a
                        auth0                       auth3
    "A"*512 (2b)        stack smashing detected     stack smashing detected

        Both failures are because of __stack_chk_fail@plt:

           0x000000000040095d <checkpass+215>:  e8 8e fd ff ff  call   0x4006f0 <__stack_chk_fail@plt>

        and:

            0x000000000040092c <checkpass+140>: e8 7f fd ff ff  call   0x4006b0 <__stack_chk_fail@plt>

    Ret.addr (4b)       stack smashing detected     stack smashing detected

        Undoubtedly, this is due to something similar.

b
    $ cat /proc/sys/kernel/randomize_va_space
    2
    
    So, yes, full randomisation.

    It helps, because we cannot find the right address anymore using objdump.

c
    Yes, then also the standard library addresses are randomised.

d
    $ objdump -p auth0
    
    auth0:     file format elf64-x86-64
    
    Program Header:
        ...
       STACK off    0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**4
             filesz 0x0000000000000000 memsz 0x0000000000000000 flags rw-

    As you can see, it has only rw permissions, not x. This helps, because otherwise we could
    overwrite the stack with code, and overwrite the return address with a stack address to
    execute our own code.
    I don't see any advantage in this though, when stack protection is enabled (because then
    it is infeasible to overwrite the return address and pass the checks), but I might be wrong.