A Little more Hardware

• Remember, memory is memory
  • The cpu will interpret contents based on usage.
  • Ascii,
  • Integer
  • Floating Point
    • See this demo.
  • Or even instructions.
• If a piece of data is loaded into the IR, it is an instruction.
• But some data could yield an illegal instruction.
  • The cpu will go to an exception handler when an illegal instruction is executed.
• Review of Setuid programs.
  • These are programs that allow normal users to become the superuser
  • They are marked by a "s" in the execute bit for user.
  • These are requred for programs that
    • Access/change resources like configuration files.
      • /bin/passwd
      • /bin/sudo
    • find /usr/bin -perm -u+s
  • Most of these allow or require additional input in interactive mode.
    • Which would make a buffer overflow possible.
    • These must be coded carefully.
    • look at chfn.c code.
      • Specifically ask_new_field
      • This uses getline. See man page.
• C style strings.
  • A null terminated array of strings.
  • const char * s
  • \0 is null.
  • c has library functions (string.h) which help with this
    • strlen
    • strcmp
    • strdup
    • And many more
  • All of these rely on the string being null terminated.
  • Take a look at stringDemo.c
• no op instruction
  • For modern pipelined systems this is a requirement.
  • reference for intel assembly.
• Finally the activation record
  • From Cooper and Torczon
  • In intel, the frame pointer is register epb.
• GDB
  • Programs compiled with a -g (in gcc/g++) can be debugged with an interactive debugger.
  • gdb is this debugger.
  • gdb progname
  • some commands
    • b(reak) linenum will break on given linenum.
      • b functionName will work too.
    • run will run until the break point is hit or the program exits.
    • s will step one instruction
    • c will continue to the break point.
    • l(ist) linenu will list starting at that line number.
    • p varname will print that variable, but only after it has been defined.
      • p/x $regname will print the register value in hex.
      • epb is the frame pointer.
      • pc is the program counter.
      • p/x $pc
    • x address examine the value of a memory address.
    • display/#i $pc display # next instructions every execution