Hi TJ,
The fix looks good to me, modulo the suggestion from David about the
comment at L345.
However, I'm not an expert in the namespaces area.
It'd be nice to hear from other people.
This has to be tested at least when the namespaces are not present.
Testing should include the J*tools (jcmd, jstack, jmap, etc.) and the
attach API tests.
Not sure, if you can run them.
But your sponsor, Erik, can probably run them for you.
Also, it would be nice to have one new Jtreg test verifying this patch
with a namespace.
Not sure, it is a strong requirement though.
Erik,
Thank you for sponsoring this patch!
David has already created the webrev:
http://cr.openjdk.java.net/~dholmes/8179498/webrev/
Thanks,
Serguei
On 5/2/17 16:51, Erik Gahlin wrote:
I am not a (R)eviewer, so I can't give it my blessings, but I can
sponsor it.
I will create a webrev so it easier to review.
Erik
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
