[PATCH xf86-input-synaptics 2/2] Ensure history delta computations are signed

[Chase Douglas]

On 02/08/2012 11:30 PM, Peter Hutterer wrote:
> On Wed, Feb 08, 2012 at 09:10:51PM +0100, Chase Douglas wrote:
>> On 02/08/2012 03:54 PM, Peter Hutterer wrote:
>>> On Tue, Feb 07, 2012 at 01:07:08PM -0800, Chase Douglas wrote:
>>>> Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
>>>> ---
>>>>  src/synaptics.c |    2 +-
>>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/src/synaptics.c b/src/synaptics.c
>>>> index a4c1e5a..106b5ee 100644
>>>> --- a/src/synaptics.c
>>>> +++ b/src/synaptics.c
>>>> @@ -1791,7 +1791,7 @@ HandleTapProcessing(SynapticsPrivate *priv, struct SynapticsHwState *hw,
>>>>  }
>>>>  
>>>>  #define HIST(a) (priv->move_hist[((priv->hist_index - (a) + SYNAPTICS_MOVE_HISTORY) % SYNAPTICS_MOVE_HISTORY)])
>>>> -#define HIST_DELTA(a, b, e) ((HIST((a)).e) - (HIST((b)).e))
>>>> +#define HIST_DELTA(a, b, e) ((int)((HIST((a)).e) - (HIST((b)).e)))
>>>
>>> this only casts the result to a signed int, the actual computation is still
>>> undefined for b > a. With the revert, HIST_DELTA is only used in one place
>>> so we could either do it there or just write a helper function like this
>>
>> Why is it undefined? We can have a negative time difference, which is
>> what the calculation was assuming, IIUC.
> 
> yeah, but IIRC (unsigned int)a - (unsigned int)b is undefined for b > a.

I thought in C that was still defined. I think it's used in many places,
including TCP timestamp wrap-around in the kernel. Maybe what I'm
remembering is slightly different? I must admit I don't know where or
how to find a definitive answer though.

-- Chase