SchOS/docs/plan/foundations.md

Foundations

The kernel work that wayland.md and hybrid.md both stop at. This is the critical path, and it is the reason those two are one plan rather than two.

The insight is small and it decides the schedule: the substrate for passing a buffer to a compositor and the substrate for handing a device to a driver process are the same substrate. Descriptor passing and shared memory, built once.

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What is actually in the tree

Measured against the source, not against a README. Each of these was checked before anything below was planned around it:

sendmsg/recvmsg, and therefore SCM_RIGHTS	Built. See item 1 below.
Writable MAP_SHARED of a file	Refused — kernel/sys/sys_mem.c:131 returns -ENODEV.
memfd_create	Does not exist.
AF_UNIX names	In a table in kernel/net/unix.c, not in the filesystem.
setsockopt	return 0; unconditionally — kernel/sys/sys_net.c:436.
exec	ET_EXEC only. No PT_INTERP, no ET_DYN.

projects/schcom/README.md said SCM_RIGHTS was done while nothing in the kernel implemented it — the most load-bearing wrong line in the tree, with an entire GUI plan drawn on top of it. The line is now true, which is not the same as it having been right, and the README says so.

The MAP_SHARED refusal is worth defending rather than fixing carelessly. The code returns -ENODEV honestly: the contract of a writable shared mapping is that stores land in the file and other mappers see them, and there is no page cache that could make that true. Quietly handing back a private copy would be the same lie in a new place.

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The six items

In dependency order. The first three are what Wayland cannot start without. All but item 4 are built.

1. sendmsg / recvmsg with SCM_RIGHTS — done

Not optional and not substitutable. wl_shm_create_pool passes a file descriptor; there is no path in the Wayland protocol that passes an integer instead.

The handle_kind field in projects/schcom/proto.h was good foresight and it cannot help here — our protocol is the flexible one, and Wayland's is not.

Built as described: struct msghdr/cmsghdr in Linux's layout, ancillary data queued alongside the byte stream in kernel/net/unix.c, descriptor installation into the receiver's file table, and the three ways a message can end — received, received without room, never received — each releasing the kernel's reference. Fourteen checks in cmd/socktest, every one of them moving data through the descriptor rather than inspecting it. Documented in ../architecture/networking.md.

The delivery rule is the part worth knowing before building on it: descriptors are pinned to the stream position of the first byte of the message that carried them, and a read that has not reached that position is cut short so it stops before it. That is what keeps "which message did these belong to" answerable.

This was also item 1 of the hybrid work, where it is how a driver process is handed the device it is allowed to touch.

Two bugs fell out of building it, neither in the new code:

• sockpair() never set the socket's domain, so every socketpair() socket claimed domain 0. Invisible for as long as the data paths asked only whether a socket was AF_INET or AF_NETLINK — zero answers "neither" correctly. SCM_RIGHTS asks "is this AF_UNIX", and zero is wrong for that one.
• pipe_poll() reports POLLHUP but not POLLIN for a pipe at end of file, so a non-blocking read of a pipe whose writers have all gone returned EAGAIN forever instead of 0. A program that polls and reads would spin waiting for an end that had already happened. sockreadable() always had the equivalent condition; the pipe path did not.

2. A shareable memory object — done

memfd_create, plus mmap(MAP_SHARED) on it genuinely working between two processes. Described in ../architecture/kernel.md; asserted by cmd/memfdtest, seventeen checks including the one that matters — a descriptor sent over a socket, mapped by a receiver that has closed every inherited copy, with stores visible in both directions.

Narrower than a general page cache, and sufficient. An anonymous shared object needs no file behind it, which is exactly why memfd exists on Linux at all — and it means this can be built on the shared-memory machinery already in kernel/sys/sys_mem.c (shmget/shmat/shmdt work, and struct vma already carries shmid and a no_free flag for pages that are not ours to release).

The general answer — a page cache, and writable MAP_SHARED of a real file — stays owed. It is a larger project and nothing on this plan needs it.

