linux-kernel - Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

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Date:	Wed, 18 Feb 2015 20:47:48 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

Hello Andrew,

thank you for your comments. Unfortunately there is no solution with
32-bit calculus. Please, see my answers below.

As fork_init is only called once there should be not performance issue
in using 64-bit calculus.

I think that my patch did not cover all problems connected to max_threads.

I just had a look at the memory hotplugging code.
Shouldn't max_threads and init_task.signal->rlim[RLIMIT_NPROC] be
recalculated after adding or removing memory?
This could be done in a hotplug callback.

max_threads can be set by writing to /proc/sys/kernel/threads-max.
Shouldn't the value be checked by the same routine and shouldn't
init_task.signal->rlim[RLIMIT_NPROC] be updated?

Best regards

Heinrich

On 18.02.2015 00:15, Andrew Morton wrote:
> On Tue, 17 Feb 2015 20:01:38 +0100 Heinrich Schuchardt <xypron.glpk@....de> 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.
>>
>> The futex implementation assumes that tids fit into the FUTEX_TID_MASK.
>> This limits the number of allowable threads.
>>
>> ...
>>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -74,6 +74,7 @@
>>  #include <linux/uprobes.h>
>>  #include <linux/aio.h>
>>  #include <linux/compiler.h>
>> +#include <linux/math64.h>
>>  
>>  #include <asm/pgtable.h>
>>  #include <asm/pgalloc.h>
>> @@ -255,6 +256,8 @@ void __init __weak arch_task_cache_init(void) { }
>>  
>>  void __init fork_init(unsigned long mempages)
>>  {
>> +	u64 temp;
> 
> That's a really poor name.  We should always try to make names
> meaningful.  Here, something like "threads" would be better.

ok.

> 
>>  #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
>>  #ifndef ARCH_MIN_TASKALIGN
>>  #define ARCH_MIN_TASKALIGN	L1_CACHE_BYTES
>> @@ -273,7 +276,16 @@ 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);
>> +	temp = div64_u64((u64) mempages * (u64) PAGE_SIZE,
>> +			 (u64) THREAD_SIZE * 8UL);
>> +
>> +	/*
>> +	 * The futex code assumes that tids fit into the FUTEX_TID_MASK.
>> +	 */
>> +	if (temp < FUTEX_TID_MASK)
>> +		max_threads = temp;
>> +	else
>> +		max_threads = FUTEX_TID_MASK;
> 
> Seems rather complicated.  How about
> 
> 	max_threads = mempages / (8 * THREAD_SIZE);

If 8 * THREAD_SIZE > mempages this gives 0.

> 	max_threads *= PAGE_SIZE;

If mempages / (8 * THREAD_SIZE) * PAGE_SIZE > INT_MAX
an overflow occurs (e.g. total memory = 96TB,  THREAD_SIZE = 4kB).

> 	max_threads = min(max_threads, FUTEX_TID_MASK);
> 
> And while we're there, I do think the comments need a refresh.  What
> does "the thread structures can take up at most half of memory" mean? 
> And what's the reasoning behind that "8"?  I suggest we just delete all
> that and make a new attempt at explaining why the code is this way.

ok.

> 
> 


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