CS31 Weekly Lab: Week 6

Week 6 lab goals:

  1. Review of compiling to/from various code formats
  2. Introduce tools for examining binary files
  3. Review tools for debugging assembly code

Create a week06 subdirectory in your weeklylab subdirectory and copy over some files:
    cd cs31/weeklylab		
    mkdir week06
    cd week06
    cp ~lammert/public/cs31/week06/* .
    Makefile simplefuncs.c

Compiling Phases and Assembly
First, let's open up simplefuncs.c in vim.

We are going to look again at how to use gcc to create an assembly version of this file, and how to create a object .o file, and how to examine its contents.

If you open up the Makefile you can see the rules for building .s, .o and executable files from simplefuncs.c. We will be compiling the 32-bit version of instructions, so we will use the -m32 flag to gcc:

gcc-4.4 -m32 -S simplefuncs.c   # just runs the assembler to create a .s text file
gcc-4.4 -m32 -c simplefuncs.c   # compiles to a relocatable object binary file (.o) 
gcc-4.4 -m32 -o simplefuncs simplefuncs.o  # creates a 32-bit executable file
To see the machine code and assembly code mappings in the .o file:
objdump -d simplefuncs.o
You can compare this to the assembly file:
cat simplefuncs.s

Tools for examining binary files
Some tools for examining binary files:
  1. strings dumps all the strings in a binary file:
      strings simplefuncs 
  2. nm (or objdump -t) to list symbol table contents:
      objdump -t  simplefuncs   # list symbol table in the executable (a.out) file
      nm --format sysv  simplefuncs  # list symbol table in the executable file
  3. gdb and ddd and disass

gdb and ddd to debug at the assembly code level
We covered using gdb to step through IA32 assembly in the week 4 Thursday Lab, but let's try it out again with the simplefuncs program.

First, let's open up simplefuncs.c in vim. Then, let's try some things out in gdb:

gdb simplefuncs
(gdb) break main
(gdb) break func1
(gdb) run
In gdb you can disassemble code using the disass command:
(gdb) disass main
You can set a break point at a specific instruction:
(gdb) break *0x08048477   # set breakpoint at specified address 
And you can step or next at the instruction level using ni or si (si steps into function calls, ni skips over them):
(gdb) ni	  # execute the next instruction then gdb gets control again 
(gdb) ni
(gdb) ni
(gdb) ni
(gdb) ni
(gdb) disass
(gdb) cont      # continue to next break point
Now we are at the call to func1, let's step into this function using si (we also have a breakpoint at this function, let's see when it is hit):
(gdb) si
(gdb) disass
(gdb) ni 
(gdb) where
(gdb) disass
(gdb) cont
You can print out the values of individual registers like this:
(gdb) print $eax
Or the memory contents at a given address, providing either the absolute numeric address or its value stored in registers:
(gdb) p *(int *)($ebp + 8)
(gdb) x     $ebp + 8
(gdb) x/d     $ebp + 8   # x/d display as decimal value
You can also view all register values:
(gdb) info registers
You can also use the display command to automatically display values each time a breakpoint is reached:
(gdb) display $eax
(gdb) display $edx
You can use the examine command (x) to display the contents of a memory location either an address of via a register value (x is shorthand for examine, and p is shorthand for the print command):
x $esp-0x8  # see what p and x display for the same value
p $esp-0x8    

p *(int *)($ebp-0x8)    # here is how to get what x gives you using print

x $esp + 0x1c    # here is examining the contents at a memory location
x 0xffffd2fc     # specifying the address in two different ways


We are going to try running this in ddd instead of gdb, because ddd has a nicer interface for viewing assembly, registers, and stepping through program execution:
ddd simplefuncs
The gdb prompt is in the bottom window. There are also menu options and buttons for gdb commands, but I find using the gdb prompt at the bottom easier to use.

You can view the register values as the program runs (choose Status->Registers to open the register window).

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