RFR: 8373867: Improve robustness of Attach API for finding tmp directory [v4]

[Sebastian Lövdahl]

On Fri, 19 Dec 2025 14:47:40 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> We can see several thread dump on the console of Distroless nonroot JDK when we attach debug (root) Distroless container image to the nonroot container as following. It is not expected.
>> 
>> 
>> $ podman run -it --rm --name debuggee -v `pwd`/LongSleep:/opt/LongSleep:Z --entrypoint java gcr.io/distroless/java25-debian13:nonroot -cp /opt/LongSleep -Xlog:attach=debug LongSleep
>> [38.252s][debug][attach] Failed to find attach file: /tmp/.attach_pid1
>> 2025-12-17 06:34:37
>> Full thread dump OpenJDK 64-Bit Server VM (25.0.1+8-LTS mixed mode, sharing):
>> 
>> Threads class SMR info:
>> _java_thread_list=0x000078a8bc13f200, length=10, elements={
>> 0x000078a8bc02bb60, 0x000078a8bc128200, 0x000078a8bc1293f0, 0x000078a8bc12ae40,
>> 0x000078a8bc12c760, 0x000078a8bc12dfe0, 0x000078a8bc12fde0, 0x000078a8bc1317d0,
>>    :
>> 
>> 
>> Attach API put `.attach_pid<pid>` file at first to clarify subsequent SIGQUIT means create AttachListener thread. That file attempt to create on current work directory of the target process, but it would fallback to /tmp if failed (e.g. attacher cannot write onto work directory).
>> 
>> In case of attaching nonroot container from root container, and also it would fail due to lack of write permission on current work directory, and cannot access /proc/<PID>/root/tmp. It causes following error on jcmd:
>> 
>> 
>> $ podman run -it --rm --pid container:debuggee --entrypoint sh gcr.io/distroless/java25-debian13:debug
>> / # /usr/lib/jvm/jcmd 1 VM.version
>> 1:
>> com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file /tmp/.java_pid1: target process 1 doesn't respond within 10500ms or HotSpot VM not loaded
>>         at jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:115)
>>         at jdk.attach/sun.tools.attach.AttachProviderImpl.attachVirtualMachine(AttachProviderImpl.java:56)
>>         at jdk.attach/com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:201)
>>         at jdk.jcmd/sun.tools.jcmd.JCmd.executeCommandForPid(JCmd.java:113)
>>         at jdk.jcmd/sun.tools.jcmd.JCmd.main(JCmd.java:97)
>> 
>> / # ls -l /proc/1/cwd
>> ls: /proc/1/cwd: cannot read link: Permission denied
>> lrwxrwxrwx 1 nonroot nonroot 0 Dec 17 06:34 /proc/1/cwd
>> 
>> 
>> 
>> After this change, we can see following exception on the console of jcmd when we encounter this situation:
>> 
>> # jcmd 1 VM.version
>> 1:
>> com.sun.tools.attach.AttachNotSupportedException: Unable to access the filesystem of the target process
>>         at jdk.attac...
>
> Yasumasa Suenaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve exception

Thank you @YaSuenag, works locally for me now too. Awesome to get a test added for it as well!

My approval is only cosmetic, but this is a great improvement nevertheless.

-------------

Marked as reviewed by slovdahl at github.com (no known OpenJDK username).

PR Review: https://git.openjdk.org/jdk/pull/28867#pullrequestreview-3602932327