I have worked quite a bit with gdb and have become pretty familiar with the common commands. gdb is a great debugger for * nix systems, but it can become a bit overwhelming with all the different commands. To make this a bit easier and more security friendly we can use ~.gdbinit. Within .gdbinit you can define alias and macros to aid in displaying and use of gdb.

With a good gdbinit file, life will be a bit more pleasent when searching for that next vulnerbility and/or reversing that next file.

Lets start with creating an alias.

define bpl
info breakpoints
end

Here is another one that save your fingers from typing info all the time.

define stack
info stack
info frame
info args
info locals
end

None of this is anything new, but in all my readings I have just recently learned about it. So I figured posting something may help out. I always knew there was a way to create macros in gdb, it just never dawned on me to look in .gdbinit !

To make life simple and if you don’t want to write your own, below is a nice cracker-friendly .gdbinit file I found.

# INSTRUCTIONS: save as ~/.gdbinit
# ______________breakpoint aliases_____________
define bpl
info breakpoints
end
document bpl
List breakpoints
end
 
define bp
set $SHOW_CONTEXT = 1
break * $arg0
end
document bp
Set a breakpoint on address
Usage: bp addr
end
 
define bpc
clear $arg0
end
document bpc
Clear breakpoint at function/address
Usage: bpc addr
end
 
define bpe
enable $arg0
end
document bpe
Enable breakpoint #
Usage: bpe num
end
 
define bpd
disable $arg0
end
document bpd
Disable breakpoint #
Usage: bpd num
end
 
define bpt
set $SHOW_CONTEXT = 1
tbreak $arg0
end
document bpt
Set a temporary breakpoint on address
Usage: bpt addr
end
 
define bpm
set $SHOW_CONTEXT = 1
awatch $arg0
end
document bpm
Set a read/write breakpoint on address
Usage: bpm addr
end
 
define bhb
set $SHOW_CONTEXT = 1
hb * $arg0
end
document bhb
Set Hardware Assisted breakpoint on address
Usage: bhb addr
end
 
# ______________process information____________
define argv
show args
end
document argv
Print program arguments
end
 
define stack
info stack
end
document stack
Print call stack
end
 
define frame
info frame
info args
info locals
end
document frame
Print stack frame
end
 
define flags
if (($eflags >> 0xB) & 1 )
printf "O "
else
printf "o "
end
if (($eflags >> 0xA) & 1 )
printf "D "
else
printf "d "
end
if (($eflags >> 9) & 1 )
printf "I "
else
printf "i "
end
if (($eflags >> 8) & 1 )
printf "T "
else
printf "t "
end
if (($eflags >> 7) & 1 )
printf "S "
else
printf "s "
end
if (($eflags >> 6) & 1 )
printf "Z "
else
printf "z "
end
if (($eflags >> 4) & 1 )
printf "A "
else
printf "a "
end
if (($eflags >> 2) & 1 )
printf "P "
else
printf "p "
end
if ($eflags & 1)
printf "C "
else
printf "c "
end
printf "\n"
end
document flags
Print flags register
end
 
define eflags
printf " OF <%d> DF <%d> IF <%d> TF <%d>",\
(($eflags >> 0xB) & 1 ), (($eflags >> 0xA) & 1 ), \
(($eflags >> 9) & 1 ), (($eflags >> 8) & 1 )
printf " SF <%d> ZF <%d> AF <%d> PF <%d> CF <%d>\n",\
(($eflags >> 7) & 1 ), (($eflags >> 6) & 1 ),\
(($eflags >> 4) & 1 ), (($eflags >> 2) & 1 ), ($eflags & 1)
printf " ID <%d> VIP <%d> VIF <%d> AC <%d>",\
(($eflags >> 0x15) & 1 ), (($eflags >> 0x14) & 1 ), \
(($eflags >> 0x13) & 1 ), (($eflags >> 0x12) & 1 )
printf " VM <%d> RF <%d> NT <%d> IOPL <%d>\n",\
(($eflags >> 0x11) & 1 ), (($eflags >> 0x10) & 1 ),\
(($eflags >> 0xE) & 1 ), (($eflags >> 0xC) & 3 )
end
document eflags
Print entire eflags register
end
 
