Effective Use Of Gdb

Effective use of GDB February 24th 2008 01/22/08

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it Brian W. Kernighan's Law of Debugging Difficulty

2

B

3

Agenda  Programming GDB  Defining commands  Controlling program flow  Inspecting and Changing Program State  Changing Debuggee state  Post-mortem analysis with GDB  Core Files  Debugging Tips, Tricks and Myths.

4

Some Definitions  Debugger – gdb  Debuggee – Program being debugged

5

Programming GDB.  Code    

Commands Conditional Breakpoints – Default Actions with Breakpoints. Sequencing Looping

 Data  Convenience Variables.  Parameters to commands

 Environment  Command files  .gdbinit

6

Defining Custom Commands  Build on top of basic gdb commands .  Useful to traverse long data structures and print values.  Sequence of GDB commands to which you can add a new name  Upto 10 arguments ($arg0... $arg9) allowed .  Supports if-else, conditional and basic looping constructs.  Convenience variables can be used.

7

Defining Custom Commands (gdb) define factorial Type commands for definition of "factorial". End with a line saying just "end". >set $fact=1 >set $n=$arg0 >while $n>1 >set $fact=$fact*$n >set $n=$n-1 >end >print $fact >end (gdb) factorial 5

.

$30 = 120

8

Convenience Variables  Use to hold values that can be reused.  Variables exist within gdb and not as a part of your debuggee.  Prefixed with `$'  No fixed type  set $foo = *obj_ptr

 show convenience

9

Inspecting Debuggee State.  Inspecting data  Interpreter on the debuggee  Changing debuggee program state

10

Interpreter on the debuggee  Possible to call functions in the debuggee  Shows return value.  Pass multiple parameters includes convenience variables  Usecases  Change Debuggee state to reset or initialize  Pretty Print large data structures  Quickly examine behaviour of a function

 Caveats  No breakpoints allowed in such functions  Called routine cannot abort or the debug session will terminate  Can't single step in such functions.

11

Changing Debuggee State  Setting values to variables  print var = value  set variable name = value  set variable name = func (....)

 Continuing from a different location  jump linespec  jump *address  set $pc = value

 Return from a function  return expr

 Giving a signal to your program  signal {signalname | signalnumber}

12

Debugging Signals    

Sending signals to the debuggee Signal handlers are not special. Trap Execution on signal delivery  signal <signalname /number>  Pass a signal to a program  Equivalent of raise.

13

Debugging Signals handle signum/name [Keywords]  Decide on what to do with the signal (action)    

{no}stop – Give control to the debugger. {no}print – Print information on the console. {no}pass – Pass the signal to the debuggee {no}ignore Ignore = nopass  noignore=pass

 Resume a program with signal 0 .

14

Core Files     

Generate core files how ? What is in a core file ? Debug a core file as you would any other program Backtraces Work ! up, down, Frame tracing works. Threads .. hmmm !

15

Generating Core Files $> ulimit -c unlimited Run your program normally get a core file. Typically a file called core or core.pid on other distros.  Configuring file info by /proc/sys/kernel/core_pattern and /proc/sys/kernel/core_uses_pid    

16

What is in a core file.  Memory and Register dump of the program being run.  Linker map dump from dynamic linker.  Stack dump at time of crash.  Default data section.  All threads are dumped.

17

/proc/kcore  Treat it as any other core file .  Get a kernel compiled with debug info in it.

18

Rudimentary Replay Debugging    

Save program state Resume from the program state. Rudimentary form of replay debugging. Caveats  File IO, Signals, Sockets , Threads etc.

 Shameless plug – Go look at LIZARD (Lizard iz a Replay Debugger) http://lizard.sourceforge.net

19

Some popular Myths  Code generated with -g is different from without it.  Do we lose MIPS with -g ?  Stepping forward in straight line code takes you back in time. Hmmm compiler bug ?  printf is my best friend !

20

Moral of the story.  Get familiar with assembly – even if it is rudimentary  Trust the debugger. It usually works.  Don't blame the compiler always . Its usually a buffer overflow some place !

 Start automating your debug tasks.  Define your commands.  Write your own gdbinit scripts with your software.  Document them !

 Structure your code better.  Remember from the command line  help  info

21

Some questions  The debugger being a user level process gets information about other user level processes . A security hole ?  Software breakpoints – How big is the instruction used?  How do breakpoints in shared libraries work across programs ?  Can I use gdb on gdb ?

22

Thank you and Questions  References  GDB User Manual  GDB Reference Manual

23

www.azingo.com 01/22/08