linux-kernel - Re: [PATCH] dma: fix up broken comparison in dma_alloc_from

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Date:	Wed, 28 Jan 2009 09:36:57 +0100 (CET)
From:	Guennadi Liakhovetski <lg@...x.de>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	adrian@...golddream.dyndns.info, lkmladrian@...il.com,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	penberg@...helsinki.fi, dbaryshkov@...il.com,
	penguin-kernel@...ove.sakura.ne.jp, hannes@...xchg.org
Subject: Re: [PATCH] dma: fix up broken comparison in dma_alloc_from_coherent

On Wed, 28 Jan 2009, Paul Mundt wrote:

> On Tue, Jan 27, 2009 at 01:48:31PM -0800, Andrew Morton wrote:
> > On Wed, 21 Jan 2009 17:11:19 +0900
> > Paul Mundt <lethal@...ux-sh.org> wrote:
> > 
> > > @@ -118,31 +118,32 @@ int dma_alloc_from_coherent(struct device *dev, ssize_t size,
> > >  	mem = dev->dma_mem;
> > >  	if (!mem)
> > >  		return 0;
> > > -	if (unlikely(size > mem->size))
> > > - 		return 0;
> > > +
> > > +	*ret = NULL;
> > > +
> > > +	if (unlikely(size > (mem->size << PAGE_SHIFT)))
> > > +		goto err;
> > 
> > Looks a bit broken on 64-bit.

Not related to the 64-bit dangers, but using bitmap_find_free_region() in 
dma_alloc_from_coherent() breaks in most non-spectacular ways again and 
again. This loop and test in bitmap_find_free_region()

	for (pos = 0; pos < bits; pos += (1 << order))
		if (__reg_op(bitmap, pos, order, REG_OP_ISFREE))
			break;
	if (pos == bits)
		return -ENOMEM;

can only return an error (-ENOMEM) if bits is a multiple of (1 << order), 
which is, for instance, true, if bits is (also) a power of 2. Which 
doesn't seem to be necessarily the case with dma_alloc_from_coherent(). 
Where shall this one be fixed - in bitmap or in DMA? The correct test in 
bitmap code seems to be

	if (pos + (1 << order) > bits)
		return -ENOMEM;

and I don't see a way to fix this in dma. Checking afterwards is too late 
- the current bitmap_find_free_region() will (with a bit of luck) quietly 
overwrite data beyond bits.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.

DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-0 Fax: +49-8142-66989-80  Email: office@...x.de
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