The Universe of Discourse

Thu, 28 Jul 2016

Controlling the KDE screen locking works now

Yesterday I wrote about how I was trying to control the KDE screenlocker's timeout from a shell script and all the fun stuff I learned along the way. Then after I published the article I discovered that my solution didn't work. But today I fixed it and it does work.

What didn't work

I had written this script:

    perl -i -lpe 's/^Enabled=.*/Enabled=False/' $HOME/.kde/share/config/kscreensaverrc
    qdbus org.freedesktop.ScreenSaver /MainApplication  reparseConfiguration
    sleep $timeout
    perl -i -lpe 's/^Enabled=.*/Enabled=True/' $HOME/.kde/share/config/kscreensaverrc
    qdbus org.freedesktop.ScreenSaver /MainApplication  reparseConfiguration

The strategy was: use perl to rewrite the screen locker's configuration file, and then use qdbus to send a D-Bus message to the screen locker to order it to load the updated configuration.

This didn't work. The System Settings app would see the changed configuration, and report what I expected, but the screen saver itself was still behaving according to the old configuration. Maybe the qdbus command was wrong or maybe the whole theory was bad.

More strace

For want of anything else to do (when all you have is a hammer…), I went back to using strace to see what else I could dig up, and tried

strace -ff -o /tmp/ss/s /usr/bin/systemsettings

which tells strace to write separate files for each process or thread. I had a fantasy that by splitting the trace for each process into a separate file, I might solve the mysterious problem of the missing string data. This didn't come true, unfortunately.

I then ran tail -f on each of the output files, and used systemsettings to update the screen locker configuration, looking to see which the of the trace files changed. I didn't get too much out of this. A great deal of the trace was concerned with X protocol traffic between the application and the display server. But I did notice this portion, which I found extremely suggestive, even with the filenames missing:

    3106  open(0x2bb57a8, O_RDWR|O_CREAT|O_CLOEXEC, 0666) = 18
    3106  fcntl(18, F_SETFD, FD_CLOEXEC)    = 0
    3106  chmod(0x2bb57a8, 0600)            = 0
    3106  fstat(18, {...})                  = 0
    3106  write(18, 0x2bb5838, 178)         = 178
    3106  fstat(18, {...})                  = 0
    3106  close(18)                         = 0
    3106  rename(0x2bb5578, 0x2bb4e48)      = 0
    3106  unlink(0x2b82848)                 = 0

You may recall that my theory was that when I click the “Apply” button in System Settings, it writes out a new version of $HOME/.kde/share/config/kscreensaverrc and then orders the screen locker to reload the configuration. Even with no filenames, this part of the trace looked to me like the replacement of the configuration file: a new file is created, then written, then closed, and then the rename replaces the old file with the new one. If I had been thinking about it a little harder, I might have thought to check if the return value of the write call, 178 bytes, matched the length of the file. (It does.) The unlink at the end is deleting the semaphore file that System Settings created to prevent a second process from trying to update the same file at the same time.

Supposing that this was the trace of the configuration update, the next section should be the secret sauce that tells the screen locker to look at the new configuration file. It looked like this:

3106  sendmsg(5, 0x7ffcf37e53b0, MSG_NOSIGNAL) = 168
3106  poll([?] 0x7ffcf37e5490, 1, 25000) = 1
3106  recvmsg(5, 0x7ffcf37e5390, MSG_CMSG_CLOEXEC) = 90
3106  recvmsg(5, 0x7ffcf37e5390, MSG_CMSG_CLOEXEC) = -1 EAGAIN (Resource temporarily unavailable)
3106  sendmsg(5, 0x7ffcf37e5770, MSG_NOSIGNAL) = 278
3106  sendmsg(5, 0x7ffcf37e5740, MSG_NOSIGNAL) = 128

There is very little to go on here, but none of it is inconsistent with the theory that this is the secret sauce, or even with the more advanced theory that it is the secret suace and that the secret sauce is a D-Bus request. But without seeing the contents of the messages, I seemed to be at a dead end.


Browsing random pages about the KDE screen locker, I learned that the lock screen configuration component could be run separately from the rest of System Settings. You use

kcmshell4 --list

to get a list of available components, and then

kcmshell4 screensaver

to run the screensaver component. I started running strace on this command instead of on the entire System Settings app, with the idea that if nothing else, the trace would be smaller and perhaps simpler, and for some reason the missing strings appeared. That suggestive block of code above turned out to be updating the configuration file, just as I had suspected:

open("/home/mjd/.kde/share/config/", O_RDWR|O_CREAT|O_CLOEXEC, 0666) = 19
fcntl(19, F_SETFD, FD_CLOEXEC)          = 0
chmod("/home/mjd/.kde/share/config/", 0600) = 0
fstat(19, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
write(19, "[ScreenSaver]\nActionBottomLeft=0\nActionBottomRight=0\nActionTopLeft=0\nActionTopRight=2\nEnabled=true\nLegacySaverEnabled=false\nPlasmaEnabled=false\nSaver=krandom.desktop\nTimeout=60\n", 177) = 177
fstat(19, {st_mode=S_IFREG|0600, st_size=177, ...}) = 0
close(19)                               = 0
rename("/home/mjd/.kde/share/config/", "/home/mjd/.kde/share/config/kscreensaverrc") = 0
unlink("/home/mjd/.kde/share/config/kscreensaverrc.lock") = 0

