RFR: 8319589: Attach from root to a user java process not supported in Mac

[Sergey Chernyshev]

Hi all,

I would like to propose a fix for JDK-8319589. This will allow jcmd and jps running as root to get the complete list of JVMs running by all users, and to attach from root to non-root JVMs.

This change affects macOS, that uses "secure" per-user temporary directories. It only affects JVMs running as root, the behavior in non-privileged JVMs remains unchanged.

Jcmd and jps rely on LocalVmManager to get the initial list of the local VMs. The LocalVmManager uses sun.jvmstat.PlatformSupport to get the list of temp directories, where it searches for user's PerfData directory such as "hsperfdata_<username\>". In macosx the temp directories are per-user, the temp path is returned by confstr(_CS_DARWIN_USER_TEMP_DIR). The per-user directories are mode 700 and so they are read-protected from non-privileged users and can be accessed by the owner and the root.

Both jps and jcmd (HotSpotAttachProvider) create MonitoredVm objects, that have PerfDataBuffer that performs attachment to the target. Only the attachable VMs are listed in jcmd output.

The proposed patch changes the list of directories returned by the PlatformSupport#getTemporaryDirectories() in VMs running as root. The list is later used in VirtualMachineImpl (jdk.attach). It changes also the way mmap_attach_shared() searches for hsperfdata_<username\>/<pid\> files to map the shared memory. Mmap_attach_shared() and VirtualMachineImpl (via PlatformSupport) list the content of /var/folders, where the temp directories are located, more specificly the temp directories are /var/folders/<BUCKET\>/<ENCODED_UUID_UID\>/T as hinted in [1]. The full list is returned by newly added PlatformSupportImpl#getTemporaryDirectories().

The attaching client's VirtualMachineImpl needs the target process's temp directory to find .java<pid\> and create .attach<pid\> files. It uses the list returned by PlatformSupportImpl#getTemporaryDirectories() and the ProcessHandle of the target process to search for user's PerfData directory, e.g. hsperfdata_<username\>, which is in the target process's temp directory, exactly where it expects to see the .java<pid\> in return on sending SIGQUIT to the target VM.

Mmap_attach_shared() traverses the /var/folders in get_user_tmp_dir() and looks for a hsperfdata_<username\> folder. If that folder is found in /var/folders/*/*/T, that means the temp folder corresponds to the <username\> and to the JVM being attached to.

The patch is tested against the range of macOS versions, the earliest version tested is 10.13 High Sierra (as 11u backport), the latest version tested is 15.5 Sequoia.

Testing:

- jtreg hotspot/jdk tier1 passed
- jtreg tier2, tier3 no regresstions
- gtest:all passed


[1] https://github.com/apple-oss-distributions/Libc/blob/Libc-997.90.3/darwin/_dirhelper.c#L210

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

Commit messages:
 - 8319589: Attach from root to a user java process not supported in Mac

Changes: https://git.openjdk.org/jdk/pull/25824/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319589
  Stats: 255 lines in 8 files changed: 241 ins; 1 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/25824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25824/head:pull/25824

PR: https://git.openjdk.org/jdk/pull/25824