# 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 | | **L3** | **A packet.** The process-backed netdev, wired to the driver's transmit and receive paths. **`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. --- ## 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. - **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. - **`probe()` is reached with invented hardware.** The BARs are numbers the harness made up and `pci_iomap` returns a zeroed page. A driver that reads a status register gets whatever was there, which is why 8139too read a MAC address of zeroes and was perfectly happy about it. - **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.