COA1 - Debugger example

This file is a walk-through of locating and fixing a bug using lldb and ghex (a hex editor) to fix cmdadd. You may find it useful to follow along on your own.

The cmdadd program is supposed to add all of its command-line arguments, so that ./cmdadd 2 3 5 8 should display Sum: 18. We start by verifying it does not work: it displays Sum: 36 instead. So we need to debug it.

We start by invoking lldb cmdadd

We see lldb tell us it loaded correctly:

 (lldb) target create "cmdadd"
             Current executable set to 'cmdadd' (x86_64).

Let’s try the same invocation we had earlier: run 2 3 5 8

lldb gives us the same output running cmdadd directly did:
```
 Process 10873 launched: '/home/mst3k/cmdadd' (x86_64)
             Sum: 36
             Process 10873 exited with status = 0 (0x00000000) 
```
The two “process” lines tell us the program started and ended; the “exit status” is the number returned by the main function, where 0 is a program’s way of saying “everything went OK”

Let’s look at the disassembled binary.

We start by trying just di -f but it does not work. We can type di sdfsdgfdsfgs or some other gibberish to get lldb to list di options, or we can look in the table above to find other uses.

We know every program starts in main, so we try di -n main. This shows us some assembly; we look in particular for callq to other functions, as main rarely does all the work itself. There are three such lines:

 0x5555555551f8 <+72>:  callq  0x1040                    ; symbol stub for: atoll
             0x555555555204 <+84>:  callq  0x1150                    ; add
             0x555555555228 <+120>: callq  0x1030                    ; symbol stub for: printf

lldb gives us comments after the ;. The “symbol stub”s are to built-in functions, which you can learn more about by opening a new terminal window and typing man 3 atoll or man 3 printf. However, since these are built-in they are not likely to be the problem.

Let’s look at the other function, add. We type di -n add and see the following:

 cmdadd`add:
             0x555555555150 <+0>:  pushq  %rbp
             0x555555555151 <+1>:  movq   %rsp, %rbp
             0x555555555154 <+4>:  subq   $0x20, %rsp
             0x555555555158 <+8>:  movq   %rdi, -0x10(%rbp)
             0x55555555515c <+12>: movq   %rsi, -0x18(%rbp)
             0x555555555160 <+16>: cmpq   $0x0, -0x18(%rbp)
             0x555555555165 <+21>: jne    0x1178                    ; <+40>
             0x55555555516b <+27>: movq   -0x10(%rbp), %rax
             0x55555555516f <+31>: movq   %rax, -0x8(%rbp)
             0x555555555173 <+35>: jmp    0x1197                    ; <+71>
             0x555555555178 <+40>: movq   -0x10(%rbp), %rax
             0x55555555517c <+44>: addq   $0x2, %rax
             0x555555555180 <+48>: movq   -0x18(%rbp), %rcx
             0x555555555184 <+52>: subq   $0x1, %rcx
             0x555555555188 <+56>: movq   %rax, %rdi
             0x55555555518b <+59>: movq   %rcx, %rsi
             0x55555555518e <+62>: callq  0x1150                    ; <+0>
             0x555555555193 <+67>: movq   %rax, -0x8(%rbp)
             0x555555555197 <+71>: movq   -0x8(%rbp), %rax
             0x55555555519b <+75>: addq   $0x20, %rsp
             0x55555555519f <+79>: popq   %rbp
             0x5555555551a0 <+80>: retq

We could try to understand what this code is doing, but it can be easier to see it in action. We start by adding a break point: br set -n add

 Breakpoint 1: where = cmdadd`add, address = 0x0000555555555150

We then run 2 3 5 8 to reach that breakpoint

 Process 10990 launched: '/home/mst3k/cmdadd' (x86_64)
             Process 10990 stopped
             * thread #1, name = 'cmdadd', stop reason = breakpoint 1.1
                 frame #0: 0x0000555555555150 cmdadd`add
             cmdadd`add:
             ->  0x555555555150 <+0>: pushq  %rbp
                 0x555555555151 <+1>: movq   %rsp, %rbp
                 0x555555555154 <+4>: subq   $0x20, %rsp
                 0x555555555158 <+8>: movq   %rdi, -0x10(%rbp)

Notice llbd disassembles the local context for us.

Let’s see what else is in scope. We know which registers are used to pass arguments, so let’s list them with reg read rdi rsi rdx rcx r8 r9
```
  rdi = 0x0000000000000000
              rsi = 0x0000000000000002
              rdx = 0x0000000000000000
              rcx = 0x1999999999999999
               r8 = 0x00007fffffffe2d2
               r9 = 0x0000000000000000
```
So we can see that add was invoked as add() or add(0) or add(0, 2) or add(0, 2, 0) or add(0, 2, 0, 0x1999999999999999) or … We’ll be able to figure out which one by seeing which registers are read before being set inside the program.

Let’s move step-by-step.

Each time we type step it executes the instruction previously pointed to and shows us the next several instructions on deck.

