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

Browse source 
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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28
docs/architecture/udev.md
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@ -80,6 +80,32 @@ and regular files in `/dev` are refused.
A queue that is full drops the message, as Linux does, and `udevadm trigger`
is how a reader recovers.
## libinput and libseat
`libinput` (Alpine) reads the evdev nodes udev tagged and `xf86-input-libinput`
gives Xorg the same library; `libinput list-devices` as an ordinary user shows
`SchOS keyboard` and `SchOS pointer`. Xorg's `40-libinput.conf` now claims both,
and the log says `Using input driver 'libinput'`.
`seatd` (job `etc/schinit/seatd.job`, socket `/run/seatd.sock`, group `video`)
and `libseat` let a program that is not root take devices from the system. The
client asks seatd for `seat0`; seatd takes the VT (`VT_SETMODE`, `KDSETMODE`,
`KDSKBMODE`, all there already), opens the device as root and passes the
descriptor back over the socket with `SCM_RIGHTS`. Checked as `schos` against
`/dev/input/event0` and `event1` with a small program linked to libseat, reading
the device's name through the descriptor it was handed.
Getting there found two kernel bugs, both in descriptor passing:
- **A socket returned by `accept()` was not an `AF_UNIX` socket as far as
`sendmsg` was concerned.** The end `connect()` queues for the listener was
allocated with its domain left at zero, and `sendmsg` refuses `SCM_RIGHTS` on
anything else. So a *server* could receive descriptors and never send one:
seatd, and equally a Wayland compositor handing a client a buffer.
- **A control buffer longer than its messages was an error.** seatd passes a
zeroed array with one descriptor at the front; Linux treats a following header
too short to be one as the end. `sendmsg` now does too.
## What is not there
- USB interfaces (`1-3:1.0`), `/dev/bus/usb` nodes, and PCI bridges as parents.
@ -89,7 +115,7 @@ and regular files in `/dev` are refused.
image, so `60-input-id` logs "returned non-zero" for the hwdb import. The
properties that matter come from `input_id`.
- Block device partitions, `by-id` / `by-uuid` links, and a `/sys/block`.
- `libinput` and `libseat` (next; see [../plan/roadmap.md](../plan/roadmap.md)).
- A compositor. libinput and libseat are in place, DRM is not; see [../plan/roadmap.md](../plan/roadmap.md).
## Testing