Add job objects to terminate child processes on VM shutdown by benhillis · Pull Request #40607 · microsoft/WSL

benhillis · 2026-05-20T17:57:25Z

Summary

Add JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE job objects to ensure that child processes spawned by the service (wsldevicehost, wslhost.exe, wslrelay.exe) are terminated when the VM shuts down. This prevents lingering processes from holding DLLs locked during package upgrades.

PR Checklist

Applies to: WSL Windows components
Tests: Existing tests pass (cmake --build . -- -m succeeds)

Detailed Description

Problem

During package upgrade, wsldevicehost.dll (and potentially wslhost.exe/wslrelay.exe) can remain running after the service exits, holding file locks that prevent the installer from replacing binaries.

Solution

Create per-VM job objects with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE:

DeviceHostProxy: Creates a job object and assigns the COM device host process (received via RegisterDeviceHost PID) using AssignProcessToJobObject.
WslCoreVm: Creates a job object and passes it to child process launches (debug console, kd relay, port relay, interop server) via PROC_THREAD_ATTRIBUTE_JOB_LIST.
SubProcess: New SetJobObject() method that uses PROC_THREAD_ATTRIBUTE_JOB_LIST for clean job assignment at process creation time.
helpers: All Launch* functions gain an optional HANDLE JobObject parameter (defaults to nullptr for backward compatibility).

When the VM is destroyed, the job handle closes and Windows automatically terminates all associated processes.

Validation Steps

Build succeeds: cmake --build . -- -m (wslservice.exe, wsl.exe)
Verified no existing callers are broken (all new parameters default to nullptr)
Code review confirmed lifetime safety and correctness

Copilot

Pull request overview

This PR introduces per-VM Windows Job Objects configured with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE so that processes spawned by the WSL service (e.g., wslhost.exe, wslrelay.exe, and device host processes) are automatically terminated when the VM shuts down, preventing lingering processes from keeping package binaries/DLLs locked during upgrades.

Changes:

Add a per-VM job object in WslCoreVm and pass it through process-launch helpers so VM-scoped child processes are tied to VM lifetime.
Add a SubProcess::SetJobObject() API that wires PROC_THREAD_ATTRIBUTE_JOB_LIST into process creation.
Add a job object in DeviceHostProxy and assign the registered device host process to it.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/windows/service/exe/WslCoreVm.h	Adds a VM-scoped job handle member to own the kill-on-close job lifetime.
src/windows/service/exe/WslCoreVm.cpp	Creates/configures the job object and passes it to debug/relay/instance launches.
src/windows/service/exe/WslCoreInstance.h	Extends instance construction to accept/store a job handle for subprocess launches.
src/windows/service/exe/WslCoreInstance.cpp	Plumbs the job handle through to interop server launch.
src/windows/common/SubProcess.h	Adds `SetJobObject()` API and stores an optional job handle.
src/windows/common/SubProcess.cpp	Implements job attribute wiring via `PROC_THREAD_ATTRIBUTE_JOB_LIST`.
src/windows/common/helpers.hpp	Extends `Launch*` helper signatures with an optional job handle parameter.
src/windows/common/helpers.cpp	Plumbs job handle into `SubProcess` launches for wslhost/wslrelay.
src/windows/common/DeviceHostProxy.h	Adds a job handle to manage device-host process lifetime.
src/windows/common/DeviceHostProxy.cpp	Creates/configures the job and assigns the device host PID to it.

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.

JohnMcPMS

Looks reasonable, but I definitely don't know enough about the process graph to speak to its completeness.

JohnMcPMS · 2026-05-21T16:27:45Z

+    m_processJobObject.reset(CreateJobObjectW(nullptr, nullptr));
+    THROW_LAST_ERROR_IF(!m_processJobObject);
+
+    JOBOBJECT_EXTENDED_LIMIT_INFORMATION jobInfo{};
+    jobInfo.BasicLimitInformation.LimitFlags = JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE;
+    THROW_IF_WIN32_BOOL_FALSE(SetInformationJobObject(m_processJobObject.get(), JobObjectExtendedLimitInformation, &jobInfo, sizeof(jobInfo)));


This is the third instance of this code; probably worth a helper.

