Re: [Qemu-devel] [PATCH 02/40] elf: Add notes implementation

[Stefan Weil <weil@mail.berlios.de>]

Am 01.11.2010 22:17, schrieb Alexander Graf:
>
> On 01.11.2010, at 16:19, Stefan Weil wrote:
>
>> Am 01.11.2010 20:51, schrieb Alexander Graf:
>>> On 01.11.2010, at 14:42, Stefan Weil wrote:
>>>
>>>
>>>> Am 01.11.2010 19:29, schrieb Blue Swirl:
>>>>
>>>>> On Mon, Nov 1, 2010 at 3:01 PM, Alexander Graf<agraf@suse.de> wrote:
>>>>>
>>>>>
>>>>>> ---
>>>>>> hw/elf_ops.h | 61 
>>>>>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>>>> hw/loader.c | 7 ++++++
>>>>>> hw/loader.h | 3 ++
>>>>>> 3 files changed, 70 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/hw/elf_ops.h b/hw/elf_ops.h
>>>>>> index 8b63dfc..645d058 100644
>>>>>> --- a/hw/elf_ops.h
>>>>>> +++ b/hw/elf_ops.h
>>>>>> @@ -189,6 +189,44 @@ static int glue(load_symbols, SZ)(struct 
>>>>>> elfhdr *ehdr, int fd, int must_swab,
>>>>>> return -1;
>>>>>> }
>>>>>>
>>>>>> +static void glue(elf_read_notes, SZ)(uint8_t *data, int data_len,
>>>>>> + ElfHandlers *handlers, int must_swab)
>>>>>> +{
>>>>>> + uint8_t *p = data;
>>>>>> +
>>>>>> + while ((ulong)&p[3]< (ulong)&data[data_len]) {
>>>>>>
>>>>>>
>>>>> Please use 'unsigned long'.
>>>>>
>>>>>
>>>> Why is a type cast used here? I see no reason for it.
>>>>
>>> Pointers can't be compared, you have to cast them to values first.
>>>
>>>
>>> Alex
>>>
>>
>> No. Pointers of same type which are not void pointers can be compared.
>>
>> There is even a data type ptrdiff_t, so you can also compare their
>> difference with zero.
>
> Let's ask someone who definitely knows :).
>
> Michael, is code like
>
> char *x = a, *y = b;
> if (x < y) {
> ...
> }
>
> valid? Or do I first have to cast x and y to unsigned longs or uintptr_t?
>
>
> Alex
>


Hopefully C did not change for code like this during the last
20 years.

Then your code is always valid, but will only return useful results
if both a and b are derived from the same base pointer
(plus an individual offset):

char *base = any_value;
a = base + offset_a;
b = base + offset_b;

Then
         x - y = a - b = (ptrdiff_t)(offset_a - offset_b) / (sizeof(*x);
and
         (x < y) == (a < b) == (offset_a < offset_b);

Regards,
Stefan