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

[Alistair Francis]

On Mon, Nov 28, 2016 at 11:57 AM, Laszlo Ersek <address@hidden> 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>

Reviewed-by: Alistair Francis <address@hidden>

Thanks,

Alistair

> ---
>  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);
>  }
>
> --
> 2.9.2
>
>