[fault-injection] Unprotected access to JVM class loader data mutex and method list by zhengyu123 · Pull Request #779 · DataDog/java-profiler · GitHub

What does this PR do?:
Hotspot JDK8 specific.

patchClassLoaderData() prepends MethodList blocks onto a class's ClassLoaderData to work around a JDK 8 jmethodID layout bug. This PR hardens that path:

Guards the cld->lock()/methodList() mutation and its matching cld->unlock() with the profiler's signal/crash-protection machinery (ProfiledThread, JmpCtxScope, sigsetjmp/SIGNAL_HANDLER_UNWIND_AFTER_LONGJMP), via the new LockState RAII helper, so a crash while the classloader-data mutex is held during a signal-unsafe window unlocks it instead of deadlocking the JVM.
Tags each patched jclass (via JVMTI Get/SetTag) with the method_count already prepended, so repeated calls for the same class (profiler restart replay, RedefineClasses/RetransformClasses) only prepend the delta instead of leaking another full set of MethodList blocks on every re-run.
Releases the classloader-data lock before calling the JVMTI tag API, since cld->lock() suppresses safepoint checks while Get/SetTag are safepoint-polling JVMTI entry points.

Motivation:
Fault injection testing found that unprotected access to the JVM class loader data mutex and method list in patchClassLoaderData() could crash or deadlock the JVM.

Additional Notes:
The SetTag() call is not serialized against a concurrent patchClassLoaderData() call for the same class, so two racing callers can both observe a stale tag and both patch. This only wastes one extra round of preallocated blocks in that rare case; unlike the original unguarded code, it can no longer grow unboundedly.

How to test the change?:
Added unit tests in ddprof-lib/src/test/cpp/hotspotSupport_ut.cpp covering the locking/crash-protection and idempotent-tagging behavior of patchClassLoaderData().

For Datadog employees:

If this PR touches code that signs or publishes builds or packages, or handles
credentials of any kind, I've requested a security review (run the dd:platform-security-review
skill, or file a request via the PSEC review form).
bewaire also runs automatically on every PR.
This PR doesn't touch any of that.
JIRA: PROF-15462

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…nd method list

Scan-Build Report

User:	runner@runnervmlun5p
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Thu Sep 17 11:03:01 2026

Bug Summary

Logic error
Bug Type	Quantity	Display?
All Bugs	1
Dereference of null pointer	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Dereference of null pointer	faultInjection.cpp	crashNow	24	2

CI Test Results

Run: #36585034644 | Commit: 11db259 | Duration: 16m 46s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

Updated: 2026-09-29 15:04:04 UTC

✅ All 40 integration tests passed

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…va-profiler into zgu/jdk8_patchClassLoaderData

More details

The static review finds no defect in the JDK 8 lock recovery or per-class delta tag paths.

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A JDK 8 class transformation keeps the old tag. The next method reload skips the required MethodList preallocation when the method count stays the same.

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Sphinx review — 7 approved findings (deduplicated from 15 raw; 0 CRITICAL/HIGH, 2 MEDIUM, 4 LOW, 1 INFO).

[fault-injection] Unprotected access to JVM class loader data mutex a…

c9143ad

…nd method list

Avoid deadlock

ac5ae41

Merge branch 'main' into zgu/jdk8_patchClassLoaderData

585d13e

zhengyu123 and others added 4 commits September 14, 2026 20:15

Fix

ece8ed9

Merge branch 'zgu/jdk8_patchClassLoaderData' of github.com:DataDog/ja…

d37a2d2

…va-profiler into zgu/jdk8_patchClassLoaderData

Add tests

8e9b967

Merge branch 'main' into zgu/jdk8_patchClassLoaderData

2ba3f13

zhengyu123 marked this pull request as ready for review September 17, 2026 19:43

zhengyu123 requested a review from a team as a code owner September 17, 2026 19:43

datadog-prod-us1-6 Bot reviewed Sep 17, 2026

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zhengyu123 marked this pull request as draft September 18, 2026 00:40

Fix

08ef73b

zhengyu123 marked this pull request as ready for review September 18, 2026 17:14

chatgpt-codex-connector Bot reviewed Sep 18, 2026

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp Outdated

datadog-prod-us1-6 Bot reviewed Sep 18, 2026

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp Outdated

zhengyu123 added 2 commits September 23, 2026 13:39

Fix class redefinition path

12092ca

Merge

fa9bc2b

jbachorik reviewed Sep 24, 2026

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zhengyu123 added 2 commits September 24, 2026 15:13

Review comments

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Merge branch 'main' into zgu/jdk8_patchClassLoaderData

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zhengyu123 requested a review from jbachorik September 24, 2026 17:26

Merge branch 'main' into zgu/jdk8_patchClassLoaderData

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zhengyu123 mentioned this pull request Sep 30, 2026

Reference-chain tracker and leak-signal engine #797

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[fault-injection] Unprotected access to JVM class loader data mutex and method list by zhengyu123 · Pull Request #779 · DataDog/java-profiler · GitHub

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[fault-injection] Unprotected access to JVM class loader data mutex and method list - #779

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[fault-injection] Unprotected access to JVM class loader data mutex and method list#779
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