Re: [PATCH 04/11] sio: fix passing negative size_t to sio_add_to_iov

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

On 03/12/2018 16:50, Vladimir Davydov wrote:
> On Fri, Nov 30, 2018 at 06:39:36PM +0300, Vladislav Shpilevoy wrote:
>> sio_add_to_iov moves struct iov position on a
>> specified offset, positive or negative. But its offset
>> argument has size_t type, which is unsigned. Make it
>> be ssize_t.
>>
>> This worked before thanks to how negative numbers are
>> stored. For example, consider
>>
>> uint8_t value = 100;
>> uint8_t offset = -5;
>>
>> Value is stored as  0110 0100.
>> Offset is stored as 1111 1011. (Yes, 1011, not 1010).
>>
>> Sum of the values above is 0001 0101 1111 - first quad
>> overflows and is truncated, so the result is
>> 0101 1111 = 95 - correct.
>> ---
>>   src/sio.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/sio.h b/src/sio.h
>> index ab0a243cd..ff383aa36 100644
>> --- a/src/sio.h
>> +++ b/src/sio.h
>> @@ -84,7 +84,7 @@ sio_move_iov(struct iovec *iov, size_t nwr, size_t *iov_len)
>>    * to adjust to a partial write.
>>    */
>>   static inline void
>> -sio_add_to_iov(struct iovec *iov, size_t size)
>> +sio_add_to_iov(struct iovec *iov, ssize_t size)
>>   {
>>   	iov->iov_len += size;
> 
> 'iov_len' has type size_t so 'size' will be converted to size_t before
> the operation, in other words this patch has, in fact, no effect.

It fixes corrupted logic - you should not accept negative numbers in
positive types. Even if they are the same internally.

> 
> Anyway, it's OK to apply unary minus to an unsigned variable: no matter
> how integer types are stored, whether the machine uses two's-complement
> or not, it should work so that (-x + x) equals 0.

I do not argue with it. But then why do we have signed types? Lets use
unsigned everywhere (<sarcasm>).

It does not matter does a compiler allow to turn a number into
the complement or not. Logic of storing negative numbers in
positive types is corrupted by definition.

> 
> That being said, I don't think we need this patch.
> 

As you wish. Removed in v2.