[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH for-2.8 2/2] loader: fix undefined behavior in r
Re: [Qemu-devel] [PATCH for-2.8 2/2] loader: fix undefined behavior in rom_order_compare()
Tue, 29 Nov 2016 19:40:58 +0100
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.0
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
>>> 220.127.116.11 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:
>>> 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 =
>> 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".