define reg
printf " eax:%08X ebx:%08X ecx:%08X ", $eax, $ebx, $ecx
printf " edx:%08X eflags:%08X\n", $edx, $eflags
printf " esi:%08X edi:%08X esp:%08X ", $esi, $edi, $esp
printf " ebp:%08X eip:%08X\n", $ebp, $eip
printf " cs:%04X ds:%04X es:%04X", $cs, $ds, $es
printf " fs:%04X gs:%04X ss:%04X ", $fs, $gs, $ss
echo \033[31m
flags
echo \033[0m
end
document reg
Print CPU registers
end
 
define func
info functions
end
document func
Print functions in target
end
 
define var
info variables
end
document var
Print variables (symbols) in target
end
 
define lib
info sharedlibrary
end
document lib
Print shared libraries linked to target
end
 
define sig
info signals
end
document sig
Print signal actions for target
end
 
define thread
info threads
end
document thread
Print threads in target
end
 
define u
info udot
end
document u
Print kernel 'user' struct for target
end
 
define dis
disassemble $arg0
end
document dis
Disassemble address
Usage: dis addr
end
 
# ________________hex/ascii dump an address______________
define ascii_char
# thanks elaine :)
set $_c=*(unsigned char *)($arg0)
if ( $_c <> 0x7E )
printf "."
else
printf "%c", $_c
end
end
document ascii_char
Print the ASCII value of arg0 or '.' if value is unprintable
end
 
define hex_quad
printf "%02X %02X %02X %02X %02X %02X %02X %02X", \
*(unsigned char*)($arg0), *(unsigned char*)($arg0 + 1), \
*(unsigned char*)($arg0 + 2), *(unsigned char*)($arg0 + 3), \
*(unsigned char*)($arg0 + 4), *(unsigned char*)($arg0 + 5), \
*(unsigned char*)($arg0 + 6), *(unsigned char*)($arg0 + 7)
end
document hex_quad
Print eight hexadecimal bytes starting at arg0
end
 
define hexdump
printf "%08X : ", $arg0
hex_quad $arg0
printf " - "
hex_quad ($arg0+8)
printf " "
 
ascii_char ($arg0)
ascii_char ($arg0+1)
ascii_char ($arg0+2)
ascii_char ($arg0+3)
ascii_char ($arg0+4)
ascii_char ($arg0+5)
ascii_char ($arg0+6)
ascii_char ($arg0+7)
ascii_char ($arg0+8)
ascii_char ($arg0+9)
ascii_char ($arg0+0xA)
ascii_char ($arg0+0xB)
ascii_char ($arg0+0xC)
ascii_char ($arg0+0xD)
ascii_char ($arg0+0xE)
ascii_char ($arg0+0xF)
 
