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@gsingh93
Created June 17, 2018 01:32
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Mirror of this GDB script for debugging x86 16-bit real mode code: http://www.capp-sysware.com/misc/stackoverflow/gdb_init_real_mode.txt
# Special mode for GDB that allows to debug/disassemble REAL MODE x86 code
#
# It has been designed to be used with QEMU or BOCHS gdb-stub
#
# 08/2011 Hugo Mercier - GPL v3 license
#
# Freely inspired from "A user-friendly gdb configuration file" widely available
# on the Internet
set confirm off
set verbose off
set prompt \033[31mreal-mode-gdb$ \033[0m
set output-radix 0d10
set input-radix 0d10
# These make gdb never pause in its output
set height 0
set width 0
# Intel syntax
set disassembly-flavor intel
# Real mode
set architecture i8086
set $SHOW_CONTEXT = 1
set $REAL_MODE = 1
# By default A20 is present
set $ADDRESS_MASK = 0x1FFFFF
# nb of instructions to display
set $CODE_SIZE = 10
define enable-a20
set $ADDRESS_MASK = 0x1FFFFF
end
define disable-a20
set $ADDRESS_MASK = 0x0FFFFF
end
# convert segment:offset address to physical address
define r2p
if $argc < 2
printf "Arguments: segment offset\n"
else
set $ADDR = (((unsigned long)$arg0 & 0xFFFF) << 4) + (((unsigned long)$arg1 & 0xFFFF) & $ADDRESS_MASK)
printf "0x%05X\n", $ADDR
end
end
document r2p
Convert segment:offset address to physical address
Set the global variable $ADDR to the computed one
end
# get address of Interruption
define int_addr
if $argc < 1
printf "Argument: interruption_number\n"
else
set $offset = (unsigned short)*($arg0 * 4)
set $segment = (unsigned short)*($arg0 * 4 + 2)
r2p $segment $offset
printf "%04X:%04X\n", $segment, $offset
end
end
document int_addr
Get address of interruption
end
define compute_regs
set $rax = ((unsigned long)$eax & 0xFFFF)
set $rbx = ((unsigned long)$ebx & 0xFFFF)
set $rcx = ((unsigned long)$ecx & 0xFFFF)
set $rdx = ((unsigned long)$edx & 0xFFFF)
set $rsi = ((unsigned long)$esi & 0xFFFF)
set $rdi = ((unsigned long)$edi & 0xFFFF)
set $rbp = ((unsigned long)$ebp & 0xFFFF)
set $rsp = ((unsigned long)$esp & 0xFFFF)
set $rcs = ((unsigned long)$cs & 0xFFFF)
set $rds = ((unsigned long)$ds & 0xFFFF)
set $res = ((unsigned long)$es & 0xFFFF)
set $rss = ((unsigned long)$ss & 0xFFFF)
set $rip = ((((unsigned long)$cs & 0xFFFF) << 4) + ((unsigned long)$eip & 0xFFFF)) & $ADDRESS_MASK
set $r_ss_sp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$esp & 0xFFFF)) & $ADDRESS_MASK
set $r_ss_bp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$ebp & 0xFFFF)) & $ADDRESS_MASK
end
define print_regs
printf "AX: %04X BX: %04X ", $rax, $rbx
printf "CX: %04X DX: %04X\n", $rcx, $rdx
printf "SI: %04X DI: %04X ", $rsi, $rdi
printf "SP: %04X BP: %04X\n", $rsp, $rbp
printf "CS: %04X DS: %04X ", $rcs, $rds
printf "ES: %04X SS: %04X\n", $res, $rss
printf "\n"
printf "IP: %04X EIP:%08X\n", ((unsigned short)$eip & 0xFFFF), $eip
printf "CS:IP: %04X:%04X (0x%05X)\n", $rcs, ((unsigned short)$eip & 0xFFFF), $rip
printf "SS:SP: %04X:%04X (0x%05X)\n", $rss, $rsp, $r_ss_sp
printf "SS:BP: %04X:%04X (0x%05X)\n", $rss, $rbp, $r_ss_bp
end
document print_regs
Print CPU registers
end
define print_eflags
printf "OF <%d> DF <%d> IF <%d> TF <%d>",\
(($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 print_eflags
Print eflags register.
