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Date: Mon, 16 Feb 2015 19:45:38 +0100
From: Heinrich Schuchardt <xypron.glpk@gmx.de>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
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        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, Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH 1/1] kernel/fork.c: avoid division by zero
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I am resending this mail because the linux-kernel list was not CCed.

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@gmx.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.

Best regards

Heinrich