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

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Date:	Fri, 26 Feb 2010 13:27:45 +0300
From:	Dan Carpenter <error27@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	"Steven J. Magnani" <steve@...idescorp.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, Feb 25, 2010 at 03:46:48PM -0800, David Rientjes wrote:
> 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)
> > {
> > 	struct dmatest_thread	*thread = data;
> > ...
> > 	unsigned int		total_tests = 0;
> > 	int			src_cnt;
> > 	int			dst_cnt;
> > 
> > ...
> > 	if (thread->type == DMA_MEMCPY)
> > 		src_cnt = dst_cnt = 1;
> > ...
> > 
> > 	while (!kthread_should_stop()
> > 	       && !(iterations && total_tests >= iterations)) {
> > 		struct dma_device *dev = chan->device;
> > 		struct dma_async_tx_descriptor *tx = NULL;
> > 		dma_addr_t dma_srcs[src_cnt];
> > 		dma_addr_t dma_dsts[dst_cnt];
> > 
> > 		...
> > 		total_tests++;
> > 
> > 		/* CODE ADDED BY ME FOR DEBUG */
> > 		printk("dmatest: Iteration %d, dma_srcs = %p\n",
> > 		       total_tests, dma_srcs);
> > 
> > 		...
> > 	}
> > 
> > With this code I get output like this:
> > 
> > dmatest: Iteration 1, dma_srcs = 2c963ee8
> > dmatest: Iteration 2, dma_srcs = 2c963ed8
> > dmatest: Iteration 3, dma_srcs = 2c963ec8
> > dmatest: Iteration 4, dma_srcs = 2c963eb8
> > ...
> > dmatest: Iteration 420, dma_srcs = 2c9624b8
> > 
> > ...and then the stack detonates and the kernel crashes with some strange
> > error or other.
> > 
> 
> This could probably become the first kernel user of the flexible array 
> library (see Documentation/flexible-arrays.txt).  Dan?
> --

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.

regards,
dan carpenter

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