[vlc-devel] [PATCH] demux/asf: fix 17579: prevent signed integer overflow

[Rémi Denis-Courmont]

Le tiistaina 1. marraskuuta 2016, 2.04.06 EET Filip Roséen a écrit :
> The previous implementation could overflow the mtime_t when
> multiplying p_sys->p_fp->i_preroll by a thousand when converting from
> the asf time unit (milliseconds) to VLCs (microseconds).
> 
> Given that you can always divide a value without running into issues
> in terms of under/overflow, these changes prevent any overflow error
> while still preserving the same logic.
> 
> In short the implementation takes advantage of the below two
> conditions being equivalent:
> 
>    1: A > ( ( B * C ) + D )
>    2: ( ( A - D ) / C ) > B

That´s true with reals, not with integers. For instance,
with A=2, B=0, C=2, D=1:

(1) <=> 2 > ((0 * 2) + 1 <=> 2 > 1 <=> true
(2) <=> ((2 - 1) / 2) > 0 <=> (1 / 2) > 0 <=> 0 > 0 <=> false

(I don´t know if it affects the correctness of the diff.)

-- 
Rémi Denis-Courmont
http://www.remlab.net/