linux-kernel - Re: [PATCH 1/1] kernel/fork.c: avoid division by zero

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Date:	Mon, 16 Feb 2015 10:49:43 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Heinrich Schuchardt <xypron.glpk@....de>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Aaron Tomlin <atomlin@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kernel/fork.c: avoid division by zero

On 02/16/2015 10:45 AM, Heinrich Schuchardt wrote:
> I am resending this mail because the linux-kernel list was not CCed.
>
Yes, sorry, that got lost. The "forward" feature on lkml.org doesn't work
as well as it used to, unfortunately, and I keep forgetting.

> On 16.02.2015 19:23, Guenter Roeck wrote:
>> On Mon, Feb 16, 2015 at 07:00:44PM +0100, Heinrich Schuchardt wrote:
>>> PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
>>> THREAD_SIZE.
>>>
>>> E.g. architecture hexagon may have page size 1M and thread size 4096.
>>>
>>> This would lead to a division by zero.
>>>
>>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
>>> ---
>>>   kernel/fork.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index cf65139..71e2583 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -273,7 +273,8 @@ void __init fork_init(unsigned long mempages)
>>>   	 * value: the thread structures can take up at most half
>>>   	 * of memory.
>>>   	 */
>>> -	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
>>> +	max_threads = (u64) PAGE_SIZE * (u64) mempages
>>> +		    / (u64) THREAD_SIZE / 8UL;
>>
>> How about
>> 	max_threads = mempages / (8 * THREAD_SIZE) * PAGE_SIZE;
>>
>> instead ? That would avoid the u64.
>
> We have to consider the case mempages < 8 * THREAD_SIZE.
>
> Let
>    mempages = 1024
>    THREAD_SIZE = 4096
>    PAGE_SIZE = 1M
>
> Your code gives 0.
> Mine gives 8192.
>
Makes sense.

Thanks,
Guenter


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