printf "\n"
end
document hexdump
Display a 16-byte hex/ASCII dump of arg0
end
 
# ________________data window__________________
define ddump
echo \033[36m
 
printf "[%04X:%08X]------------------------", $ds, $data_addr
printf "---------------------------------[ data]\n"
echo \033[34m
set $_count=0
while ( $_count < $arg0 ) set $_i=($_count*0x10) hexdump ($data_addr+$_i) set $_count++ end end document ddump Display $arg0 lines of hexdump for address $data_addr end define dd if ( ($arg0 & 0x40000000) || ($arg0 & 0x08000000) || ($arg0 & 0xBF000000) ) set $data_addr=$arg0 ddump 0x10 else printf "Invalid address: %08X\n", $arg0 end end document dd Display 16 lines of a hex dump for $arg0 end define datawin if ( ($esi & 0x40000000) || ($esi & 0x08000000) || ($esi & 0xBF000000) ) set $data_addr=$esi else if ( ($edi & 0x40000000) || ($edi & 0x08000000) || ($edi & 0xBF000000) ) set $data_addr=$edi else if ( ($eax & 0x40000000) || ($eax & 0x08000000) || \ ($eax & 0xBF000000) ) set $data_addr=$eax else set $data_addr=$esp end end end ddump 2 end document datawin Display esi, edi, eax, or esp in data window end # ________________process context______________ define context echo \033[36m printf "----------------------------------------" printf "---------------------------------[ regs]\n" echo \033[32m reg echo \033[36m printf "[%04X:%08X]------------------------", $ss, $esp printf "---------------------------------[stack]\n" echo \033[34m hexdump $sp+0x30 hexdump $sp+0x20 hexdump $sp+0x10 hexdump $sp datawin echo \033[36m printf "[%04X:%08X]------------------------", $cs, $eip printf "---------------------------------[ code]\n" echo \033[37m x /6i $pc echo \033[36m printf "---------------------------------------" printf "----------------------------------------\n" echo \033[0m end document context Print regs, stack, ds:esi, and disassemble cs:eip end define context-on set $SHOW_CONTEXT = 1 end document context-on Enable display of context on every program stop end define context-off set $SHOW_CONTEXT = 1 end document context-on Disable display of context on every program stop end # ________________process control______________ define n ni end document n Step one instruction end define go stepi $arg0 end document go Step # instructions end define pret finish end document pret Step out of current call end define init set $SHOW_CONTEXT = 1 set $SHOW_NEST_INSN=0 tbreak _init r end document init Run program; break on _init() end define start set $SHOW_CONTEXT = 1 set $SHOW_NEST_INSN=0 tbreak _start r end document start Run program; break on _start() end define sstart set $SHOW_CONTEXT = 1 set $SHOW_NEST_INSN=0 tbreak __libc_start_main r end document sstart Run program; break on __libc_start_main(). Useful for stripped executables. end define main set $SHOW_CONTEXT = 1 set $SHOW_NEST_INSN=0 tbreak main r end document main Run program; break on main() end # ________________eflags commands_______________ define cfc if ($eflags & 1) set $eflags = $eflags&~1 else set $eflags = $eflags|1 end end document cfc change Carry Flag end define cfp if (($eflags >> 2) & 1 )
set $eflags = $eflags&~0x4
else
set $eflags = $eflags|0x4
end
end
document cfp
change Carry Flag
end
 
define cfa
if (($eflags >> 4) & 1 )
set $eflags = $eflags&~0x10
else
set $eflags = $eflags|0x10
end
end
document cfa
change Auxiliary Carry Flag
end
 
define cfz
if (($eflags >> 6) & 1 )
set $eflags = $eflags&~0x40
else
set $eflags = $eflags|0x40
end
end
document cfz
change Zero Flag
end
 
define cfs
if (($eflags >> 7) & 1 )
set $eflags = $eflags&~0x80
else
set $eflags = $eflags|0x80
end
end
document cfs
change Sign Flag
end
 
define cft
if (($eflags >>8) & 1 )
set $eflags = $eflags&100
else
set $eflags = $eflags|100
end
end
document cft
change Trap Flag
end
 
define cfi
if (($eflags >> 9) & 1 )
set $eflags = $eflags&~0x200
else
set $eflags = $eflags|0x200
end
end
document cfi
change Interrupt Flag
end
 
define cfd
if (($eflags >>0xA ) & 1 )
set $eflags = $eflags&~0x400
else
set $eflags = $eflags|0x400
end
end
document cfd
change Direction Flag
end
 
define cfo
if (($eflags >> 0xB) & 1 )
set $eflags = $eflags&~0x800
else
set $eflags = $eflags|0x800
end
end
document cfo
change Overflow Flag
end
 
# --------------------patch---------------------
define nop
set * (unsigned char *) $arg0 = 0x90
end
document nop
Patch byte at address arg0 to a NOP insn
Usage: nop addr
end
 
define null
set * (unsigned char *) $arg0 = 0
end
document null
Patch byte at address arg0 to NULL
Usage: null addr
end
 
define int3
set * (unsigned char *) $arg0 = 0xCC
end
document int3
Patch byte at address arg0 to an INT3 insn
Usage: int3 addr
end
 
