How are strings encoded in an ELF file?

Question

I wanted to demonstrate that passwords in clear are easy to read from a program:

#include <stdio.h>
#include <string.h>

int main(int argc, char** argv)
{
    char password[] = "a big refreshing lemonade";
    return strcmp(argv[1], password);
}

But it does not work as expected:

$ gcc foo.c
$ hexdump -C a.out | grep -C2 'lem'
000006c0  00 00 00 48 89 45 f8 31  c0 48 b8 61 20 62 69 67  |...H.E.1.H.a big|
000006d0  20 72 65 48 ba 66 72 65  73 68 69 6e 67 48 89 45  | reH.freshingH.E|
000006e0  d0 48 89 55 d8 48 b8 20  6c 65 6d 6f 6e 61 64 48  |.H.U.H. lemonadH|
000006f0  89 45 e0 66 c7 45 e8 65  00 48 8b 45 c0 48 83 c0  |.E.f.E.e.H.E.H..|
00000700  08 48 8b 00 48 8d 55 d0  48 89 d6 48 89 c7 e8 6d  |.H..H.U.H..H...m|

I notice some weird characters. Why is that?

Answer 1

It's because the strings aren't being stored as static data.

For example if you had this:

const char* password = "a big refreshing lemonade";

Or even this:

static char password[] = "a big refreshing lemonade";

It is stored contiguously in the binary (You see "a big refreshing lemonade" next to each other) in the constants section.

If you look at the assembly output, you see this:

 6:test.c        ****     char password[] = "a big refreshing lemonade";
23                            .loc 1 6 0
24 001e 48B86120              movabsq $7309940773697495137, %rax
24      62696720
24      7265
25 0028 48BA6672              movabsq $7453010330678293094, %rdx
25      65736869
25      6E67
26 0032 488945D0              movq    %rax, -48(%rbp)
27 0036 488955D8              movq    %rdx, -40(%rbp)
28 003a 48B8206C              movabsq $7233183901389515808, %rax
28      656D6F6E
28      6164
29 0044 488945E0              movq    %rax, -32(%rbp)
30 0048 66C745E8              movw    $101, -24(%rbp)
30      6500

Where you see a lot of movabsq, which loads a 64 bit constant. So, what it does load 8 bytes at a time into password.

You'll notice that the first constant (7309940773697495137) is the little-endian form of "a big re"