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Date: Tue, 17 Feb 2015 10:14:59 +0100
From: Heinrich Schuchardt <xypron.glpk@gmx.de>
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To: Vladimir Davydov <vdavydov@parallels.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        David Rientjes <rientjes@google.com>,
        Kees Cook <keescook@chromium.org>, Aaron Tomlin <atomlin@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] kernel/fork.c: avoid division by zero
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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