sitting in `sysroot/` when that image is built
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becomes part of a distributable artifact. A GPL-2.0 or non-redistributable
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file staged there does not merely ride along, it makes the kernel itself
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undistributable.
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A previous acceptance test — a ported 1993 shooter, GPL-2.0 engine on
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non-redistributable shareware data — was carried under exactly these rules and
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has since been removed. It is worth recording *why* it was safe: it was never in
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`PORTS`, and its 33 MB of data was excluded from the ramdisk. It is equally
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worth recording that the safety depended on two separate opt-outs staying
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correct, which is a lot of standing risk for a test that had stopped finding
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bugs. See `docs/roadmap.md` Phase 7.
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---
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## 5. Specifications are not code
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A published specification carries no copyright over an independent
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implementation of it. The xHCI, USB, PCI, fbdev and ELF specifications were all
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read and implemented directly, which is why those drivers have no licence
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entanglement at all despite equivalents existing in both Linux and BSD.
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When a subsystem is small enough to write from the spec, that is usually the
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cheapest option in the long run — not because copying is hard, but because a
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copied file is a file to be understood, attributed and maintained forever.
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