On Mon, 16 Jun 2025 09:46:11 GMT, Sergey Chernyshev <schernys...@openjdk.org>
wrote:
> 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. Previously, JDK-8197387 introduced the
> same approach on Linux.
>
> 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 pa...
src/jdk.attach/macosx/classes/sun/tools/attach/VirtualMachineImpl.java line 242:
> 240: for (String dir :
> platformSupport.getTemporaryDirectories(pid)) {
> 241: Path fullPath = Path.of(dir, HSPERFDATA_PREFIX +
> user, String.valueOf(pid));
> 242: if(Files.exists(fullPath)) {
Suggestion:
if (Files.exists(fullPath)) {
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/25824#discussion_r2151898308
