This is just a note to myself, and to any others who may have run into the same issue as I did.

After switching some Java servers over from the previous Ubuntu LTS release (10.04) to the latest one (12.04), I noticed that the "jmap" command had stopped working. For example, "jmap -heap 4711" aborted with

    Attaching to process ID 4711, please wait...
    Error attaching to process: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process

It turns out a new security feature has been enabled in the Linux kernel a while ago. The system call "ptrace" is now only allowed from the parent of the process. This makes things like jmap useless, since the target process is never a child of the jmap process. Fortunately the feature can be disabled. Just do the following as root:

    # echo 0 > /proc/sys/kernel/yama/ptrace_scope

Now you can use jmap as before.

If you want to disable it permanently, put the following line in /etc/sysctl.conf:

    kernel.yama.ptrace_scope=0

This will disable it every time the system boots.