end
# dump content of bytes in memory
# arg0 : addr
# arg1 : nb of bytes
define _dump_memb
if $argc < 2
printf "Arguments: address number_of_bytes\n"
else
set $_nb = $arg1
set $_i = 0
set $_addr = $arg0
while ($_i < $_nb)
printf "%02X ", *((unsigned char*)$_addr + $_i)
set $_i++
end
end
end
# dump content of memory in words
# arg0 : addr
# arg1 : nb of words
define _dump_memw
if $argc < 2
printf "Arguments: address number_of_words\n"
else
set $_nb = $arg1
set $_i = 0
set $_addr = $arg0
while ($_i < $_nb)
printf "%04X ", *((unsigned short*)$_addr + $_i)
set $_i++
end
end
end
# display data at given address
define print_data
if ($argc > 0)
set $seg = $arg0
set $off = $arg1
set $raddr = ($arg0 << 16) + $arg1
set $maddr = ($arg0 << 4) + $arg1
set $w = 16
set $i = (int)0
while ($i < 4)
printf "%08X: ", ($raddr + $i * $w)
set $j = (int)0
while ($j < $w)
printf "%02X ", *(unsigned char*)($maddr + $i * $w + $j)
set $j++
end
printf " "
set $j = (int)0
while ($j < $w)
set $c = *(unsigned char*)($maddr + $i * $w + $j)
if ($c > 32) && ($c < 128)
printf "%c", $c
else
printf "."
end
set $j++
end
printf "\n"
set $i++
end
end
end
define context
printf "---------------------------[ STACK ]---\n"
_dump_memw $r_ss_sp 8
printf "\n"
set $_a = $r_ss_sp + 16
_dump_memw $_a 8
printf "\n"
printf "---------------------------[ DS:SI ]---\n"
print_data $ds $rsi
printf "---------------------------[ ES:DI ]---\n"
print_data $es $rdi
printf "----------------------------[ CPU ]----\n"
print_regs
print_eflags
printf "---------------------------[ CODE ]----\n"
set $_code_size = $CODE_SIZE
# disassemble
# first call x/i with an address
# subsequent calls to x/i will increment address
if ($_code_size > 0)
x /i $rip
set $_code_size--
end
while ($_code_size > 0)
x /i
set $_code_size--
end
end
document context
Print context window, i.e. regs, stack, ds:esi and disassemble cs:eip.
end
define hook-stop
compute_regs
if ($SHOW_CONTEXT > 0)
context
end
end
document hook-stop
!!! FOR INTERNAL USE ONLY - DO NOT CALL !!!
end
# add a breakpoint on an interrupt
define break_int
set $offset = (unsigned short)*($arg0 * 4)
set $segment = (unsigned short)*($arg0 * 4 + 2)
break *$offset
end
define break_int_if_ah
if ($argc < 2)
printf "Arguments: INT_N AH\n"
else
set $addr = (unsigned short)*($arg0 * 4)
set $segment = (unsigned short)*($arg0 * 4 + 2)
break *$addr if ((unsigned long)$eax & 0xFF00) == ($arg1 << 8)
end
end
document break_int_if_ah
Install a breakpoint on INT N only if AH is equal to the expected value
end
define break_int_if_ax
if ($argc < 2)
printf "Arguments: INT_N AX\n"
else
set $addr = (unsigned short)*($arg0 * 4)
set $segment = (unsigned short)*($arg0 * 4 + 2)