# --------------------cflow---------------------
define print_insn_type
if ($arg0 == 0)
printf "UNKNOWN";
end
if ($arg0 == 1)
printf "JMP";
end
if ($arg0 == 2)
printf "JCC";
end
if ($arg0 == 3)
printf "CALL";
end
if ($arg0 == 4)
printf "RET";
end
if ($arg0 == 5)
printf "INT";
end
end
document print_insn_type
This prints the human-readable mnemonic for the instruction typed passed as
a parameter (usually $INSN_TYPE).
end
 
define get_insn_type
set $INSN_TYPE = 0
set $_byte1=*(unsigned char *)$arg0
if ($_byte1 == 0x9A || $_byte1 == 0xE8 )
# "call"
set $INSN_TYPE=3
end
if ($_byte1 >= 0xE9 && $_byte1 <= 0xEB) # "jmp" set $INSN_TYPE=1 end if ($_byte1 >= 0x70 && $_byte1 <= 0x7F) # "jcc" set $INSN_TYPE=2 end if ($_byte1 >= 0xE0 && $_byte1 <= 0xE3 ) # "jcc" set $INSN_TYPE=2 end if ($_byte1 == 0xC2 || $_byte1 == 0xC3 || $_byte1 == 0xCA || $_byte1 == 0xCB || $_byte1 == 0xCF) # "ret" set $INSN_TYPE=4 end if ($_byte1 >= 0xCC && $_byte1 <= 0xCE) # "int" set $INSN_TYPE=5 end if ($_byte1 == 0x0F ) # two-byte opcode set $_byte2=*(unsigned char *)($arg0 +1) if ($_byte2 >= 0x80 && $_byte2 <= 0x8F) # "jcc" set $INSN_TYPE=2 end end if ($_byte1 == 0xFF ) # opcode extension set $_byte2=*(unsigned char *)($arg0 +1) set $_opext=($_byte2 & 0x38) if ($_opext == 0x10 || $_opext == 0x18) # "call" set $INSN_TYPE=3 end if ($_opext == 0x20 || $_opext == 0x28) # "jmp" set $INSN_TYPE=1 end end end document get_insn_type This takes an address as a parameter and sets the global $INSN_TYPE variable to 0, 1, 2, 3, 4, 5 if the instruction at the address is unknown, a jump, a conditional jump, a call, a return, or an interrupt. end define step_to_call set $_saved_ctx = $SHOW_CONTEXT set $SHOW_CONTEXT = 0 set $SHOW_NEST_INSN=0 set logging file /dev/null set logging on set logging redirect on set $_cont = 1 while ( $_cont > 0 )
stepi
get_insn_type $pc
if ($INSN_TYPE == 3)
set $_cont = 0
end
end
 
if ( $_saved_ctx > 0 )
context
else
x /i $pc
end
 
set $SHOW_CONTEXT = 1
set $SHOW_NEST_INSN=0
set logging redirect off
set logging off
set logging file gdb.txt
end
document step_to_call
This single steps until it encounters a call instruction; it stops before
the call is taken.
end
 
define trace_calls
set $SHOW_CONTEXT = 0
set $SHOW_NEST_INSN=0
set $_nest = 1
set listsize 0
set logging overwrite on
set logging file ~/gdb_trace_calls.txt
set logging on
set logging redirect on
 
while ( $_nest > 0 )
get_insn_type $pc
 
# handle nesting
if ($INSN_TYPE == 3)
set $_nest = $_nest + 1
else
if ($INSN_TYPE == 4)
set $_nest = $_nest - 1
end
end
 
# if a call, print it
if ($INSN_TYPE == 3)
set $x = $_nest
while ( $x > 0 )
printf "\t"
set $x = $x - 1
end
x /i $pc
end
 
#set logging file /dev/null
stepi
#set logging file ~/gdb_trace_calls.txt
end
 
set $SHOW_CONTEXT = 1
set $SHOW_NEST_INSN=0
set logging redirect off
set logging off
set logging file gdb.txt
 