3. AF_UNIX sockets in the filesystem namespace — done

Clients find a Wayland compositor by bind()ing $XDG_RUNTIME_DIR/wayland-0 as a real path, and libwayland flock()s the lock file beside it. A name table can express neither.

bind() creates a T_SOCK node and connect() resolves the path and matches the socket bound to that inode — not to a string, which is what made two spellings of one path miss each other. ls -l shows srwxr-xr-x, stat() reports S_IFSOCK, and unlink() removes it. Asserted by cmd/socktest, section 15. flock() is still not implemented; see below.

kernel/net/unix.c's own header comment already predicted this:

"What is lost is ls -l /tmp/schcom showing an s, and unlink() of a socket path failing with ENOENT. Both are worth fixing when the VFS next gets opened up; neither blocks anything."

They block something now. This is that moment, and it is a VFS change rather than a socket change: a socket inode type threaded through iget, dirlookup and stat, plus flock.

4. setsockopt that does not lie

At minimum SO_PASSCRED and SO_SNDBUF. Returning 0 for an option you ignored is worse than ENOPROTOOPT, because the caller adjusts its behaviour on the strength of the lie.

Small, and grouped here because it is in the same file as item 3 and will be found by the same testing.

5. Dynamic linking — done

PT_INTERP and ET_DYN in kernel/fs/exec.c. A program that names an interpreter gets it loaded at the bottom of the mmap area and started instead, with AT_PHDR, AT_PHENT, AT_PHNUM, AT_BASE and AT_ENTRY to find the program by; a PIE program is placed where a fixed one would be. The interpreter is musl's own: ports/musl now builds libc.so as well, shipped as /lib/ld-musl-x86_64.so.1, which is libc and loader in one file.

The kernel half that was missing besides exec was mmap(MAP_FIXED) of a file. It handed back zeroed pages whatever the descriptor said, and kept no record of them for fork — and a dynamic linker maps every library segment after the first exactly that way.

Every port still links -static, on purpose: this makes the other kind possible, it does not convert anything. cmd/dynhello is the test — a program against libc.so and a library of its own, checking TLS, the heap and a forked child that still has its libraries.

mprotect is accepted and not enforced, so RELRO is not read-only yet.

6. The event descriptors — done

epoll, eventfd, timerfd and signalfd, in kernel/sys/sys_event.c. libwayland's event loop is all four in one epoll set, which is why they arrive together; not in the original list because they were stubs that looked like syscalls, returning ENOSYS from table.inc entries that read as done.

They wait the way poll() does, a tick at a time, so the latency is 10 ms. cmd/evtest checks each one, then all four in one set, level- and edge-triggered.

Building it found a signal bug: a default-fatal signal killed on arrival even when it was blocked. A blocked signal now stays pending, which is the contract signalfd is built on.

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Verification

Each item gets a test before the thing that needs it is built, because a substrate that is subtly wrong fails inside the consumer and looks like the consumer's bug.

• Item 1: two processes, one passes an open descriptor, the other reads through it. Plus the leak case: a sender that exits with descriptors in flight.
• Item 2: two processes map the same memfd, one writes, the other sees it.
• Item 3: bind() a socket, then stat() and ls -l the path and see an s, then unlink() it.
• Items 1–3 together: a third test that does all of it at once, because that combination is what Wayland actually performs and the three passing separately does not prove it.

These belong in bootcheck as a new switch, following the -Sock model.

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What this does not deliver

• No page cache, so writable MAP_SHARED of a real file stays refused.
• No dlopen until item 5 is done, which means no Mesa and no third-party shared libraries.
• SO_* coverage is the two options named, not the full set.
• No credential passing (SCM_CREDENTIALS) — only descriptors. Nothing on this plan needs it, and a compositor that wants to know who connected will.
• No flock(). libwayland locks wayland-0.lock beside the socket to stop two compositors claiming one display. The socket itself is now a real path, so the lock file can be created and opened; what is missing is the advisory lock, which is a separate mechanism and has no other caller yet.