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8364281: Reduce JNI usage in Linux attach provider #26712

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@marschall marschall commented Aug 9, 2025 •
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Reduce the usage of JNI in the Linux attach provider by making more use of the Path API.

  • use the Path API to read file permissions
  • use the Path API to write to and read from Unix domain sockets
  • use JDK internal APIs to access the euid and eguid

After this the sole usage of JNI in the Linux attach provider will be to send a signal using kill.

I ran the tier1 and serviceability test suites and they both pass. I verified I can attach to and attach using a locally built JDK. I didn't do any Docker related tests.

I split the changes into two commits:

  • The first commit ports the permission check code more or less directly from C to Java using integer based checks.
  • The second commit changes the permission check code to use higher level Java abstractions (UserPrincipal, Set, ...)

I hope this makes it easier to review and verify the changes preserve the semantics.

To the best of my knowledge uid_t is an unsigned 32 bit integer. VM#geteuid() returns it as long, unsigned. However UnixUserPrincipals#fromUid expects an int, I believe casting is the correct way to convert in this case.

I tried to keep unrelated changes to VirtualMachineImpl to a minimum. I did however replace all usages of java.io.File with java.nio.file.Path and made two methods static. We could get rid of the socket_path instance variable if we instead use socket_address to flag a disconnect.

Ultimately the same changes can be applied to the macOS an AIX implementations.

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Issue

  • JDK-8364281: Reduce JNI usage in Linux attach provider (Enhancement - P4)

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Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/26712/head:pull/26712
$ git checkout pull/26712

Update a local copy of the PR:
$ git checkout pull/26712
$ git pull https://git.openjdk.org/jdk.git pull/26712/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 26712

View PR using the GUI difftool:
$ git pr show -t 26712

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Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/26712.diff

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@marschall
8364281: Reduce JNI usage in Linux attach provider
c644495 
- use Path API to read file permissions
- use Path API for Unix domain sockets
@marschall
8364281: Reduce JNI usage in Linux attach provider
80b045a 
- use idiomatic Java APIs for permission checks
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bridgekeeper bot commented Aug 9, 2025

👋 Welcome back marschall! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

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openjdk bot commented Aug 9, 2025

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

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openjdk bot commented Aug 9, 2025

@marschall The following label will be automatically applied to this pull request:

  • serviceability

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

@openjdk openjdk bot added serviceability [email protected] rfr Pull request is ready for review labels Aug 9, 2025
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Thanks for the work so far, nice to get rid of JNI. 🙂

There is a problemlisted test related to this that could make sense to run manually (https://bugs.openjdk.org/browse/JDK-8341518, it's failing in Oracle's CI but I have not been able to reproduce the failure myself, discussed a bit in #21331 too).

I'll see if I can find time to check out this change locally and run some tests, otherwise, feel free to run it yourself.

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Thanks for the work so far, nice to get rid of JNI. 🙂

There is a problemlisted test related to this that could make sense to run manually (https://bugs.openjdk.org/browse/JDK-8341518, it's failing in Oracle's CI but I have not been able to reproduce the failure myself, discussed a bit in #21331 too).

I'll see if I can find time to check out this change locally and run some tests, otherwise, feel free to run it yourself.

TestJcmdWithSideCar fails with

java.nio.file.NoSuchFileException: /proc/7/cwd/.attach_pid7
  at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
  at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
  at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
  at java.base/sun.nio.fs.UnixPath.toRealPath(UnixPath.java:856)
  at jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:106)
  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)

I must confess I'm quite confused by this as this ultimately means that

Files.createFile(path).toRealPath()

Fails with NoSuchFileException on the Path returned by createFile.

Is there a better way to run TestJcmdWithSideCar other than removing it from ProblemList?

As for the issue on Oralce CI, is there I way I ran run TestJcmdWithSideCar on a feature branch of mine on Oralce CI without opening a PR?

@marschall marschall marked this pull request as draft August 15, 2025 16:08
@openjdk openjdk bot removed the rfr Pull request is ready for review label Aug 15, 2025
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