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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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8
docs/architecture/desktop.md
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@ -14,10 +14,10 @@ Wayland needs — udev, `/sys/devices`, libinput, a DRM device — is in [../pla
Every Wayland compositor worth running — labwc, and everything built on wlroots — needs
DRM/KMS, libinput, udev and libseat. DRM/KMS is the largest of them by far: a kernel graphics
subsystem, and in practice a great deal taken from Linux. Xorg runs on a plain framebuffer
through `fbdev` and reads input through `evdev`, both interfaces SchOS already had. That is
the shorter road to a desktop by a long way, and it brings the window managers with it.
DRM/KMS is not planned; libinput, udev and libseat are worth having for their own sake and
may come later.
through `fbdev`, which SchOS already had. That is the shorter road to a desktop by a long way,
and it brings the window managers with it. udev, libinput and libseat are now in the image
([udev.md](udev.md)); DRM/KMS is the part still missing, and it is the GPU plan
([../plan/gpu.md](../plan/gpu.md)).
## What it stands on