unit5 ss notes

8/6/2019 Unit5 Ss Notes

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7.3 In t erac t ive debugg ing

sys tems

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In t roduc t ion

An interactive debugging systemprovides programmers with facilities thataid in the testing and debugging of

programs.


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Approaches t o debugging

Print statements Printing to log files Sprinkling the code with assertions Using post-mortem dumps Having programs that provide function call stacks on

termination Profiling Heap checking

System call tracing tools Interactive source-level debugging Reverse execution ...

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Debugger bas ic pr inc ip les

The Heisenberg principle This principle says that the debugger must intrude on the

debuggee in a minimal way.

Truthful debugging

The debugger must be truthful so the programmer alwaystrusts it. Providing context information

The debugger should provide the program contextinformation during debugging.

Program context information. source code, stack back-track variable values, thread information, ...


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Debugging funct ions andcapab i l i t ies

Breakpoints

The programmer may define a breakpointwhich causesexecution to be suspended when a specified point in theprogram is reached.

After execution is suspended, other debugging commands canbe used to analyze the progress of the program and todiagnose errors detected.

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Debugging funct ions and

capab i l i t ies

Tracing

Tracing can be used to track the flow the executionlogic and data modifications.

Step into Step over Run to cursor Run until return ...


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capab i l i t ies

Traceback

Trackback can show the path by which the currentstatement was reached (a list of all the functions calledin order).

This kind of information should be displayedsymbolically, and should be related to the sourceprogram.

call stack

registers

watch


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Program-display capabilities

It must be possible to display the program being debugged. The user should be able to control the level at which this

display occurs.

source level

instruction

level

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capab i l i t ies

Capabilities of dealing with optimized code

If the compiler has so distorted the final instruction streamthrough optimizations, the programmer cannot easily mapthis back onto the original source code

switching to a mode where the actual source code isinterpreted may remove a major source of confusion for theprogrammer.

Fix-and-run

The goal of this approach is to minimize the turnaroundtime between creating a fix to test and the ability to actuallytest the system with the modification in place.


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Capabilities of handling multilingual situations

Many application systems involve the use of severaldifferent programming language.

A debugging system must be sensitive to the specificlanguage being debugged so that procedural,arithmetic, and conditional logic can be coded in thesyntax of that language.

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Hardw are/OS debugging support

The debuggers ability to control theexecution of the debuggee depends both

on hardware support built into the processorand

on sophisticated, specialized debuggingmechanisms built into the operating system.


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Hardw are/OS debugging support

A way to specify a breakpoint---a specific location inthe executing code such that when the processor reachesthis location, execution will stop.

This can be provided for by simply writing some illegalinstruction into the code stream for the debuggee.

A notification system, also called an interrupt or a trap,that will notify the operating system (and thereby thedebugger) that an important event has occurred with

respect to the running process. The ability to read and write directly out of and into

the hardware registers when the interrupt occurs.

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Hardw are debugging suppor t

Breakpoint support

Single-step support

Fault-detection support Watchpoint support

... (such as Multithread control,Multiprocessor control)


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Breakpoin t suppor t

Breakpoints are usually implemented as a specialinstruction that causes a trap to the operation system,which then can notify a special program that hasregistered itself as a debugger.

On architectures with varying length instructions, it isnormal for the trap---or breakpoint---instruction to be thelength of the shortest possible instruction.

This makes it much simpler for the debugger to guaranteebreakpoints are placed on instruction boundaries.

Breakpoint instruction of Intel x86 is a one-byte instructionINT 3.

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Single-st ep support

Single-step means that the processor isinstructed to execute a single machineinstruction when it is next processing

instructions for the debuggee.

Most processors provide a mode bit thatcontrols single-step operation.


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Faul t -detec t ion suppor t

When some faults of the debuggee aredetected by the processor, such as divide by zero and memory

access violations,

by OS, such as I/O failure, or by some higher level software systems, such as stack

corruption or array bounds check violations,

the OS notifies the debugger that its debuggeehas received a fault before the debuggeeactually is allowed control again.

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Watchpoint suppo rt

Watchpoints are notifications presentedto the debugger when specified portionsof the debuggees address space are

modified.