break *$addr if ((unsigned long)$eax & 0xFFFF) == $arg1
end
end
document break_int_if_ax
Install a breakpoint on INT N only if AX is equal to the expected value
end
define stepo
## we know that an opcode starting by 0xE8 has a fixed length
## for the 0xFF opcodes, we can enumerate what is possible to have
set $lip = $rip
set $offset = 0
# first, get rid of segment prefixes, if any
set $_byte1 = *(unsigned char *)$rip
# CALL DS:xx CS:xx, etc.
if ($_byte1 == 0x3E || $_byte1 == 0x26 || $_byte1 == 0x2E || $_byte1 == 0x36 || $_byte1 == 0x3E || $_byte1 == 0x64 || $_byte1 == 0x65)
set $lip = $rip + 1
set $_byte1 = *(unsigned char*)$lip
set $offset = 1
end
set $_byte2 = *(unsigned char *)($lip+1)
set $_byte3 = *(unsigned char *)($lip+2)
set $noffset = 0
if ($_byte1 == 0xE8)
# call near
set $noffset = 3
else
if ($_byte1 == 0xFF)
# A "ModR/M" byte follows
set $_mod = ($_byte2 & 0xC0) >> 6
set $_reg = ($_byte2 & 0x38) >> 3
set $_rm = ($_byte2 & 7)
#printf "mod: %d reg: %d rm: %d\n", $_mod, $_reg, $_rm
# only for CALL instructions
if ($_reg == 2 || $_reg == 3)
# default offset
set $noffset = 2
if ($_mod == 0)
if ($_rm == 6)
# a 16bit address follows
set $noffset = 4
end
else
if ($_mod == 1)
# a 8bit displacement follows
set $noffset = 3
else
if ($_mod == 2)
# 16bit displacement
set $noffset = 4
end
end
end
end
# end of _reg == 2 or _reg == 3
else
# else byte1 != 0xff
if ($_byte1 == 0x9A)
# call far
set $noffset = 5
else
if ($_byte1 == 0xCD)
# INTERRUPT CASE
set $noffset = 2
end
end
end
# end of byte1 == 0xff
end
# else byte1 != 0xe8
# if we have found a call to bypass we set a temporary breakpoint on next instruction and continue
if ($noffset != 0)
set $_nextaddress = $eip + $offset + $noffset
printf "Setting BP to %04X\n", $_nextaddress
tbreak *$_nextaddress
continue
# else we just single step
else
nexti
end
end
document stepo
Step over calls
This function will set a temporary breakpoint on next instruction after the call so the call will be bypassed
You can safely use it instead nexti since it will single step code if it's not a call instruction (unless you want to go into the call function)
end
define step_until_iret
set $SHOW_CONTEXT=0
set $_found = 0
while (!$_found)
if (*(unsigned char*)$rip == 0xCF)
set $_found = 1
else
stepo
end
end
set $SHOW_CONTEXT=1
context
end
define step_until_ret
set $SHOW_CONTEXT=0
set $_found = 0
while (!$_found)
set $_p = *(unsigned char*)$rip
if ($_p == 0xC3 || $_p == 0xCB || $_p == 0xC2 || $_p == 0xCA)
set $_found = 1
else
stepo
end
end
set $SHOW_CONTEXT=1
context
end
define step_until_int
set $SHOW_CONTEXT = 0
while (*(unsigned char*)$rip != 0xCD)
stepo
end
set $SHOW_CONTEXT = 1
context
end
# Find a pattern in memory
# The pattern is given by a string as arg0
# If another argument is present it gives the starting address (0 otherwise)
define find_in_mem
if ($argc >= 2)
set $_addr = $arg1
else
set $_addr = 0
end
set $_found = 0
set $_tofind = $arg0
while ($_addr < $ADDRESS_MASK) && (!$_found)
if ($_addr % 0x100 == 0)
printf "%08X\n", $_addr
end
set $_i = 0
set $_found = 1
while ($_tofind[$_i] != 0 && $_found == 1)
set $_b = *((char*)$_addr + $_i)
set $_t = (char)$_tofind[$_i]
if ($_t != $_b)
set $_found = 0
end
set $_i++
end
if ($_found == 1)
printf "Code found at 0x%05X\n", $_addr
end
set $_addr++
end
end
document find_in_mem
Find a pattern in memory
The pattern is given by a string as arg0
If another argument is present it gives the starting address (0 otherwise)
end
define step_until_code
set $_tofind = $arg0
set $SHOW_CONTEXT = 0
set $_found = 0
while (!$_found)
set $_i = 0
set $_found = 1
while ($_tofind[$_i] != 0 && $_found == 1)
set $_b = *((char*)$rip + $_i)
set $_t = (char)$_tofind[$_i]
if ($_t != $_b)
set $_found = 0
end
set $_i++
end
if ($_found == 0)
stepo
end
end
set $SHOW_CONTEXT = 1
context
end
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