An implementation overview By Aldo Núñez

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 Run/Attach  Control  Inspect  Why?  Find out the cause of a problem (bug)

 Kick off a process  Attach to an already running process

 Breakpoints  Stepping  Changing instruction pointer  Suspend and resume threads

 Callstack  Loaded modules  Threads  Memory  Registers  Variables  Expressions

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 A process that runs, controls, and inspects another process  Special relationship between debugger and debuggee  System notifies debugger of events taking place in debuggee

OS Debug events Break/Run mode Change state HW Single Step Breakpoints Registers

Loop event ← WaitForDebugEvent( timeout ) if got event ContinueDebugEvent( event.pid,event.tid, DISCARD_EXCEPTION ) Until event.code = EXIT_PROCESS

 Start Process  Exit Process  Start Thread  Exit Thread  Load Module  Unload Module  Exception  Message

Debug Events WaitForDebugEvent ContinueDebugEvent Launch CreateProcess TerminateProcess Attach DebugActiveProcess DebugActiveProcessStop Registers GetThreadContext SetThreadContext Memory ReadProcessMemory WriteProcessMemory Threads SuspendThread ResumeThread

Enable SS GetThreadContext context.Eflags or 0x100 SetThreadContext ContinueDebugEvent SS Event event ← WaitForEvent event.code = EXCEPTION event.exception.code = EXCEPTION_SINGLE_STEP

Enable BP ReadProcessMemory WriteProcessMemory data = 0xCC (int 3) FlushInstructionCache ContinueDebugEvent BP Event event ← WaitForEvent event.code = EXCEPTION event.exception.code = EXCEPTION_BREAKPOINT Except. address = original EIP = original + 1

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 A debugger for D programs  A set of independent libraries  A Visual Studio plug-in

 Interest in debuggers since 2005  Started September 2009  Source code released August 2010  Integrated into Visual D September 2010

 Benefits to making separate components  Targeted testing  Mix and match for different purposes  Use with any shell program  Benefits to making VS plug-in  Well tested shell program already written  High level debug programming model

Debug EngineExecExprSyms

Exec Control debuggee Read and change state Expr Evaluate D Expressions Formatting Syms Read debug info DE Combine other components Expose AD7 interface

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 Abstracts run, control, and inspection services  Built first to make it as solid as possible  Many APIs are locked to thread that started debuggee  Because of underlying Windows API

 WaitForEvent, Continue from event  Launch, Terminate  Attach, Detach  Read, Write Memory  Set, Remove Breakpoint  Step, Cancel Step  Async Break

 Software breakpoint abstraction  Hardware breakpoint abstraction  Breakpoint sharing  Resuming from breakpoint

 Stepping over a single instruction  Can easily step over most instructions with native single step (SS)  Others require setting a BP after the instruction  REP string instructions

 State machines for complex stepping  In, Over, Out, Go/Resume  Instruction, Statement  Control low-level SS and BP  Receive notification of SS and BP events  Can be canceled

 Instruction steppers handle 18 scenarios  3x Instruction type: (simple, call, REP)  2x At a BP  3x Movement: (Go, Step In, Step Over)  Range stepper uses instruction steppers over an address range  Step Out stepper runs to a BP at return address

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 Evaluates D expressions  Input is textual expression  Output is a result value record  Declarations, symbols, and input values come from outside  IValueBinder, IDeclaration  Handles formatting values  Enumerates children of values  Based on DMD front end

MakeTypeEnv( &typeEnv ); MakeNameTable( &nameTable ); ParseText( L”a[2] + 3”, typeEnv, nameTable, &expr ); expr->Bind( options, binder ); expr->Evaluate( options, binder, &result );

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 Reads debug info for a program  Maps of source files to lines  Maps of source code lines to addresses  Functions – address and scopes  Symbols – name, type, value, storage  Types  Reads specific formats  Currently, CodeView 4.10, output by DMD

CODEVIEW  Fixed record fields  Numeric constant compression  Common type encoding  Sorted symbols  Nested Lexical blocks DWARF  Flexible  Attributes: key-value  Explicit base type definition  Location expressions  Compression  Flatten tree  Abbreviations  Byte code for tables

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 A plug-in to the VS Debugger package (vsdebug.dll)  Standalone DLL doesn’t depend on any other package  Expected to implement AD7 interface  Knows how to debug one kind of program  DEs are multiplexed during a debug session

 A programming model for debugging processes  Single-threaded calls from VS Debugger to DE  Simplifies design  COM interfaces  Debug Engine is a COM co-class

 IDebugEngine2  IDebugThread2  IDebugBoundBreakpoint2  IDebugExpression2  IDebugStackFrame2  IDebugDisassemblyStream2  IDebugEvent2

 What is debugging?  What is a debugger?  What is Mago?  Execution Agent  Expression Evaluator  Symbol Reader  Debug Engine  D and the debugger

 Rewrite in D eventually  Only EE and parts of DE know about D  EE Test input generated by D program  Expression and expected value  Uses compile-time reflection

void main( string[] args ) { writeln( " " ); if ( set == 1 ) UnaryList!(byte, ubyte, short, ushort, int, uint, long, ulong).Operation( op ); else if ( set == 2 ) UnaryList!(float, double, real, ifloat, idouble, ireal, cfloat, cdouble, creal).Operation( op ); writeln( " " ); } template Unary(T) { void Unary( void function( T ) func ) { foreach ( t; Vals!T.vals ) { Id++; writefln( " ", Prefix, Id ); func( t ); writefln( " " ); } template UnaryList(T...) { void Operation( Op op ) { foreach ( t; T ) { switch ( op ) { case Op.Negate: Unary!(t)(&UnOp!(t).Negate); break; case Op.BitNot: static if (__traits( compiles, Unary!(t)( &UnOp!(t).BitNot ))) Unary!(t)( &UnOp!(t).BitNot ); break; } template Vals(T) { static T[] vals; static this() { static if ( !__traits( isFloating, T ) ) // Add values like T.max, cast(T) -1, cast(T) 0 else // Add values like T.nan, -T.infinity, cast(T) 0 }

template UnOp(T) { static if ( __traits( isIntegral, T ) ) void BitNot( T t ) { writeln( " " ); CastTerm( t ); writeln( " " ); PrintType!(typeof( ~t ))(); PrintTerm( ~t ); writeln( " " ); } void PrintType(X)() { static if ( __traits( isArithmetic, X ) ) writefln( " ", typeid( X ) ); else writefln( " ", typeid( X ) ); } void CastTerm(X)( X x ) { writeln( " " ); PrintType!X(); PrintTerm( x ); writeln( " " ); } void PrintTerm(X)( X x ) { static if ( is(X==creal) || is(X==cdouble) || is(X==cfloat) ) { writeln( " " ); writefln( " ", x.re ); writefln( " ", x.im ); writeln( " " ); } else if ( is( X == ireal ) || is( X == idouble ) || is( X == ifloat ) ) { writefln( " ", x ); } else if ( __traits( isFloating, X ) ) { writefln( " ", x ); } else if ( is( X == ulong ) || (is( X == long ) && (x < 0)) ) { writefln( " ", x ); } else writefln( " ", x ); }

 Interface for sharing between modules  Take DMD front end or mimic it  80-bit floating point in expression eval  Disassembling variable length instructions

 Mago:  D:  Visual D IDE by Rainer Schuetze:  Visual Studio Debugger Extensibility: us/library/bb aspx us/library/bb aspx  x86 and x64 Manuals:  DWARF Format: