64 bit linux unaligned stack crash and printf params repetition.

[artur-augustyniak]

Hi,
AFAIK problem with xmm registers and doubling printf arguments affects only x86_64 linux (and OSX in extension). Tested with a lot of your code from youtube assembly course, seems to work well on 32/64 bit Windows, Linux nad OSX:
For linux stub i had to extend stub space in asmloader itself 0x400 instead 0x200.
Affects 86_64 linux stub:

  ; remove args passed via registers from stack to avoid printing
  ; them twice in case of more than 6 printf arguments
  ; no easy way to detect exact number of arguments so we have to save the stack
  ; to rebuild it in case of less then 6 params
  mov [rel saved_stack_top], rdi
  mov [rel saved_stack_nd], rsi
  mov [rel saved_stack_rd], rdx
  mov [rel saved_stack_fourth], rcx
  mov [rel saved_stack_fifth], r8
  mov [rel saved_stack_sixth], r9
  add rsp, 48`

Next problem was rax usage for printf float args count:

  ; When calling printf stub uses rax, so this register is nonzero.
  ; This causes printf to "see" floating point arguments and starts using SSE.
  ; At the same time if the stack does not have the proper 16 byte alignment asmloader will crash.
  ; After first [rbx + 3 * 8] call rax may be nonzero again
  ; so code like:
  ;
  ; [bits 64]
  ;
  ; call no_arg_procedure
  ;
  ; push 0
  ; call [rbx + 0 * 8]
  ;
  ; no_arg_procedure:                  ; no_arg_procedure()
  ;
  ;    call _print
  ;         db "abc", 0xa, 0
  ;     _print:
  ;     call [rbx + 3 * 8]
  ;     add rsp, 8
  ;
  ;     call _print2
  ;         db "cde", 0xa, 0
  ;         _print2:
  ;         call [rbx + 3 * 8]
  ;     add rsp, 8
  ;     ret
  ;  ;end
  ;
  ; will leave stack unaligned and nonzero rax, this will crash, so:
  xor rax, rax
  call r15`

We also need to restore stack add rsp, 48 can hurt some ret address in case less than 6 arguments passed to printf,:

  ; restore 6 stack elements
  push QWORD [rel saved_stack_sixth]
  push QWORD [rel saved_stack_fifth]
  push QWORD [rel saved_stack_fourth]
  push QWORD [rel saved_stack_rd]
  push QWORD [rel saved_stack_nd]
  push QWORD [rel saved_stack_top]`

Regards