 (lldb) step
             Process 10990 stopped
             * thread #1, name = 'cmdadd', stop reason = instruction step into
                 frame #0: 0x0000555555555151 cmdadd`add + 1
             cmdadd`add:
             ->  0x555555555151 <+1>:  movq   %rsp, %rbp
                 0x555555555154 <+4>:  subq   $0x20, %rsp
                 0x555555555158 <+8>:  movq   %rdi, -0x10(%rbp)
                 0x55555555515c <+12>: movq   %rsi, -0x18(%rbp)

If we type step several more times, we eventually reach

 (lldb) step
             Process 10990 stopped
             * thread #1, name = 'cmdadd', stop reason = instruction step into
                 frame #0: 0x0000555555555160 cmdadd`add + 16
             cmdadd`add:
             ->  0x555555555160 <+16>: cmpq   $0x0, -0x18(%rbp)
                 0x555555555165 <+21>: jne    0x555555555178            ; <+40>
                 0x55555555516b <+27>: movq   -0x10(%rbp), %rax
                 0x55555555516f <+31>: movq   %rax, -0x8(%rbp)

Up to this point we’ve moves %rsp, copied it into %rbp, and moved two registers into that memory: %rdi into -0x10(%rdp) and %rsi into -0x18(%rdp). That implies we have a two-argument function (just rdi and rsi were accessed) and this function was invoked as add(0, 2).

We’re now about to run a cmpq and a jne; that is, to compare to values and jump if they are not equal.

if (value1 != value2) goto somewhere

One of the values is 0; the others is -0x18(%rbp). Just for practice, let’s see what is in -0x18(%rbp). That’s a memory address, so we need a memory read; it’s being compared with cmpq so we need -s 8 and -c 1; but what is the address? to figure that out we find our what’s in %rbp

(lldb) register read rbp
                 rbp = 0x00007fffffffde40

and add -0x18 to it to get 0x7fffffffde28, then use this in a memory read:

(lldb) me rea -s8 -c1 -fx 0x7fffffffde28
            0x7fffffffde28: 0x0000000000000002

(note: we could also have done me rea -s8 -c1 -fx 0x00007fffffffde40-0x18 to get the same result)

So we are comparing 2 to 0 and jumping if not equal, so we expect to jump. Let’s verify this:

(lldb) step
            Process 11266 stopped
            * thread #1, name = 'cmdadd', stop reason = instruction step into
                frame #0: 0x0000555555555165 cmdadd`add + 21
            cmdadd`add:
            ->  0x555555555165 <+21>: jne    0x555555555178            ; <+40>
                0x55555555516b <+27>: movq   -0x10(%rbp), %rax
                0x55555555516f <+31>: movq   %rax, -0x8(%rbp)
                0x555555555173 <+35>: jmp    0x555555555197            ; <+71>
            (lldb) step
            Process 11266 stopped
            * thread #1, name = 'cmdadd', stop reason = instruction step into
                frame #0: 0x0000555555555178 cmdadd`add + 40
            cmdadd`add:
            ->  0x555555555178 <+40>: movq   -0x10(%rbp), %rax
                0x55555555517c <+44>: addq   $0x2, %rax
                0x555555555180 <+48>: movq   -0x18(%rbp), %rcx
                0x555555555184 <+52>: subq   $0x1, %rcx

stepping some more we see that we are
1. loading the first argument in rax
2. adding 2 to it
3. loading the second argument into rcx
4. subtracting 1 from it
5. loading rax into rdi, the 1^st argument spot for a call
6. loading rax into rsi, the 2^nd argument spot for a call
7. calling <+0>: that is, this very function, add.
So we have a recursive function; add(0, 2) invoked add; let’s see what it’s arguments will be:
```
(lldb) reg read rdi rsi
                 rdi = 0x0000000000000002
                 rsi = 0x0000000000000001
```

We’re in a recursive function, and have a breakpoint at its beginning, so we can repeatedly type continue and register read rdi rsi to see what arguments it has each time it is invoked.

Tracking these, we see (removing some messages for brevity)

(lldb) continue
            (lldb) reg read rdi rsi
                 rdi = 0x0000000000000004
                 rsi = 0x0000000000000000
            (lldb) continue
            (lldb) reg read rdi rsi
                 rdi = 0x0000000000000004
                 rsi = 0x0000000000000003
            (lldb) continue
            (lldb) reg read rdi rsi
                 rdi = 0x0000000000000006
                 rsi = 0x0000000000000002
            (lldb) continue
            (lldb) reg read rdi rsi
                 rdi = 0x0000000000000008
                 rsi = 0x0000000000000001
            (lldb) continue
            (lldb) reg read rdi rsi
                 rdi = 0x000000000000000a
                 rsi = 0x0000000000000000

            ...

Combining this with what we’ve already seen, we have the following sequence of calls:

add(0, 2)
add(2, 1)
add(4, 0)
add(4, 3)
add(6, 2)
add(8, 1)
add(10, 0)
add(10, 5)
add(12, 4)
add(14, 3)
add(16, 2)
add(18, 1)
add(20, 0)
add(20, 8)
add(22, 7)
add(24, 6)
add(26, 5)
add(28, 4)
add(30, 3)
add(32, 2)
add(34, 1)
add(36, 0)

Looking at the above, I notice we seem to be counting down each of the arguments, and counting up by twos. If all of the counting up steps were by 1 instead, we’d be OK. So I need to figure out where the down-by-2 is in the binary: di -n add -b includes this line
```
0x55555555517c <+44>: 48 83 c0 02        addq   $0x2, %rax
```
Let’s fix that. I wanted 01 not 02. So I open up a hex editor, such as ghex cmdadd In that, I find the instruction I don’t want by
1. Ctrl+F to open the find dialog
2. Type 48 83 c0 02
3. Find Next
4. Verify that there is only one instruction with this encoding by pressing “Find Next” again. If there were more than one, we’d pick a larger string, perhaps including the bytes of the movq on either side.
  
  Note this is not at byte 0x55555555517c of the executable. The loader relocates these bytes when we run the program.
5. Edit the incorrect byte (changing the 02 to 01) and save the file
Test the fix: ./cmdadd 2 3 5 8 should now display Sum: 18.