Set the portions of the address space asread-only.

The processor traps on any modification ofdata in these addresses and stop itself before

the write occurs.


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OS debugging supp ort

To control a debuggee process, a debuggerneeds a mechanism to notify the OS about theexecutable program it wishes to control. Once this is done, the debugger needs to modify that

debuggees code in order to modify the instructionstream with the breakpoint instructions;

And then, the debugger needs to tell the OS to run thedebuggee but notify the debugger if any sort of

exceptions occur before the debuggee itself getnotified or terminated.

Once the debuggee stops, the debugger needs to beable to gather information from the debuggees registerset and data memory.

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Breakpo in t da ta s t ruc t u res

At least two levels of breakpointrepresentation are needed: the logical breakpoints,

set by users, associated with a point in the source code.

the physical breakpoints, it relates directly to executable machine instructions, the points in the address space where actual hardware

breakpoint instructions get written.

it stores the original instruction. the original instruction is restored if the breakpoint is to

be removed.


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logical physical

File foo.c: line 9

File foo.c: line 9

File foo.c: line 9

File foo.c: line 5

0x458F0

0x400F0

Logical-to-physical: set, modify, or delete breakpoints

Physical-to-logical: whenever a hardware breakpoint event occurs

physical breakpoint

structure

Originalinstruction

Reference count

Physical address

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Breakpo in t se t t ing and

ac t i va t ion

Breakpoint setting

request symbol table agent to map given file nameand line number information into physical address.

create logical breakpoint object containing thisinformation. create physical breakpoint object, or increment

reference count if this object already exists

physical breakpoint agent must put breakpointinstruction and save original instruction at that location


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Condi t iona l breakpoin t

A conditional breakpoint may or may notstop the debuggee when the breakpointfires depending on the value of anassociated condition.

Conditions are Boolean expressions that areevaluated by the debugger when thebreakpoint is activated.

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St ep over and s tep in t o

Step overis a statement step in the context ofthe current function scope running anydescendent function full speed to completion.

For statement step over, an internal breakpoint is seton the functions return address to allow the functionand all its descendents at full speed.

Step intois a statement step that goes into anydescendent functions found during the currentoperations.


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float compute(int a,int b)

{

float c = b;

for(int i = 0; i < a; i++)

c = c * b;

return c;

}

main()

{

int a,b;

float c;

printf(Enter a, b:\n);

scanf(%d %d,&a,&b);

c = compute(a,b);

printf(result: %f\n,c);

}

Step over ends here

after running

compute

Last step ended

here. From here do

a step into or a step

over.

Step into stops here

when entering

compute().

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Sourc e-level s t ep into

How to implement step into ? Main concern.

A source statement may be translated into several machine instructions. Approaches

Use machine step with repeatedly checking at each instruction step to see ifthe current address matches a source statements starting address. Drawback: very slow. (1000 times slower than full-speed run to a breakpoint)

Set a temporary breakpoint at the address of the beginning of the nextsource statement.

How to handle branching instruction?if (expression) {

...

} else {

...

}

either machine single-step instruction, or decode the branch target and set a temporary breakpoint at that address


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St ep over

The algorithm for step over is like that of stepinto but it notes a call instruction duringinstruction decode as special, and once thecalled function has been entered, a breakpointis inserted at the return address of this functionby looking at the current stack frame.

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Stac k t race

When a fault or breakpoint occurs, theuser needs a stack track to know howdid the program get here?

A stack traceis a list of the procedure

activation records currently on the callstack.

call stackSTACK

frame 0

frame 1

frame 2

call P1

call P2


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Stac k t race

Once the debugger knowsthe boundaries of a activation record and

the procedure-calling conventions,

a stack trace can showthe procedure names, and

formal arguments.

The debugger can also retrieve the values ofthe local variables.

P0 return address

arguments from P0 to P1

activation record 0 locals

registers saved by P0

P1 return address

arguments from P1 to P2activation record 1 locals


activation record 2 locals


P0 calls P1

P1 calls P2


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Rela t ionsh ip w i th o t her par ts o f t he sys tem

The debugger must also exist in a waythat is consistent with the security andintegrity components of the system.

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