# clean up trace file
shell grep -v ' at ' ~/gdb_trace_calls.txt > ~/gdb_trace_calls.1
shell grep -v ' in ' ~/gdb_trace_calls.1 > ~/gdb_trace_calls.txt
end
document trace_calls
Creates a runtime trace of the calls made target in ~/gdb_trace_calls.txt.
Note that this is very slow because gdb "set redirect on" does not work!
end
 
define trace_run
set $SHOW_CONTEXT = 0
set $SHOW_NEST_INSN=1
set logging overwrite on
set logging file ~/gdb_trace_run.txt
set logging on
set logging redirect on
set $_nest = 1
 
while ( $_nest > 0 )
 
get_insn_type $pc
# jmp, jcc, or cll
if ($INSN_TYPE == 3)
set $_nest = $_nest + 1
else
# ret
if ($INSN_TYPE == 4)
set $_nest = $_nest - 1
end
end
 
stepi
end
 
set $SHOW_CONTEXT = 1
set $SHOW_NEST_INSN=0
set logging file gdb.txt
set logging redirect off
set logging off
 
# clean up trace file
shell grep -v ' at ' ~/gdb_trace_run.txt > ~/gdb_trace_run.1
shell grep -v ' in ' ~/gdb_trace_run.1 > ~/gdb_trace_run.txt
 
end
document trace_run
Creates a runtime trace of the target in ~/gdb_trace_run.txt. Note
that this is very slow because gdb "set redirect on" does not work!
end
 
# _____________________misc_____________________
# this makes 'context' be called at every BP/step
define hook-stop
if ( $SHOW_CONTEXT > 0 )
context
end
if ( $SHOW_NEST_INSN > 0 )
set $x = $_nest
while ($x > 0 )
printf "\t"
set $x = $x - 1
end
end
end
 
define assemble
printf "Hit Ctrl-D to start, type code to assemble, hit Ctrl-D when done.\n"
printf "It is recommended to start with\n"
printf "\tBITS 32\n"
printf "Note that this command uses NASM (Intel syntax) to assemble.\n"
shell nasm -f bin -o /dev/stdout /dev/stdin | od -v -t x1 -w16 -A n
end
document assemble
Assemble Intel x86 instructions to binary opcodes. Uses nasm.
Usage: assemble
end
 
define gas_asm
printf "Type code to assemble, hit Ctrl-D until results appear :)\n"
printf "Note that this command uses GAS (AT&T syntax) to assemble.\n"
shell as -o ~/__gdb_tmp.bin
shell objdump -d -j .text --adjust-vma=$arg0 ~/__gdb_tmp.bin
shell rm ~/__gdb_tmp.bin
end
document gas_asm
Assemble Intel x86 instructions to binary opcodes using gas and objdump
Usage: gas_asm address
end
 
# !scary bp_alloc macro!
# The idea behind this macro is to break on the following code:
# 0x4008e0aa : sub $0xc,%esp
# 0x4008e0ad : call 0x4008e0b2
# 0x4008e0b2 : pop %ebx
# 0x4008e0b3 : add $0xa3f6e,%ebx
# At 0x4008e0b3, %ebx contains the address that has just been allocated
# The bp_alloc macro generates this breakpoint and *should* work for
# the forseeable future ... but if it breaks, set a breakpoint on
# __libc_malloc and look for where where the return value gets popped.
 
define bp_alloc
tbreak *(*__libc_malloc + F) if $ebx == $arg0
end
document bp_alloc
This sets a temporary breakpoint on the allocation of $arg0.
It works by setting a breakpoint on a specific address in __libc_malloc().
USE WITH CAUTION -- it is extremely platform dependent.
Usage: bp_alloc addr
end
 
define dump_hexfile
dump ihex memory $arg0 $arg1 $arg2
end
document dump_hexfile
Write a range of memory to a file in Intel ihex (hexdump) format.
Usage: dump_hexfile filename start_addr end_addr
end
 
define dump_binfile
dump memory $arg0 $arg1 $arg2
end
document dump_binfile
Write a range of memory to a binary file.
Usage: dump_binfile filename start_addr end_addr
end
 
