Recitation 2: Assembly & gdb Andrew Faulring Section A 16 September 2002

Andrew Faulring Office hours: –NSH 2504 (lab) / 2507 (conference room) –Normally Thursday 5–6 –THIS WEEK: Wednesday 5–6

Today’s Plan Preparing for Lab2 –due Thursday, 26 11:59PM Assembly programming C to ASM –Using gdb ASM to C –Reverse engineering (like in Lab2)

Machine Model EIPEIP Registers CPU Memory Object Code Program Data Addresses Data Instructions Stack Condition Codes

Special Registers %eaxReturn Value %eipInstruction Pointer %ebpBase (Stack Frame) Pointer %espStack Pointer

Simple Addressing Modes $1010 (R)Mem[R] $10(R)Mem[R + 10] $0x10(R)Mem[R + 16]

Indexed Addressing Modes Generic Form D(Rb, Ri, S)Mem[Reg[Rb]+S*Reg[Ri]+D] Examples (Rb,Ri)Mem[Reg[Rb]+Reg[Ri]] D(Rb,Ri)Mem[Reg[Rb]+Reg[Ri]+D] (Rb,Ri,S)Mem[Reg[Rb]+S*Reg[Ri]]

Example 1: Arithmetic int func1(int a, int b) { int x, y; x = a + b; y = 2*x - b; return x*y; }

func1 assembly func1: push %ebp # save frame pointer mov %esp,%ebp # frame ptr = stack ptr mov 0xc(%ebp),%eax # %eax = b mov 0x8(%ebp),%ecx # %ecx = a add %eax,%ecx # %ecx = x = a + b lea (%ecx,%ecx,1),%edx # %edx = 2 * x sub %eax,%edx # %edx = y = 2 * x - b mov %ecx,%eax # %eax = x imul %edx,%eax # %eax = x * y mov %ebp,%esp # restore stack pointer pop %ebp # restore frame pointer ret

gdb GNU debugger Your friend for Lab2 Usage –gdb

gdb commands run : starts the program, can include command line arguments disas : disassembles code into asm print : used to print values of variables & memory x : examine memory contents step, next : step through code break : set breakpoints in the code

Using gdb with func1 break func1 run disas where print/x $eax print/x $ecx

Example 2: Control int func2(int a, int b) { if(a>b) return a; else return b; }

ASM for func2 func2: push %ebp # save frame pointer mov %esp,%ebp # frame ptr = stack ptr mov 0x8(%ebp),%edx # %edx = a mov 0xc(%ebp),%eax # %eax = b cmp %eax,%edx # a > b jle.L1 # a <= b mov %edx,%eax # return a.L1: # otherwise %eax = b mov %ebp,%esp # restore stack pointer pop %ebp # restore frame pointer ret

Example 3: int func3(int a, int b) { int r = 0xDEADBEEF; switch(a) { case 0: r = a; break; case 1: r = b; break; case 2: r = a+b; break; case 3: r = a-b; break; case 4: r = a*b; break; } return r; }

ASM for func3 # $edx = a, $ecx = b, $eax = 0xdeadbeef 0x : mov 0x8(%ebp),%edx 0x : mov 0xc(%ebp),%ecx 0x : mov $0xdeadbeef,%eax # go to default case, if a > 4 0x804845e : cmp $0x4,%edx 0x : ja 0x804848b # execute the jump … 0x : jmp *0x (,%edx,4) 0x804846a : lea 0x0(%esi),%esi

ASM for func3 case 0: return a 0x : mov %edx,%eax 0x : jmp 0x804848b case 1: return b 0x : mov %ecx,%eax 0x : jmp 0x804848b 0x : lea (%ecx,%edx,1),%eax case 2: return a+b 0x : lea (%ecx,%edx,1),%eax 0x804847b : jmp 0x804848b 0x804847d : lea 0x0(%esi),%esi

ASM for func3 case 3: a-b 0x : mov %edx,%eax 0x : sub %ecx,%eax 0x : jmp 0x804848b case 4: a*b 0x : mov %edx,%eax 0x : imul %ecx,%eax

Addresses of the cases case 0: 0x case 1: 0x case 2: 0x case 3: 0x case 4: 0x

The Jump Table 0x : jmp *0x (,%edx,4) (gdb) x/5xw 0x x : 0x x x x x : 0x %edx = a Jump to instruction with address MEM[0x a*4]

Example 4: asm => c Dump of assembler code for function func4: 0x80483c0 : push %ebp 0x80483c1 : mov %esp,%ebp 0x80483c3 : mov 0x8(%ebp),%ecx 0x80483c6 : xor %eax,%eax 0x80483c8 : xor %edx,%edx 0x80483ca : cmp %ecx,%eax 0x80483cc : jge 0x80483d7 0x80483ce : mov %esi,%esi 0x80483d0 : add %edx,%eax 0x80483d2 : inc %edx 0x80483d3 : cmp %ecx,%edx 0x80483d5 : jl 0x80483d0 0x80483d7 : mov %ebp,%esp 0x80483d9 : pop %ebp 0x80483da : ret 0x80483db : nop End of assembler dump.

Examining func4 : pushl %ebp # save frame pointer movl %esp,%ebp # frame ptr = stack ptr movl 0x8(%ebp),%ecx # put arg1 into %ecx xorl %eax,%eax # zero %eax xorl %edx,%edx # zero %edx

Examining func4 cmpl %ecx,%eax # compare arg1 (%ecx) and %eax jge.L4 # jump to.L4 if arg1 <= %eax (0).L6: addl %edx,%eax # %eax = %eax + %edx incl %edx # %edx = %edx + 1 cmpl %ecx,%edx # compare arg1 (%ecx) and %edx jl.L6 # jump to.L06 if %edx < arg1.L4: movl %ebp,%esp # restore stack pointer popl %ebp # restore frame pointer ret

Name the variables %ecx: x (first argument) %eax: result %edx: i

func4 int func4(int x) // %ecx = x { int result = 0; // %eax = result int i; // %edx = i for (i = 0; i < x; i++) result += i; return result; }