gdb can't access memory address error

Question

here is my disas code:

   0x0804844d <+0>:     push   %ebp    0x0804844e <+1>:     mov    %esp,%ebp    0x08048450 <+3>:     and    $0xfffffff0,%esp    0x08048453 <+6>:     sub    $0x20,%esp    0x08048456 <+9>:     movl   $0x8048540,(%esp)    0x0804845d <+16>:    call   0x8048310 <puts@plt>    0x08048462 <+21>:    lea    0x1c(%esp),%eax    0x08048466 <+25>:    mov    %eax,0x4(%esp)    0x0804846a <+29>:    movl   $0x8048555,(%esp)    0x08048471 <+36>:    call   0x8048320 <scanf@plt>    0x08048476 <+41>:    mov    0x1c(%esp),%eax    0x0804847a <+45>:    cmp    $0x208c,%eax    0x0804847f <+50>:    jne    0x804848f <main+66>    0x08048481 <+52>:    movl   $0x8048558,(%esp)    0x08048488 <+59>:    call   0x8048310 <puts@plt>    0x0804848d <+64>:    jmp    0x804849b <main+78> => 0x0804848f <+66>:    movl   $0x8048569,(%esp)    0x08048496 <+73>:    call   0x8048310 <puts@plt>    0x0804849b <+78>:    mov    $0x0,%eax    0x080484a0 <+83>:    leave      0x080484a1 <+84>:    ret  

what i'm tring to examine is $0x208c. When I type x/xw 0x208c it gives me back error which says Cannot access memory at address 0x208c. When i type Info registers and look at eax it says the value which i provided. So basically this program compares two values and depending on that prints something out.The problem is that this is homework from university and I have not got code. Hope you can help. Thank you.

Answer 1

When I type x/xw 0x208c it gives me back error which says Cannot access memory at address 0x208c

The disassembly for your program says that it does something like this:

puts("some string"); int i; scanf("%d", &i);  // I don't know what the actual format string is.                   // You can find out with x/s 0x8048555 if (i == 0x208c) { ... } else { ... } 

In other words, the 0x208c is a value (8332) that your program has hard-coded in it, and is not a pointer. Therefore, GDB is entirely correct in telling you that if you interpret 0x208c as a pointer, that pointer does not point to readable memory.

i finally figured out to use print statement instead of x/xw

You appear to not understand the difference between print and examine commands. Consider this example:

int foo = 42; int *pfoo = &foo; 

With above, print pfoo will give you the address of foo, and x pfoo will give you the value stored at that address (i.e. the value of foo).

Answer 2

I found out that it is impossible to examine mmaped memory that does not have PROT_READ flag. This is not the OPs problem, but it was mine, and the error message is the same.

Instead of

mmap(0, size, PROT_WRITE | PROT_EXEC, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0); 

do

mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0); 

and voila, the memory can be examined.