# _________________cracker tips_________________
# The 'tips' command is used to provide tutorial-like info to the user
define tips
printf "Tip Topic Commands:\n"
printf "\ttip_display : Automatically display values on each break\n"
printf "\ttip_patch : Patching binaries\n"
printf "\ttip_strip : Dealing with stripped binaries\n"
printf "\ttip_syntax : ATT vs Intel syntax\n"
end
document tips
Provide a list of tips from crackers on various topics.
end
 
define tip_patch
printf "\n"
printf " PATCHING MEMORY\n"
printf "Any address can be patched using the 'set' command:\n"
printf "\t`set ADDR = VALUE` \te.g. `set *0x8049D6E = 0x90`\n"
printf "\n"
printf " PATCHING BINARY FILES\n"
printf "Use `set write` in order to patch the target executable\n"
printf "directly, instead of just patching memory.\n"
printf "\t`set write on` \t`set write off`\n"
printf "Note that this means any patches to the code or data segments\n"
printf "will be written to the executable file. When either of these\n"
printf "commands has been issued, the file must be reloaded.\n"
printf "\n"
end
document tip_patch
Tips on patching memory and binary files
end
 
define tip_strip
printf "\n"
printf " STOPPING BINARIES AT ENTRY POINT\n"
printf "Stripped binaries have no symbols, and are therefore tough to\n"
printf "start automatically. To debug a stripped binary, use\n"
printf "\tinfo file\n"
printf "to get the entry point of the file. The first few lines of\n"
printf "output will look like this:\n"
printf "\tSymbols from '/tmp/a.out'\n"
printf "\tLocal exec file:\n"
printf "\t `/tmp/a.out', file type elf32-i386.\n"
printf "\t Entry point: 0x80482e0\n"
printf "Use this entry point to set an entry point:\n"
printf "\t`tbreak *0x80482e0`\n"
printf "The breakpoint will delete itself after the program stops as\n"
printf "the entry point.\n"
printf "\n"
end
document tip_strip
Tips on dealing with stripped binaries
end
 
define tip_syntax
printf "\n"
printf "\t INTEL SYNTAX AT&T SYNTAX\n"
printf "\tmnemonic dest, src, imm mnemonic src, dest, imm\n"
printf "\t[base+index*scale+disp] disp(base, index, scale)\n"
printf "\tregister: eax register: %%eax\n"
printf "\timmediate: 0xFF immediate: $0xFF\n"
printf "\tdereference: [addr] dereference: addr(,1)\n"
printf "\tabsolute addr: addr absolute addr: *addr\n"
printf "\tbyte insn: mov byte ptr byte insn: movb\n"
printf "\tword insn: mov word ptr word insn: movw\n"
printf "\tdword insn: mov dword ptr dword insn: movd\n"
printf "\tfar call: call far far call: lcall\n"
printf "\tfar jump: jmp far far jump: ljmp\n"
printf "\n"
printf "Note that order of operands in reversed, and that AT&T syntax\n"
printf "requires that all instructions referencing memory operands \n"
printf "use an operand size suffix (b, w, d, q).\n"
printf "\n"
end
document tip_syntax
Summary of Intel and AT&T syntax differences
end
 
define tip_display
printf "\n"
printf "Any expression can be set to automatically be displayed every time\n"
printf "the target stops. The commands for this are:\n"
printf "\t`display expr' : automatically display expression 'expr'\n"
printf "\t`display' : show all displayed expressions\n"
printf "\t`undisplay num' : turn off autodisplay for expression # 'num'\n"
printf "Examples:\n"
printf "\t`display/x *(int *)$esp` : print top of stack\n"
printf "\t`display/x *(int *)($ebp+8)` : print first parameter\n"
printf "\t`display (char *)$esi` : print source string\n"
printf "\t`display (char *)$edi` : print destination string\n"
printf "\n"
end
document tip_display
Tips on automatically displaying values when a program stops.
end
# __________________gdb options_________________
set confirm off
set verbose off
#set prompt \033[01;m\033] niel@gdb $ \033[0m
set prompt \033[31mgdb $ \033[0m
set output-radix 0x10
set input-radix 0x10
# These make gdb never pause in its output
set height 0
set width 0
# why do these not work???
set $SHOW_CONTEXT = 1
set $SHOW_NEST_INSN=0
 
set disassembly-flavor intel
 
#EOF

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