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

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Message-Id: <20150217151511.95143a25ff83c165f4199d08@linux-foundation.org>
Date:	Tue, 17 Feb 2015 15:15:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Heinrich Schuchardt <xypron.glpk@....de>
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

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.

>  #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);
	max_threads *= PAGE_SIZE;
	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.

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