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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
132 lines
5.9 KiB
Markdown
132 lines
5.9 KiB
Markdown
# Syscalls, and the Linux personality
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Two things happen at this boundary. SchOS has its own syscall numbering, and it
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also answers Linux's — which is why musl, toybox and mksh run unmodified.
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---
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## One table, everything generated
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**Syscalls are declared in exactly one place:**
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[`kernel/sys/table.inc`](../../kernel/sys/table.inc). One line each.
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```c
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SYSCALL(nr, linux_nr, name) // implemented as sys_<name>()
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SYSCALL_ALIAS(linux_nr, name) // a second Linux number for the same handler
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```
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Three things are generated from it by macro expansion, and none is
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hand-maintained:
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| Generated | Where |
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|---|---|
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| The `SYS_*` constants | `include/syscall.h` |
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| The dispatch table | `kernel/sys/syscall.c` |
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| The Linux translation map | `kernel/sys/linux_abi.c` |
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Adding a syscall is therefore two edits: a line here, and a body written with
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`SYSCALL_DEFINE(<name>)` in one of the `kernel/sys/sys_*.c` files.
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**This replaced a two-place scheme and the reason is worth keeping.** The Linux
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number used to live in a second hand-written table. Adding a syscall meant three
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edits across two files, and forgetting the second one left the call *unreachable
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from every musl-linked program* while looking perfectly registered in the first.
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A syscall can no longer be half-registered.
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## How a Linux binary runs
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The translation is a flat array indexed by Linux syscall number, whose value is
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the native `SYS_*` number — or 0, meaning not implemented, which dispatches to
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`-ENOSYS`.
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It is enabled **per process**, and selected automatically by `exec()`:
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```c
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// musl binaries have entry >= 0x100000; SchOS-native ones are linked at 0x4000
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if (elf.entry >= LINUX_ABI_ENTRY_THRESHOLD)
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myproc()->syscall_map = linux_syscall_map;
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```
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A link address is a crude discriminator and it is honest about being one. It
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works because the two classes of binary in this system are linked by two
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different linker scripts at two very different addresses.
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There is also `enable_linux_abi()` as an explicit call, which is why
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`cmd/schinit/main.c` carries a warning not to use it: doing so mid-process would
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renumber the syscalls the caller still has to make.
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### Native vs. Linux numbering
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Some calls have no Linux equivalent and are marked `NO_LINUX`: `sleep`,
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`uptime`, `reboot`, `shutdown`, `getmouse`, `vga_sync`, `schfs_index`,
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`schfs_query`. These are reachable only from first-party programs linked
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against `lib/`.
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They all land on the same scratch index in the translation array and overwrite
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each other there, which is harmless — no real Linux syscall uses that number, so
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the slot is never dispatched.
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### Where the two disagree on a name
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`shutdown` is the one collision. SchOS has a native `shutdown()` that powers the
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machine off, and Linux number 48 is the socket call. Two syscalls cannot share a
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`sys_` function name, so Linux 48 maps to `sockshutdown`.
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## What is implemented
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Roughly 190 native numbers covering:
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| Process | fork, exec, exit, wait/waitpid/wait4/waitid, clone, kill, sessions, process groups |
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| Identity | the full real/effective/saved set for both uid and gid, groups, umask |
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| Memory | mmap, munmap, mprotect, brk/sbrk, madvise, System V shared memory |
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| Filesystem | the ordinary set, the whole `*at()` family, statfs, xattr, mount/umount2 |
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| I/O | read/write, readv/writev, pread/pwrite, sendfile, poll/select/ppoll/pselect6, epoll, ioctl, fcntl |
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| Sockets | socket, bind, listen, accept, connect, sendto, recvfrom, socketpair, setsockopt, getsockopt |
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| Signals | rt_sigaction, rt_sigreturn, rt_sigsuspend, rt_sigpending, rt_sigtimedwait, sigaltstack |
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| Time | gettimeofday, clock_gettime, clock_getres, nanosleep, clock_nanosleep, alarm, timers |
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| Notification | eventfd2, timerfd_create/settime/gettime, signalfd4 |
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### Argument fetching
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`fetchint`, `fetchaddr`, `fetchstr` and `argptr` validate a user pointer through
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the page tables and nothing else. They previously rejected any address
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`>= proc->sz`, on the assumption that every valid user pointer lives in the
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heap — which is wrong for stack addresses and for every `mmap`'d buffer, i.e.
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for most pointers real programs pass.
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`uva2ka()` translates a user virtual address and **returns the in-page offset**,
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which it did not originally do. Every caller that read or wrote through a
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non-page-aligned pointer — `TIOCGPGRP`, `TCGETS`, `getcwd`, signal masks,
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`times()`, `sysinfo()` — was touching the wrong address whenever the pointer was
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not page-aligned, i.e. almost always.
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**`uva2ka()` guarantees only the page containing that address.** A structure
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copied in bulk through one translation runs off the end of the mapped page when
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the pointer lands near a boundary — and `P2V` is a flat offset, so "the next
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page" in kernel space has nothing to do with the user's next page. That bug
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corrupted unrelated kernel memory and crashed every freshly `exec`'d shell. Copy
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field by field, each through its own translation, or use `copyin`/`copyout`.
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---
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## What it does not do
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- **`sendmsg`/`recvmsg` carry `SCM_RIGHTS` and nothing else.**
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`SCM_CREDENTIALS` is refused rather than ignored. See
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[networking.md](networking.md#descriptor-passing).
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- **No `memfd_create`.**
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- **No `getsockname` or `getpeername`**, and `accept()` does not report an
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AF_INET peer's address.
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- **`setsockopt` handles six options** and answers `ENOPROTOOPT` for everything
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else. `SO_RCVTIMEO`/`SO_SNDTIMEO` are among the refused.
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- **`clone` does not share an address space**, so it is `fork` with extra steps.
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- **`sched_setaffinity` is accepted and ignored.**
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- **`getrusage` does not distinguish `RUSAGE_CHILDREN`** from `RUSAGE_SELF`, and
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fills in only the calling process's own tick count.
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- **The ABI is chosen by link address.** It works, and it is a heuristic rather
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than a fact about the binary.
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- **`/proc/<pid>/exe` is a regular file, not a symlink**, so `readlink()` on it
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fails and `ps -o comm` prints `[init]` rather than `init`. Default `ps` and
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`top` output is unaffected.
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