linux-kernel - RE: [PATCH] char: random: casting prevents missing calculations

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Message-Id: <1494187086-4529-1-git-send-email-karim.eshapa@gmail.com>
Date:   Sun,  7 May 2017 21:58:06 +0200
From:   Karim Eshapa <karim.eshapa@...il.com>
To:     tytso@....edu
Cc:     arnd@...db.de, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, Karim Eshapa <karim.eshapa@...il.com>
Subject: RE: [PATCH] char: random: casting prevents missing calculations

On Sun, 7 May 2017 20:36:55 +0200, Arnd Bergmann wrote:
>On Sun, May 7, 2017 at 2:47 PM, Karim Eshapa <karim.eshapa@...il.com> wrote:
>> Cast (long)jiffies and (long)state->last_time beacause
>> they tends to unsigned long. may cause a problem specially
>> when comparison happens (< 0).
>>
>> Signed-off-by: Karim Eshapa <karim.eshapa@...il.com>
>>
>I don't understand what you are saying above, and the patch does not
>appear to have any effect since the destination variable is already of
>type 'long'.
>
>What problem did you observe?
>

I mean if jiffies = 0xf0000000 and 
state->last_time was = 0x70000000 then 

sample.jiffies = jiffies; here you assign signed long with
unsigned with last bit = 1
..
...
if (!state->dont_count_entropy) {
	delta = sample.jiffies - state->last_time;
	state->last_time = sample.jiffies;
	...
	...
	
	if (delta < 0) 
	so, here this condition will be
	true while it has to be false. 
}

So, I think that may cause a problem.

Thanks,
Karim