Assembly Language Debugging with gdb.

Useful commands

• Signal Handlers

info signal SIGUSR1
handle SIGUSR1 noprint nostop

• Run gdb in Text User Interface (TUI) mode

gdb -tui <program>

• Change the layout to Assembly

layout asm

• Or split the layout to C and Assembly

layout split

• Apply the following customization

set disassembly-flavor att/intel
set print asm-demangle
set disassemble-next-line on  //ask gdb to show us the next instruction every time

• Puts break point on main and invoke run

start

• Examine memory: x/FMT ADDRESS.
  • ADDRESS is an expression for the memory address to examine.
  • FMT == [NUM][FORMAT][SIZE]
    • [Format] ::= [ o(octal), x(hex), d(decimal), u(unsigned decimal), t(binary), f(float), a(address), i(instruction), c(char), s(string), z(hex, zero padded on the left)].
    • [Size] ::= [b(byte), h(halfword), w(word), g(giant, 8 bytes)]
    • The specified number of objects of the specified size are printed according to the format. If a negative number is specified, memory is examined backward from the address.
    • Example

      x/10hb $rsp
      x/Ni $pc
      

• Use nexti,stepi instead of next, step which traverse the source lines
• Printing registers

info registers
info all-registers
info registers regname …
info registers eflags

• Printing & setting xmm registers

(gdb) print $xmm1
$1 = {
  v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
  v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
  v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
  v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
  v4_int32 = {0, 20657912, 11, 13},
  v2_int64 = {88725056443645952, 55834574859},
  uint128 = 0x0000000d0000000b013b36f800000000
}

To set values of such registers, you need to tell GDB which view of the register you wish to change, as if you were assigning value to a struct member:

 (gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF

References

• Debugging with GDB