0.26.0-1 gave the host CAP_SYS_NICE, which made it invisible to KWin — every KDE desktop session died, on five packaging channels #136

Merged

enricobuehler merged 2 commits from worktree-kwin-capability-identification into main 2026-08-09 08:04:45 +00:00

Conversation 0 Commits 2 Files Changed 13

+293 -132

2 Commits

Author	SHA1	Message	Date
enricobuehler	4d383811c0	fix(packaging): the same CAP_SYS_NICE broke KDE on FIVE channels, not one — Bazzite included ... The Arch fix in the previous commit was incomplete. 0.26.0-1 granted the host CAP_SYS_NICE through every Linux channel we ship, and each one breaks KWin identification the same way: * packaging/rpm/punktfunk.spec .......... %caps(cap_sys_nice=ep) in %files <- Fedora AND Bazzite via rpm-ostree layering * packaging/bazzite/build-sysext.sh ..... setcap on the staging tree, recorded by mksquashfs * packaging/debian/build-deb.sh ......... setcap in the postinst * packaging/nix/nixos-module.nix ........ security.wrappers with capabilities = "cap_sys_nice=ep" * scripts/steamdeck/install.sh .......... setcap on $BIN, six lines after writing the .desktop whose Exec= it thereby voids Bazzite was NOT a separate fault, as first reported here — it is this one. Verified by mounting the published punktfunk-0.26.0-1-x86-64.raw: `getcap usr/bin/punktfunk-host` reports cap_sys_nice=ep, stored as security.capability in the squashfs. The claim in packaging/arch/build-sysext.sh that "file capabilities don't survive this squashfs path" is false and is corrected here; mksquashfs records them, which is exactly why the image shipped one. NixOS deserves its own note: a security.wrappers entry does not dodge the problem. The wrapper raises the capability into its AMBIENT set before exec'ing the store binary, precisely so it survives — which lands CAP_SYS_NICE in the exec'd process's permitted set and fails the readlink identically to a file capability. ExecStart now points at the store path directly, which is also the path packages.nix substitutes into the .desktop's Exec=, so the two finally agree. Measured blast radius of holding a capability, same-uid reader, CachyOS kernel 7.1.6: /proc/PID/exe ....... EPERM <- KWin's identification. Desktop sessions die. /proc/PID/root/* .... EPERM <- xdg-desktop-portal reads .flatpak-info here to resolve an app id; the wlroots and Hyprland backends go through it /proc/PID/environ ... EPERM /proc/PID/cgroup .... OK /proc/PID/status .... OK /proc/PID/cmdline ... OK Compositor backends, by exposure: KWin is broken outright (proven, field-confirmed). gamescope has no identity gate and was never affected, which matches the field — only Desktop mode was reported. Mutter drives Mutter's own D-Bus API, not the portal, and looks unaffected. wlroots and Hyprland go through the ScreenCast portal, whose app-id resolution reads a path the capability blocks — a real exposure, not something I reproduced end to end. The sysext build now HARD-FAILS if a capability is staged, rather than trusting that the RPM payload never carries one: a merged sysext's /usr is read-only squashfs, so a bad image cannot be repaired on the box, and the spec was one %caps() away from baking one in again. Docs corrected, because they advertised the capability as a feature: * docs-site running-as-a-service "GPU scheduling priority" — rewritten: the host carries no capability, why it must not, and how to clear a 0.26.0-1 install (Bazzite needs a new image) * docs-site configuration.md — the PYROWAVE_QUEUE_PRIORITY row no longer claims the packages grant it * packaging/bazzite/README.md — §6.5 still described the kde-desktop-setup.sh behaviour from before it stopped writing KWIN_WAYLAND_NO_PERMISSION_CHECKS and started REMOVING it; plus a note that 0.26.0-1 Desktop mode cannot be repaired in place * packaging/arch/README.md — the false "capabilities don't survive the sysext" line * CHANGELOG v0.26.0 PW1 — annotated with the 0.26.0-2 correction rather than rewritten, and the owed PyroWave-under-load A/B now says it needs a gamescope-only box Verified: bash -n on all five changed shell files; nix-instantiate --parse on nixos-module.nix and packages.nix; the published 0.26.0-1 sysext mounted and its capability read; getcap on an uncapped file exits 0 with empty output, so the new build assertion cannot false-positive.	2026-08-09 09:56:36 +02:00
enricobuehler	42ee6c5628	fix(packaging): the host's CAP_SYS_NICE made it invisible to KWin, killing every KDE session ... 0.26.0-1 setcap'd `cap_sys_nice=ep` on /usr/bin/punktfunk-host so the encoder could open an elevated global-priority Vulkan queue. On every KDE box that ended desktop streaming outright: KWin virtual output failed: KWin does not expose zkde_screencast_unstable_v1 to this client reported from CachyOS on NVIDIA and on AMD, surviving a clean reinstall of host and client, and worked around only by KWIN_WAYLAND_NO_PERMISSION_CHECKS=1. The two cannot coexist. KWin hands out its restricted protocols — zkde_screencast_unstable_v1, which mints our virtual output, and org_kde_kwin_fake_input, which injects input — only to a client it can IDENTIFY, by resolving that client's /proc/<pid>/exe and matching it against an installed .desktop's Exec=. The kernel refuses that readlink to any reader whose effective set is not a superset of the target's PERMITTED set (cap_ptrace_access_check), and KWin holds no capabilities. So the instant the binary carries one, KWin's executablePath() is empty, nothing matches, and the global is never advertised — presenting exactly as a missing or mis-installed .desktop file. Measured on CachyOS (kernel 7.1.6), same-uid reader, cap_sys_nice=ep on the target: no capability .............................. readlink /proc/<pid>/exe OK capability ................................. EPERM capability + prctl(PR_SET_DUMPABLE, 1) ..... EPERM <- dumpable is NOT the gate capability dropped + PR_SET_DUMPABLE(1) .... OK <- only an uncapped process works The third row also rules out the reflex fix of moving the grant to systemd AmbientCapabilities=, which lands CAP_SYS_NICE in the very same permitted set. Nothing short of not holding the capability restores identification, so the host does not get one. The cost is pacing only. pf-zerocopy's device create already walks REALTIME -> HIGH -> default when a priority class is refused, and pf-frame's thread nice is a documented best-effort no-op without the capability — so this is 0.25.0's behaviour exactly, which is the behaviour that worked. * packaging/arch/punktfunk-host.install: grant -> revoke. post_upgrade strips the capability from boxes that already ran 0.26.0-1's scriptlet. A pacman upgrade writes a new inode and file capabilities do not survive that, so this is belt-and-braces for reinstall/downgrade paths. * pf-vdisplay kwin.rs: all three "KWin does not expose zkde_screencast" errors now read /proc/self/status and, if this process holds ANY capability, name it with its CapPrm mask and the `setcap -r` that repairs it. The failure stays impossible to diagnose from the Wayland side otherwise, and it is not unique to our own packaging — a hand-rolled setcap does it too. Verified on 192.168.1.21 (CachyOS): the capability/dumpable matrix above; cargo check and cargo clippy --all-targets -- -D warnings clean for pf-vdisplay; both new unit tests pass; and the hint itself exercised end-to-end, silent uncapped and firing with CapPrm=0x0000000000800000 under cap_sys_nice=ep. The shipped punktfunk-host-0.26.0-1-x86_64.pkg.tar.zst was unpacked to confirm its .INSTALL carries the setcap on both post_install and post_upgrade. Ships as 0.26.0-2 — packaging plus one crate, no version bump.	2026-08-09 09:37:30 +02:00