Fix the partial kernel/userspace stackwalking

[jbachorik]

What does this PR do?:
It fixes the mixed kernel/userspace stackwalking

Motivation:
The current logic seems flawed when using perf_event_open scheduler and it fails to get the non-kernel part of the stacktrace.
The fix is trivial and it does fix the issue.

How to test the change?:
One must have a system with perf_event_open available. Collected stacktraces should not contain a significant number of UNKNOWN_PACKAGE frames.

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 ticket: PROF-9142

Unsure? Have a question? Request a review!

[github-actions]

🔧 Report generated by pr-comment-cppcheck

CppCheck Report

Warnings (5)

• Obsolete function 'alloca' called.
• Member variable 'ScopeDesc::_stream' is not initialized in the constructor.
• Member variable 'ScopeDesc::_method_offset' is not initialized in the constructor.
• Member variable 'ScopeDesc::_bci' is not initialized in the constructor.
• Member variable 'ElfParser::_vaddr_diff' is not initialized in the constructor.

Style Violations (164)

• Local variable 'jni' shadows outer function
• Local variable 'size' shadows outer function
• Local variable 'prev' shadows outer function
• The class 'NativeFunc' does not declare a constructor although it has private member variables which likely require initialization.
• Local variable 'count' shadows outer function
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• Consider using std::transform algorithm instead of a raw loop.
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• struct member 'CpuTime::system' is never used.
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• The function 'enableEvents' overrides a function in a base class but is not marked with a 'override' specifier.
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• Unused variable: kept_classes
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• Variable 'error' is assigned a value that is never used.
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• Unused private function: 'StackFrame::withinCurrentStack'
• The comparison 'EMPTY_FRAME_SIZE > 0' is always false.
• The comparison 'EMPTY_FRAME_SIZE > 0' is always false.
• The scope of the variable 'prev_sp' can be reduced.
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• The scope of the variable 'delayedCounter' can be reduced.
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• Variable 'ctx' can be declared with const
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[github-actions]

🔧 Report generated by pr-comment-scanbuild

Scan-Build Report

User:	runner@fv-az523-512
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 14.0.0-1ubuntu1.1
Date:	Fri Feb 9 12:37:45 2024

Bug Summary

Bug Type	Quantity	Display?
All Bugs	6
Logic error
Assigned value is garbage or undefined	1
Dereference of null pointer	2
Result of operation is garbage or undefined	1
Unused code
Dead initialization	1
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Assigned value is garbage or undefined	dwarf.cpp	parseInstructions	235	20
Unused code	Dead initialization	livenessTracker.cpp	cleanup_table	45	1
Unused code	Dead nested assignment	vmStructs.cpp	checkNativeBinding	818	1
Logic error	Dereference of null pointer	flightRecorder.cpp	flush	1409	8
Logic error	Dereference of null pointer	safeAccess.h	load	35	18
Logic error	Result of operation is garbage or undefined	dwarf.h	getSLeb	142	25

[richardstartin]

Good catch, let's upstream this one