The document discusses debugging tools and techniques, focusing on GDB (the GNU Project debugger). It covers: 1. How debuggers work under the hood using ptrace to control programs and set breakpoints. 2. The basics of using GDB - starting and running programs, setting breakpoints, printing variables, and examining stack traces. 3. Additional GDB commands for inspection, as well as examples of inline assembly code, calling conventions, and disassembling code.

1. Something On GDB And Debugging
dutor
Gdutor@gmail.com
TAIR
September 26, 2013

2. Outline
1
Debugger Under the Hood

3. Outline
1
Debugger Under the Hood
2
GDB the Basics

4. Outline
1
Debugger Under the Hood
2
GDB the Basics
3
Bonus

5. Outline
1
Debugger Under the Hood
2
GDB the Basics
3
Bonus
4
A Little Assembly

6. Debugger Under the Hood Basics Bonus A Little Assembly
What and How
what debuggers do
how to take control of the target
how to set up breakpoints
how to map instructions with source code
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7. Debugger Under the Hood Basics Bonus A Little Assembly
ptrace
long ptrace(request, pid, addr, data)
tracer/tracee
signal/waitpid
PTRACE_TRACEME
PTRACE_ATTACH
PTRACE_CONT/SINGLESTEP
PTRACE_PEEKDATA/POKEDATA
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8. Debugger Under the Hood Basics Bonus A Little Assembly
breakpoints
hardware, the debug registers
software, INT 3
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9. Debugger Under the Hood Basics Bonus A Little Assembly
Debug Info
DWARF, designed for various languages and ABIs
source lines and instructions
names and adresses
utilities
readelf
strip
nm/strings
c++filt
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10. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
$
$
$
$
cc test . cpp -g
gdb a . out
gdb a . out core .1234
gdb -- args a . out arg1 arg2
breakpoints
print/x
others
help
$ gdb
( gdb )
( gdb )
( gdb )
( gdb )
file a . out
start arg1 arg2
run arg1 arg2
run < if > of
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11. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
breakpoints
print/x
others
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
next
step
continue
nexti
stepi
finish
until LINE
help
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12. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
breakpoints
print/x
others
help
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
set p
end
break
break test . cpp :32
break foo if p == NULL
break 32 thread 0
info breakpoints
disable 1
enable 2
delete 3
ignore 4 123
commands 5
= malloc (1024)
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13. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
breakpoints
print/x
( gdb )
( gdb )
( gdb )
( gdb )
print n
print / x p
print object
x &n
others
help
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14. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
breakpoints
print/x
others
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
( gdb )
backtrace
info locals
info threads
thread 16
list
list foo
list -
help
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15. Debugger Under the Hood Basics Bonus A Little Assembly
Basic Usage of GDB
start
running
breakpoints
print/x
( gdb ) help print
( gdb ) help set
( gdb ) help set print
others
help
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16. Debugger Under the Hood Basics Bonus A Little Assembly
Miscs and Bonus
p $rip
set follow-fork-mode child
i reg
set scheduler-locking on
p {tair::StorageManager}0x608048
symbol-file
p *array@10
add-symbol-file
display/i $rip
i sharedlibrary
x/40a $rsp
gcc test.cpp -g -g3
l *0x608048
maintenance info sections
watch expr
set logging on
return 0
set print pretty on
enable breakpoints
gdb -p 1234 -ex ‘set n=0’ -batch
gcore
Ctrl-X Ctrl-A
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17. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Syntax
instruction src, des The first operand is the source, the second is the destination
%register Register names are prefixed with a %, %% in inlined assembly
$literal Literal values are prefixed with $
instruction{b,w,l,q} The instruction suffix denotes the operand size
seg:off(base, index, scale) Memory access
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18. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Common Used Registers
rax, eax, ax, al
rbx, ebx, bx, bl
rcx, ecx, cx, cl
rdx, edx, dx, dl
rsi, esi, si
rdi, edi, di
rbp, rsp
r8-r15
xmm0-xmm7
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19. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Stack Frame
+------+
+-----------------------+
|
|
|
|
return address
|
|
|
v
+-----------------------+
|
|
%ebp----> |
old %ebp
|
|
|
+-----------------------+
|
|
%esp----> |
local variables
|
|
|
+-----------------------+
stack |
|
|
argument 2
|
| growing
|
+-----------------------+
direction |
|
|
argument 1
|
|
|
+-----------------------+
|
|
|
return address
|
|
|
+-----------------------+
|
+--------------+
old %ebp
| <-----%ebp
|
+-----------------------+
|
|
local variables
| <-----%esp
|
+-----------------------+
|
|
|
_|_
|
Red Zone
|
/
|
|
’
+-----------------------+
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20. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Calling Conventions
Arguments, rdi, rsi, rdx, rcx, r8, r9, xmm0-xmm7
Arguments on stack
Return, rax, xmm0
Stack Frame, rbp, rsp
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21. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Data Representatoin
Integers
Floats
Structures
Classes
Arrays
PC-relative Reference
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22. That’s All.

23. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Basic Format
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asm ( < assembler template >
: [ " constraints " ( var ) ] [ , " constraints " ( var ) ]
: [ " constraints " ( var ) ] [ , " constraints " ( var ) ]
: [ " register " ] [ , " register " ] [ , " memory " ]
);
/* output operands */
/* input operands */
/* clobbered registers */
Examples
asm("nop":::)
asm("incl %%eax")
asm("movl $1, %0":"m"(ret))
asm("movl %0, %%eax"::"m"(ret))
asm("addl %1, %0":"+"(a):"r"(b))
asm("incl global_var":::"memory")
nop
Access register directly
Write to C variable
Read from C variable
a =a+b
Write to arbitrary memory
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24. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Constraints
r register operands, any of the followings.
a %rax, etc.
b %rbx, etc.
c %rcx, etc.
d %rdx, etc.
S %rsi, etc.
D %rdi, etc.
q register operands, any of a, b, c, d.
m memory operands.
f floating poing register.
[0-9] matching constraints, both for input
and output.
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25. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Constraints
r register operands, any of the followings.
a %rax, etc.
b %rbx, etc.
c %rcx, etc.
d %rdx, etc.
S %rsi, etc.
D %rdi, etc.
q register operands, any of a, b, c, d.
m memory operands.
f floating poing register.
[0-9] matching constraints, both for input
and output.
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asm (
" int $0x80 n t "
: " = a " ( ret )
: " 0 " (4) , " b " ( fd ) , " c " ( buf ) , " d " ( n )
);
Something On GDB And Debugging

26. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Constraints Modifiers
= operand is write-only.
+ operand is used as both read and write.
Write-Only Example
asm ( " movl $1 , %0 " : " = r " ( ret ) ) ;
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27. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Constraints Modifiers
= operand is write-only.
+ operand is used as both read and write.
Read-Write Example
asm ( " incl %0 " : " + r " ( ret ) ) ;
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28. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Call C Functions
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int
main ()
{
char * fmt = " Hello , % s n " ;
char * s = " World " ;
int ret = 0;
asm ( " callq printf n t "
: " = a " ( ret )
: " D " ( fmt ) , " S " ( s ) ) ;
printf ( " ret : % d n " , ret ) ;
return 0;
}
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29. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Examples
Do System Call
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/* syscall write */
inline int as_write ( int fd , char * buf , size_t n ) {
int ret ;
asm (
" int $0x80 n t "
: " = a " ( ret )
: " 0 " (1) , " D " ( fd ) , " S " ( buf ) , " d " ( n )
);
return ret ;
}
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30. Debugger Under the Hood Basics Bonus A Little Assembly
Inlined Assembly Examples Disassembling
Disassembling
Using GDB
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