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libinput, seatd and libseat; sockets from accept() can pass descriptors

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Alpine's libinput, libseat, seatd, xf86-input-libinput and usbutils join the
image, and seatd runs from schinit. libinput lists the keyboard and pointer as
an ordinary user, Xorg now drives both through libinput, and a program linked
to libseat running as schos opens seat0 through seatd, is handed
/dev/input/event0 and reads its name through the descriptor.

Two kernel bugs stood in the way, and both would have stopped a Wayland
compositor too. The socket connect() queues for a listener was allocated with
its domain at zero, so sendmsg refused SCM_RIGHTS on the accepted end: a server
could receive descriptors and never send one. And sendmsg treated a control
buffer longer than its messages as an error, where Linux takes a following
header too short to be one as the end; seatd passes a zeroed CMSG_SPACE array
with one descriptor at the front.

Found with /proc/strace and a temporary kinfo in sendmsg's EINVAL sites.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao
This commit is contained in:
Nexoniarz 2026-10-26 16:51:39 +01:00
commit b411836478
8 changed files with 94 additions and 11 deletions
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14
docs/plan/roadmap.md
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@ -62,12 +62,14 @@ compositor worth running is years of work that Alpine already packages.) The des
Wayland compositor (labwc, sway, weston, a GNOME or KDE session) is the larger step. What Wayland — and a modern X — need
from the system, in the order they are useful:
1. **udev — done for the kernel's own devices** ([udev.md](../architecture/udev.md)). eudev runs from `schinit`, the uevent
netlink socket and the `/sys/devices` tree exist, devtmpfs takes udev's links, and Xorg finds its keyboard and mouse
through it. **Left:** PCI and USB devices in the same tree, with `add`/`remove` events from the hotplug thread, so a USB
keyboard or a GPU appears to udev; libinput then enumerates evdev devices through it.
2. **libinput and libseat** (seatd is in Alpine): evdev with the properties libinput reads (`ID_INPUT_*` from udev), and a way
for a non-root compositor to take the keyboard and the display from the kernel — the `VT_PROCESS` handshake exists.
1. **udev — done** ([udev.md](../architecture/udev.md)). eudev runs from `schinit`, the uevent netlink socket and the
`/sys/devices` tree exist, PCI and USB devices are in it with `add`/`remove` events from USB hot-plug, devtmpfs takes udev's
links, and Xorg finds its keyboard and mouse through it. **Left:** USB interfaces, PCI bridges as parents, `hwdb`.
2. **libinput and libseat — done.** `libinput list-devices` sees the keyboard and the pointer as an ordinary user, Xorg now
drives both through `xf86-input-libinput`, and `seatd` runs from `schinit`: a non-root program opens `seat0` through
`libseat`, is handed `/dev/input/event0` over the socket, and reads it. Getting there found a real kernel bug (sockets
returned by `accept()` could not carry file descriptors) and a second one in `sendmsg`'s control-buffer parsing; both would
have stopped a Wayland compositor and its clients too. **Not yet tried:** a compositor.
3. **A DRM device.** Wayland compositors want `/dev/dri/card0` with KMS, and Mesa wants `renderD128`. SchOS has neither: it has
`/dev/fb0`. This is the GPU plan ([gpu.md](gpu.md)) — Linux's own `amdgpu` run by LDLS and `/dev/dri` served by that
process — and, for machines without a real GPU driver, a *simple-framebuffer* DRM shim (Linux's `simpledrm`, which is a