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

[TJ Fontaine]

Hey,

I’ve attached a version rebased on jdk10, it also (currently) applies cleanly to jdk9.

While there is no supplied test or harness for this patch, how I built and tested is
available at https://github.com/tjfontaine/jdkbuild (there’s also a preview of my
follow on patch for pathmap_open as well).

Thanks!

TJ

On 4/28/17, 3:47 PM, "serviceability-dev on behalf of TJ Fontaine" <serviceability-dev-bounces at openjdk.java.net on behalf of tj.fontaine at oracle.com> wrote:

    I had no doubt we’d end up on the conversation of 10 -> 9 -> 8u, I started with 8u merely because it was representative of today’s customer pain. I’ll be sure to work on retargeting it as well.
    
    Thanks!
    
    TJ
    
    On 4/28/17, 3:42 PM, "David Holmes" <david.holmes at oracle.com> wrote:
    
        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
        >
        
    
    
    

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