And the following secret sauce was revealed as:

    sendmsg(7, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\0\1\30\0\0\0\v\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/freedesktop/DBus\0\0\0\6\1s\0\24\0\0\0org.freedesktop.DBus\0\0\0\0\2\1s\0\24\0\0\0org.freedesktop.DBus\0\0\0\0\3\1s\0\f\0\0\0GetNameOwner\0\0\0\0\10\1g\0\1s\0\0", 144}, {"\23\0\0\0org.kde.screensaver\0", 24}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 168
    sendmsg(7, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\1\1\206\0\0\0\f\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/freedesktop/DBus\0\0\0\6\1s\0\24\0\0\0org.freedesktop.DBus\0\0\0\0\2\1s\0\24\0\0\0org.freedesktop.DBus\0\0\0\0\3\1s\0\10\0\0\0AddMatch\0\0\0\0\0\0\0\0\10\1g\0\1s\0\0", 144}, {"\201\0\0\0type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0='org.kde.screensaver'\0", 134}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 278
    sendmsg(7, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\0\1\0\0\0\0\r\0\0\0j\0\0\0\1\1o\0\f\0\0\0/ScreenSaver\0\0\0\0\6\1s\0\23\0\0\0org.kde.screensaver\0\0\0\0\0\2\1s\0\23\0\0\0org.kde.screensaver\0\0\0\0\0\3\1s\0\t\0\0\0configure\0\0\0\0\0\0\0", 128}, {"", 0}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 128
    sendmsg(7, {msg_name(0)=NULL,

(I had to tell give strace the -s 256 flag to tell it not to truncate the string data to 32 characters.)

Binary gibberish

A lot of this is illegible, but it is clear, from the frequent mentions of DBus, and from the names of D-Bus objects and methods, that this is is D-Bus requests, as theorized. Much of it is binary gibberish that we can only read if we understand the D-Bus line protocol, but the object and method names are visible. For example, consider this long string:


With qdbus I could confirm that there was a service named org.freedesktop.DBus with an object named / that supported a NameOwnerChanged method which expected three QString arguments. Presumably the first of these was org.kde.screensaver and the others are hiding in other the 134 characters that strace didn't expand. So I may not understand the whole thing, but I could see that I was on the right track.

That third line was the key:

sendmsg(7, {msg_name(0)=NULL,
            msg_iov(2)=[{"… /ScreenSaver … org.kde.screensaver … org.kde.screensaver … configure …", 128}, {"", 0}],
        MSG_NOSIGNAL) = 128

Huh, it seems to be asking the screensaver to configure itself. Just like I thought it should. But there was no configure method, so what does that configure refer to, and how can I do the same thing?

But org.kde.screensaver was not quite the same path I had been using to talk to the screen locker—I had been using org.freedesktop.ScreenSaver, so I had qdbus list the methods at this new path, and there was a configure method.

When I tested

qdbus org.kde.screensaver /ScreenSaver configure

I found that this made the screen locker take note of the updated configuration. So, problem solved!

(As far as I can tell, org.kde.screensaver and org.freedesktop.ScreenSaver are completely identical. They each have a configure method, but I had overlooked it—several times in a row—earlier when I had gone over the method catalog for org.freedesktop.ScreenSaver.)

The working script is almost identical to what I had yesterday:

        perl -i -lpe 's/^Enabled=.*/Enabled=False/' $HOME/.kde/share/config/kscreensaverrc
        qdbus org.freedesktop.ScreenSaver /ScreenSaver configure
        sleep $timeout
        perl -i -lpe 's/^Enabled=.*/Enabled=True/' $HOME/.kde/share/config/kscreensaverrc
        qdbus org.freedesktop.ScreenSaver /ScreenSaver configure

That's not a bad way to fail, as failures go: I had a correct idea about what was going on, my plan about how to solve my problem would have worked, but I was tripped up by a trivium; I was calling MainApplication.reparseConfiguration when I should have been calling ScreenSaver.configure.

What if I hadn't been able to get strace to disgorge the internals of the D-Bus messages? I think I would have gotten the answer anyway. One way to have gotten there would have been to notice the configure method documented in the method catalog printed out by qdbus. I certainly looked at these catalogs enough times, and they are not very large. I don't know why I never noticed it on my own. But I might also have had the idea of spying on the network traffic through the D-Bus socket, which is under /tmp somewhere.

I was also starting to tinker with dbus-send, which is like qdbus but more powerful, and can post signals, which I think qdbus can't do, and with gdbus, another D-Bus introspector. I would have kept getting more familiar with these tools and this would have led somewhere useful.

Or had I taken just a little longer to solve this, I would have followed up on Sumana Harihareswara’s suggestion to look at Bustle, which is a utility that logs and traces D-Bus requests. It would certainly have solved my problem, because it makes perfectly clear that clicking that apply button invoked the configure method:

I still wish I knew why strace hadn't been able to print out those strings through.

[Other articles in category /Unix] permanent link