Fix ASan heap-buffer-overflow in signal handlers

[jbachorik]

What does this PR do?:

Use SafeAccess for signal-handler memory reads in vmStructs.h to prevent crashes when reading from partially initialized thread structures during GC worker thread creation.

Motivation:

Fixes ASan heap-buffer-overflow errors in CI where the CTimer signal handler was delivering signals to newly created GC worker threads before their VMThread structure was fully initialized. The TLS pointer returned a valid address but offset calculations resulted in reading memory before the allocated region ("72 bytes to the left").

Additional Notes:

While doing this change I took a sweep on other signal-handler-accessible methods where SafeAccess makes sense:

• VMThread::state() - Uses SafeAccess::load32()
• VMThread::inDeopt() - Uses SafeAccess::loadPtr()
• JavaFrameAnchor::lastJavaSP() - Uses SafeAccess::loadPtr()
• JavaFrameAnchor::lastJavaFP() - Uses SafeAccess::loadPtr()
• JavaFrameAnchor::lastJavaPC() - Uses SafeAccess::loadPtr()
• JavaFrameAnchor::fromEntryFrame() - Uses SafeAccess::loadPtr()

How to test the change?:

• All gtests pass locally
• CI ASan tests should pass without heap-buffer-overflow errors: https://github.com/DataDog/java-profiler/actions/runs/21221768151

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 review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

🤖 Generated with Claude Code

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervmmtnos
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Wed Jan 21 18:50:35 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	10
Unused code
Dead assignment	2
Dead initialization	7
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead assignment	libraryPatcher_linux.cpp	patch_library_unlocked	93	1
Unused code	Dead assignment	stackWalker.cpp	walkVM	551	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_JavaProfiler_getStatus0	119	1
Unused code	Dead initialization	profiler.cpp	runInternal	1644	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	658	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	691	1
Unused code	Dead initialization	flightRecorder.cpp	resolveMethod	351	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_OTelContext_setProcessCtx0	475	1
Unused code	Dead initialization	vmStructs_dd.cpp	initOffsets	56	1
Unused code	Dead nested assignment	vmStructs_dd.cpp	checkNativeBinding	388	1

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervmmtnos
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Wed Jan 21 19:22:20 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	10
Unused code
Dead assignment	2
Dead initialization	7
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead assignment	libraryPatcher_linux.cpp	patch_library_unlocked	93	1
Unused code	Dead assignment	stackWalker.cpp	walkVM	551	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_JavaProfiler_getStatus0	119	1
Unused code	Dead initialization	profiler.cpp	runInternal	1644	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	658	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	691	1
Unused code	Dead initialization	flightRecorder.cpp	resolveMethod	351	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_OTelContext_setProcessCtx0	475	1
Unused code	Dead initialization	vmStructs_dd.cpp	initOffsets	56	1
Unused code	Dead nested assignment	vmStructs_dd.cpp	checkNativeBinding	388	1

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervmmtnos
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Wed Jan 21 19:46:58 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	10
Unused code
Dead assignment	2
Dead initialization	7
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead assignment	libraryPatcher_linux.cpp	patch_library_unlocked	93	1
Unused code	Dead assignment	stackWalker.cpp	walkVM	551	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_JavaProfiler_getStatus0	119	1
Unused code	Dead initialization	profiler.cpp	runInternal	1644	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	658	1
Unused code	Dead initialization	flightRecorder.cpp	cleanupUnreferencedMethods	691	1
Unused code	Dead initialization	flightRecorder.cpp	resolveMethod	351	1
Unused code	Dead initialization	javaApi.cpp	Java_com_datadoghq_profiler_OTelContext_setProcessCtx0	475	1
Unused code	Dead initialization	vmStructs_dd.cpp	initOffsets	56	1
Unused code	Dead nested assignment	vmStructs_dd.cpp	checkNativeBinding	388	1

[r1viollet]

LGTM