Re: [Qemu-devel] [PATCH for-2.8 2/2] loader: fix undefined behavior in rom_order_compare()

[Michael S. Tsirkin]

On Mon, Nov 28, 2016 at 08:57:01PM +0100, Laszlo Ersek wrote:
> According to ISO C99 / N1256 (referenced in HACKING):
> 
> > 6.5.8 Relational operators
> >
> > 4 For the purposes of these operators, a pointer to an object that is
> >   not an element of an array behaves the same as a pointer to the first
> >   element of an array of length one with the type of the object as its
> >   element type.
> >
> > 5 When two pointers are compared, the result depends on the relative
> >   locations in the address space of the objects pointed to. If two
> >   pointers to object or incomplete types both point to the same object,
> >   or both point one past the last element of the same array object, they
> >   compare equal. If the objects pointed to are members of the same
> >   aggregate object, pointers to structure members declared later compare
> >   greater than pointers to members declared earlier in the structure,
> >   and pointers to array elements with larger subscript values compare
> >   greater than pointers to elements of the same array with lower
> >   subscript values. All pointers to members of the same union object
> >   compare equal. If the expression /P/ points to an element of an array
> >   object and the expression /Q/ points to the last element of the same
> >   array object, the pointer expression /Q+1/ compares greater than /P/.
> >   In all other cases, the behavior is undefined.
> 
> Our AddressSpace objects are allocated generally individually, and kept in
> the "address_spaces" linked list, so we mustn't compare their addresses
> with relops.
> 
> Convert the pointers subjected to the relop in rom_order_compare() to
> "uintptr_t":
> 
> > 7.18.1.4 Integer types capable of holding object pointers
> >
> > 1 [...]
> >
> >   The following type designates an unsigned integer type with the
> >   property that any valid pointer to void can be converted to this type,
> >   then converted back to pointer to void, and the result will compare
> >   equal to the original pointer:
> >
> >   /uintptr_t/
> >
> >   These types are optional.
> 
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Alistair Francis <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Cc: Peter Maydell <address@hidden>
> Cc: address@hidden
> Fixes: 3e76099aacb4dae0d37ebf95305369e03d1491e6
> Signed-off-by: Laszlo Ersek <address@hidden>
> ---
>  hw/core/loader.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/core/loader.c b/hw/core/loader.c
> index c0d645a87134..766e48f2aec2 100644
> --- a/hw/core/loader.c
> +++ b/hw/core/loader.c
> @@ -818,7 +818,7 @@ static QTAILQ_HEAD(, Rom) roms = 
> QTAILQ_HEAD_INITIALIZER(roms);
>  
>  static inline bool rom_order_compare(Rom *rom, Rom *item)
>  {
> -    return (rom->as > item->as) ||
> +    return ((uintptr_t)(void*)rom->as > (uintptr_t)(void*)item->as) ||
>             (rom->as == item->as && rom->addr >= item->addr);
>  }

Can't hurt but why cast to void *?
Should not be needed.

> -- 
> 2.9.2