If your computer's /tmp is non-executable, you will run into problems with CrashPlan.

For example, the temp directory on my laptop is mounted using this line in /etc/fstab:

tmpfs  /tmp  tmpfs  size=1024M,noexec,nosuid,nodev  0  0

This configuration leads to two serious problems with CrashPlan.

CrashPlan client not starting up

The first one is that while the daemon is running, the client doesn't start up and doesn't print anything out to the console.

You have to look in /usr/local/crashplan/log/ui_error.log to find the following error message:

Exception in thread "main" java.lang.UnsatisfiedLinkError: Could not load SWT library. Reasons:
  Can't load library: /tmp/.cpswt/libswt-gtk-4234.so
  Can't load library: /tmp/.cpswt/libswt-gtk.so
  no swt-gtk-4234 in java.library.path
  no swt-gtk in java.library.path
  /tmp/.cpswt/libswt-gtk-4234.so: /tmp/.cpswt/libswt-gtk-4234.so: failed to map segment from shared object: Operation not permitted

  at org.eclipse.swt.internal.Library.loadLibrary(Unknown Source)
  at org.eclipse.swt.internal.Library.loadLibrary(Unknown Source)
  at org.eclipse.swt.internal.C.<clinit>(Unknown Source)
  at org.eclipse.swt.internal.Converter.wcsToMbcs(Unknown Source)
  at org.eclipse.swt.internal.Converter.wcsToMbcs(Unknown Source)
  at org.eclipse.swt.widgets.Display.<clinit>(Unknown Source)
  at com.backup42.desktop.CPDesktop.<init>(CPDesktop.java:266)
  at com.backup42.desktop.CPDesktop.main(CPDesktop.java:200)

To fix this, you must tell the client to use a different directory, one that is executable and writable by users who need to use the GUI, by adding something like this to the GUI_JAVA_OPTS variable of /usr/local/crashplan/bin/run.conf:

-Djava.io.tmpdir=/home/username/.crashplan-tmp

Backup waiting forever

The second problem is that once you're able to start the client, backups are stuck at "waiting for backup" and you can see the following in /usr/local/crashplan/log/engine_error.log:

Exception in thread "W87903837_ScanWrkr" java.lang.NoClassDefFoundError: Could not initialize class com.code42.jna.inotify.InotifyManager
  at com.code42.jna.inotify.JNAInotifyFileWatcherDriver.<init>(JNAInotifyFileWatcherDriver.java:21)
  at com.code42.backup.path.BackupSetsManager.initFileWatcherDriver(BackupSetsManager.java:393)
  at com.code42.backup.path.BackupSetsManager.startScheduledFileQueue(BackupSetsManager.java:331)
  at com.code42.backup.path.BackupSetsManager.access$1600(BackupSetsManager.java:66)
  at com.code42.backup.path.BackupSetsManager$ScanWorker.delay(BackupSetsManager.java:1073)
  at com.code42.utils.AWorker.run(AWorker.java:158)
  at java.lang.Thread.run(Thread.java:744)

This time, you must tell the server to use a different directory, one that is executable and writable by the CrashPlan engine user (root on my machine), by adding something like this to the SRV_JAVA_OPTS variable of /usr/local/crashplan/bin/run.conf:

-Djava.io.tmpdir=/var/crashplan