sockets: a descriptor can cross between processes

sendmsg/recvmsg with SCM_RIGHTS. Item 1 of docs/plan/foundations.md and
mechanism 1 of docs/plan/hybrid.md, which are the same primitive seen from two
directions: Wayland's wl_shm_create_pool passes a file descriptor with no
integer-shaped alternative, and handing a driver process the device it is
allowed to touch is the same act. Both plans stopped here.

Linux's structures byte for byte -- msghdr, cmsghdr, the CMSG_ macros -- because
the programs that need this are not ours. libwayland and toybox both call
recvmsg with Linux's headers and would not notice a friendlier door.

The delivery rule is the whole design. A batch of descriptors is pinned to the
stream position of the first byte of the message that carried it, and that has
two halves: a reader arriving at that position is handed them, and a reader that
has not reached it is stopped SHORT of it. Leave out the second half and a
program asking for 4096 bytes swallows three messages and is handed one
message's descriptors, with nothing in the result saying which. socktest checks
exactly that: two one-byte messages, an eight-byte read, one byte back.

The kernel holds a reference to every descriptor in flight, so a sender may
close its own copy the moment sendmsg returns. Giving that reference back is the
part that leaks if it is not written down, and there are three ways a message
ends:

  - received with room            installed in the receiver's table
  - received without room, or by
    plain read(2)                 closed; MSG_CTRUNC for the first case
  - never received, socket closed closed by sockclose

The middle row looks like a choice and is not: holding kernel references for a
receiver that has proved it is not looking is a leak with extra steps.

No garbage collector. A socket descriptor sent over that same socket and never
received holds a reference to the socket that holds the reference to it, and
neither close reaches zero. Linux carries several hundred lines for this.
Nothing on either plan passes a socket over a socket -- Wayland passes memory,
the driver work passes devices -- so it is a bounded leak that is documented
rather than a silent one.

Fourteen checks in cmd/socktest, and every one moves data through the descriptor
rather than inspecting it. The descriptor passed is always a pipe end, because a
pipe announces its own lifetime: a read of a pipe whose writers are all gone
returns 0, which turns "was the kernel's reference released" -- otherwise
invisible from userspace -- into a value a test can compare.

Two bugs fell out, neither in the new code:

  - sockpair() never set the socket's domain, so every socketpair() socket
    claimed domain 0. Invisible while the data paths asked only whether a socket
    was AF_INET or AF_NETLINK, since 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 at end of file, so a non-blocking
    read of a pipe whose writers had all gone returned EAGAIN forever instead of
    0 -- a program that polls and reads spins waiting for an end that already
    happened. sockreadable() always had the equivalent condition; the pipe path
    did not. Found because the test helper for "is this descriptor closed" is a
    non-blocking read of a pipe.

projects/schcom/README.md said SCM_RIGHTS was done for a long time while nothing
in the kernel implemented it, and an entire GUI plan was drawn on that line.
It is true now, which is not the same as it having been right, and the README
says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/sys/table.inc

@@ -140,6 +140,10 @@ SYSCALL(192,         55, getsockopt)
 // takes three arguments, so reading a fourth register from an accept() caller
 // reads whatever it last left there and invents a flag out of stack litter.
 SYSCALL(198,        288, accept4)
+// sendmsg/recvmsg exist for the one thing only they can do: carry a descriptor
+// from one process to another (SCM_RIGHTS). docs/plan/foundations.md item 1.
+SYSCALL(199,         46, sendmsg)
+SYSCALL(200,         47, recvmsg)
 
 // --- time ---
 SYSCALL( 28,         37, alarm)