https://bugs.kde.org/show_bug.cgi?id=526940
Bug ID: 526940
Summary: Login Screen KCM: Apply does nothing for wallpaper
packages with many files ("Remote peer disconnected")
Classification: Plasma
Product: plasma-login-manager
Version First 6.6.4
Reported In:
Platform: RedHat Enterprise Linux
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Target Milestone: ---
Created attachment 197013
--> https://bugs.kde.org/attachment.cgi?id=197013&action=edit
strace -f of kcmplasmalogin_authhelper (run manually as root) while pressing
Apply. See line 1376: recvmsg of performAction, msg_flags has MSG_CTRUNC.
Environment:
- Rocky Linux 10.2, Wayland
- plasma-login-manager 6.6.4-1.el10_2
- kcm-plasmalogin 6.6.4-1.el10_2
- polkit 125-4.el10_2.1, systemd 257-23.el10_2.2
Steps to reproduce:
1. System Settings -> Login Screen -> Configure Appearance
2. Select a wallpaper package that contains many per-resolution entries (e.g.
rocky-arizona, rocky-abstract-1, rocky-quartz-forest from
/usr/share/wallpapers; each entry is a symlink to one of two images)
3. Press Apply
Actual result:
- No password prompt appears
- /etc/plasmalogin.conf and /var/lib/plasmalogin/wallpapers are not modified
- No error shown in the UI
- Console output of kcmshell6 kcm_plasmalogin: kf.auth: "Remote peer
disconnected"
Expected result:
The wallpaper is saved, or at least an error is shown to the user.
Working case:
Selecting a single image file (jpg/png) with "Add image" works.
Analysis:
- busctl monitor: the client (systemsettings) sends
org.kde.kf6auth.performAction for org.kde.kcontrol.kcmplasmalogin.save to the
helper. About 3 ms later the client gets
org.freedesktop.DBus.Error.NoReply "Remote peer disconnected".
- strace of /usr/libexec/kf6/kauth/kcmplasmalogin_authhelper (started
manually): the helper receives the performAction message. The D-Bus message
header declares 67 unix fds (UNIX_FDS field = 0x43), but the recvmsg control
buffer is 80 bytes, which only holds 16 fds, and msg_flags contains MSG_CTRUNC.
The helper does not process the request further and exits with status 0 after
its idle timeout. No coredump, no AVC.
- The helper never sends a CheckAuthorization request to polkit (only
systemsettings itself sends non-interactive checks with flags=0), which is why
no authentication dialog appears.
- A second machine with identical package versions works, because its
configured wallpaper is a single file.
Likely cause: the KCM passes one file descriptor per file of the wallpaper
package, which exceeds the per-message fd limit (apparently 16, libdbus
default) on the helper side.
Suggested fix: pass fewer fds (e.g. only the files actually needed, resolving
symlinks and de-duplicating), and/or raise the limit on the helper connection.
At minimum, report the failure to the user instead of failing silently.
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