How to see the alignment of a struct in C?-CodePudding

I have an assignment where I need to log in to a simple program using a buffer overflow. The program takes two args with gets() and strcpy()'s them into v.username and v.password.

struct {
        int32_t goodcanary;
        char password[25];
        int32_t canary;
        char good_username[25];
        char good_password[25];
        char username[25];
    } v;

The canary is a 10 digit number which I know. Since a int32_t is only 4 bytes I converted the number to ascii and want to overflow password which is input2 and since canary comes after password in memory I thought I could put it at the end of input2 after it overflows. I can't get it working and I know that structs have padding but I don't know exactly how it works and where the padding is. Here is the memory locations of each member of the struct: screenshot

How can I see where the padding is and how do I know where to put the canary value?

Thank you.

CodePudding user response：

The rules are basically as follows:

On systems with alignment requirements, a struct must always be allocated starting at an even address. For this reason, a struct needs to end with trailing padding bytes so that an array of such structs doesn't get the next array item misaligned.
On a computer with 4 byte alignment requirement for integers, each member in your struct that has such a requirement must start at an address evenly divisible by 4.
Character arrays have no alignment requirement so they may get allocated at any address.

You can use offsetof (stddef.h) to print the byte position of any struct member. Example:

#include <stdio.h>
#include <stdint.h>
#include <stddef.h>

typedef struct 
{
    int32_t goodcanary;
    char password[25];
    int32_t canary;
    char good_username[25];
    char good_password[25];
    char username[25];
} v;

#define print_offset(x) printf("Offset %s: %zu\n", #x, offsetof(v,x))

int main (void)
{
  printf("Size of struct: %zu\n", sizeof(v));
  print_offset(goodcanary);
  print_offset(password);
  print_offset(canary);
  print_offset(good_username);
  print_offset(good_password);
  print_offset(username);
  return 0;
}

Output (x86_64 computer):

Size of struct: 112
Offset goodcanary: 0
Offset password: 4
Offset canary: 32
Offset good_username: 36
Offset good_password: 61
Offset username: 86

Since canary starts at 32, there must have been 32-25-4=3 padding bytes inserted. And they must have been inserted after password since goodcanary is 4 byte aligned with no trailing padding necessary.

Or if you will:

offsetof(v, canary) - sizeof((v){0}.password) - offsetof(v,password)