Re: Format String Attacks

[Matthias Meixner <meixner () RBG INFORMATIK TU-DARMSTADT DE>]

Ajax wrote:
>
[...]
> /* init AP to the next arg we pop from the stack */
> #define va_start(AP, LASTARG)                                           \
>  (AP = ((__gnuc_va_list) __builtin_next_arg (LASTARG)))
>
> /* advance the AP pointer and return the next arg */
> #define va_arg(AP, TYPE)                                                \
>  (AP = (__gnuc_va_list) ((char *) (AP) + __va_rounded_size (TYPE)),     \
>   *((TYPE *) (void *) ((char *) (AP) - __va_rounded_size (TYPE))))
>
> Note how this works; AP is treated as, essentially, void *AP[], an array
> of pointers to arbitrary types.  This creates a natural terminating
> condition, where the last element in the array is NULL (_not_ a pointer to
> NULL).


Wrong. AP is not an array of pointers pointing to the arguments, but a pointer
to the beginning of the arguments on the stack. va_arg is shifting this
pointer further by the size of the object on the stack
   [(AP = (__gnuc_va_list) ((char *) (AP) + __va_rounded_size (TYPE))]
each time an argument is read.

So there is no pointer array, that could be terminated by a NULL-pointer.

- Matthias Meixner

--
Matthias Meixner                   meixner () rbg informatik tu-darmstadt de
Technische Universität Darmstadt
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