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

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Date:	Tue, 17 Feb 2015 10:14:59 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Vladimir Davydov <vdavydov@...allels.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	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 17.02.2015 08:56, Vladimir Davydov wrote:
> On Mon, Feb 16, 2015 at 07:00:44PM +0100, Heinrich Schuchardt wrote:
>> 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;
> 
> This won't compile for a 32 bit arch, will it? Should you use div_u64?
> 
> Thanks,
> Vladimir
> 
Hello Vladimir,

I compiled the kernel successfully with the patch on a 32bit arm system
and Kernel 3.19.0.

Obviously that was good luck.
http://gcc.gnu.org/onlinedocs/gccint/Integer-library-routines.html
describes that library function __udivti3 might be used on other
platforms. And using library functions is not allowable in the kernel.

Thank you for reviewing. I will update the patch.

Best regards

Heinrich
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