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

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Message-ID: <20150218193814.GB32179@roeck-us.net>
Date:	Wed, 18 Feb 2015 11:38:14 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Heinrich Schuchardt <xypron.glpk@....de>,
	"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
Subject: Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

On Tue, Feb 17, 2015 at 03:15:11PM -0800, 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.
> 
> >  #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);

Apparently it is possible that
	mempages < 8 * THREAD_SIZE
which would result in max_threads == 0.

How about the following ?

	max_threads = mempages / DIV_ROUND_UP(8 * THREAD_SIZE, PAGE_SIZE);

Guenter
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