Add JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE job objects to ensure that child processes spawned by the service (wsldevicehost, wslhost.exe, wslrelay.exe) are terminated when the VM shuts down. This prevents lingering processes from holding DLLs locked during package upgrades. Changes: - DeviceHostProxy: Create a job object and assign the COM device host process to it in RegisterDeviceHost. - WslCoreVm: Create a per-VM job object and pass it to LaunchDebugConsole, LaunchKdRelay, LaunchPortRelay, and WslCoreInstance. - SubProcess: Add SetJobObject() which uses PROC_THREAD_ATTRIBUTE_JOB_LIST to assign the process to the job at creation time. - helpers: Add optional JobObject parameter to Launch* functions. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Follow-up to the per-VM job object work: WSLCVirtualMachine::LaunchPortRelay spawns wslrelay.exe directly via SubProcess. Without a per-VM job object, if wslcsession.exe crashes or is force-killed before signaling m_vmTerminatingEvent, the spawned wslrelay.exe is orphaned and keeps wslrelay.exe locked. Create a JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE job in WSLCVirtualMachine::Initialize and pass it to LaunchPortRelay via SubProcess::SetJobObject, matching the pattern used in WslCoreVm. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Member destruction order is reverse of declaration order. Declare m_processJobObject before m_portRelayChannelRead/Write so the pipes are destroyed first and the job object (which kills wslrelay.exe) is destroyed last. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Replace four near-identical CreateJobObject + SetInformationJobObject blocks with a single helpers::CreateKillOnCloseJob() call: - WslCoreVm - DeviceHostProxy - WSLCVirtualMachine - WSLCSessionManager Addresses code review feedback from JohnMcPMS. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings May 20, 2026 17:57

Copilot started reviewing on behalf of benhillis May 20, 2026 18:00 View session

Copilot AI reviewed May 20, 2026

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Comment thread src/windows/common/SubProcess.cpp

Comment thread src/windows/common/DeviceHostProxy.cpp

benhillis force-pushed the benhill/job-object-child-processes branch from af651e7 to ad5556a Compare May 20, 2026 18:39

benhillis marked this pull request as ready for review May 20, 2026 23:54

benhillis requested a review from a team as a code owner May 20, 2026 23:54

Copilot AI review requested due to automatic review settings May 20, 2026 23:54

Copilot started reviewing on behalf of benhillis May 20, 2026 23:55 View session

Copilot AI reviewed May 21, 2026

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JohnMcPMS previously approved these changes May 21, 2026

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benhillis dismissed JohnMcPMS’s stale review via b99308c May 21, 2026 16:22

Copilot AI review requested due to automatic review settings May 21, 2026 16:23

Copilot started reviewing on behalf of benhillis May 21, 2026 16:23 View session

JohnMcPMS reviewed May 21, 2026

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Ben Hillis and others added 4 commits May 21, 2026 09:29

fix formatting

fa24192

benhillis force-pushed the benhill/job-object-child-processes branch from abbd937 to 5d87a5f Compare May 21, 2026 16:29

Copilot AI reviewed May 21, 2026

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Comment thread src/windows/common/SubProcess.cpp

JohnMcPMS approved these changes May 21, 2026

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benhillis merged commit 35f6566 into master May 21, 2026
11 checks passed

benhillis deleted the benhill/job-object-child-processes branch May 21, 2026 23:29

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Add job objects to terminate child processes on VM shutdown#40607

Add job objects to terminate child processes on VM shutdown#40607
benhillis merged 5 commits into
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benhill/job-object-child-processes

benhillis commented May 20, 2026

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benhillis commented May 20, 2026

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