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

[Laszlo Ersek]

On 11/29/16 17:29, Michael S. Tsirkin wrote:
> 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.

Just to comply with the word of the standard above; it says "any valid
pointer to void".

> 
>> -- 
>> 2.9.2