linux-kernel - Re: Buggy variable-length array code...or compiler?

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Message-Id: <1267211701.3124.204.camel@iscandar.digidescorp.com>
Date:	Fri, 26 Feb 2010 13:15:01 -0600
From:	"Steven J. Magnani" <steve@...idescorp.com>
To:	Dan Carpenter <error27@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>, linux-kernel@...r.kernel.org,
	microblaze-uclinux@...e.uq.edu.au, dan.j.williams@...el.com,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Buggy variable-length array code...or compiler?

On Thu, 25 Feb 2010, Steven J. Magnani wrote:
> > 
> > > When I run a memcpy dmatest with a Microblaze 2.6.33 noMMU kernel, the
> > > system crashes after about 400 iterations. After much head scratching, I
> > > believe I've narrowed the problem to this fragment of code in
> > > drivers/dma/dmatest.c:
> > > 
> > > static int dmatest_func(void *data)
> > > {
> > > ...
> > >     int                     src_cnt;
> > >     int                     dst_cnt;
> > > ...
> > >     if (thread->type == DMA_MEMCPY)
> > >             src_cnt = dst_cnt = 1;
> > > ...
> > >     while (!kthread_should_stop()
> > >            && !(iterations && total_tests >= iterations)) {
> > > ...
> > >             dma_addr_t dma_srcs[src_cnt];
> > >             dma_addr_t dma_dsts[dst_cnt];
> > > ...

On Thu, Feb 25, 2010 at 03:46:48PM -0800, David Rientjes wrote:
> > This could probably become the first kernel user of the flexible array 
> > library (see Documentation/flexible-arrays.txt).  Dan?
> > --

On Fri, 2010-02-26 at 13:27 +0300, Dan Carpenter wrote:
> I think the max that src_cnt can be is 3 and the most dst_cnt can be is 2.  
> We could just put that there.

src_cnt is dependent on module parameters.

The bug goes away if dma_srcs and dma_dsts are declared outside the
loop, but that requires knocking the loop in another tabstop. At that
point dmatest_func() would be begging for refactoring.

Regards,
------------------------------------------------------------------------
 Steven J. Magnani               "I claim this network for MARS!
 www.digidescorp.com              Earthling, return my space modulator!"

 #include <standard.disclaimer>



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