[PATCH] attach in linux should be relative to /proc/pid/root and namespace aware

[David Holmes]

Hi TJ,

Thanks for the patch (I haven't looked at it yet). FYI at the moment, 
unless this is considered a high priority bug for JDK 9 it has to be 
targeted to JDK 10, and then possibly backported to 9 and 8u.

Cheers,
David

On 29/04/2017 8:23 AM, TJ Fontaine wrote:
> I have attached a patch that allows jcmd to work against a java process running
> inside a Docker container. Apologies if this is not in the correct format. It was
> built against jdk8u. I couldn’t seem to find an existing JIRA for it.
>
> Diagnostic commands (i.e. jcmd, jstack, etc) fail to attach to a target JVM
> that is inside a container (e.g. Docker).
>
> A Linux container often isolates a process in a PID and Mount namespace that is
> separate from the "root container" (analogous to the hypervisor/dom0 in
> hardware virtualization environments, or the global zone on Solaris). A target
> JVM that is isolated in either a PID namespace, or a Mount namespace will fail
> the attach sequence.
>
> When the target JVM is in its own PID namespace the pid of the process is
> distinct from what the real pid of the process as it relates to the root
> container. For example, in the root container you can observe a JVM with a pid
> of 17734, however if that JVM is running inside a Docker container the pid
> inside its PID namespace is likely 1. So when the target JVM receives the
> SIGQUIT it looks in /proc/self/cwd/ for .attach_pid1 however the external
> attaching JVM has created the file /proc/17734/cwd/.attach_pid17734. Given this
> discrepancy the target JVM will output to stderr thread status, since
> /proc/self/cwd/.attach_pid1 doesn't exist and won't continue with the attach
> sequence.
>
> The solution is to parse /proc/pid/status for the field NSpid (available since
> Linux 4.1) which contains a list of pids, where the last entry is the "inner
> most" PID namespace value. (Namespaces can be stacked, unlike Solaris Zones
> which have a virtualization depth of 1)
>
> The rest of the Linux attach sequence assumes a shared mount namespace by
> waiting for /tmp/.java_pid17734 to appear. But if the attaching process is in a
> separate namespace because the target JVM is in a mount namepsace (or in a
> chroot as well) the unix domain socket for attaching won't appear.
>
> Instead the attach sequence should resolve file names relative to
> /proc/17734/root which has a materialized view of the rootfs for the target.
>
> Thanks!
>
> TJ
>