Handle soft links on library paths

[zhengyu123]

What does this PR do?:
Handle soft link paths in path comparison.

Motivation:
Java profiler patches library's pthread_create() @plt entry, but not itself. If java profiler's library is on a path with soft link, path comparison may fail that results it accidentally patches itself.

Additional Notes:

How to test the change?:

• Regular tests
• Drop libjavaprofiler.so to a path with soft link, run `java -agentpath:${PATH_WITH_SOFT_LINK}/libjavaProfiler.so=start,filter=1,wall=10ms,cpu=10ms,file=profile.jfr $JAVA_APPLICATION

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.
• JIRA: PROF-13517

Unsure? Have a question? Request a review!

[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:	Tue Jan 20 20:47:48 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

[Copilot]

Pull request overview

This PR addresses a bug where the Java profiler could accidentally patch itself when its library path contains symbolic links. The fix uses realpath() to resolve symbolic links before comparing library paths.

Changes:

• Added realpath() call to resolve the profiler's own library path in initialize()
• Added realpath() call to resolve each library's path before comparing with the profiler's path in patch_library_unlocked()
• Handles special cases like virtual files (e.g., [vdso]) that cannot be resolved

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[Copilot]

Missing error handling: If realpath() fails (returns nullptr), _profiler_name will be set to nullptr. This could cause crashes later when _profiler_name is used in strcmp() at line 96. The code should handle this error case, either by falling back to the original path from strdup() or by logging an error and handling the null case appropriately.

Suggested change

	_profiler_name = realpath(info.dli_fname, nullptr);
	char* resolved_path = realpath(info.dli_fname, nullptr);
	if (resolved_path != nullptr) {
	_profiler_name = resolved_path;
	} else {
	// Fall back to the original path if realpath fails
	_profiler_name = strdup(info.dli_fname);
	}

[Copilot]

@jbachorik I've opened a new pull request, #335, to work on those changes. Once the pull request is ready, I'll request review from you.