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Date:   Thu, 29 Dec 2016 03:49:06 +0200
From:   Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Mugunthan V N <mugunthanvnm@...com>,
        Sekhar Nori <nsekhar@...com>, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org
Subject: Re: [PATCH] net: ethernet: ti: davinci_cpdma: fix access to
 uninitialized variable in cpdma_chan_set_descs()

On Wed, Dec 28, 2016 at 05:42:13PM -0600, Grygorii Strashko wrote:
Grygorii,

> Now below code sequence causes "Unable to handle kernel NULL pointer
> dereference.." exception and system crash during CPSW CPDMA initialization:
> 
> cpsw_probe
> |-cpdma_chan_create (TX channel)
>   |-cpdma_chan_split_pool
>     |-cpdma_chan_set_descs(for TX channels)
>     |-cpdma_chan_set_descs(for RX channels) [1]
> 
> - and -
> static void cpdma_chan_set_descs(struct cpdma_ctlr *ctlr,
> 				 int rx, int desc_num,
> 				 int per_ch_desc)
> {
> 	struct cpdma_chan *chan, *most_chan = NULL;
> 
> ...
> 
> 	for (i = min; i < max; i++) {
> 		chan = ctlr->channels[i];
> 		if (!chan)
> 			continue;
> ...
> 
> 		if (most_dnum < chan->desc_num) {
> 			most_dnum = chan->desc_num;
> 			most_chan = chan;
> 		}
> 	}
> 	/* use remains */
> 	most_chan->desc_num += desc_cnt; [2]
> }
> 
> So, most_chan value will never be reassigned when cpdma_chan_set_descs() is
> called second time [1], because there are no RX channels yet and system
> will crash at [2].

How did you get this?
I just remember as I fixed it before sending patchset.

Maybe it was some experiment with it.
I just wonder and want to find actual reason what's happening.

Look bellow:

cpsw_probe
|-cpdma_chan_create (TX channel)
  |-cpdma_chan_split_pool
    |-cpdma_chan_set_descs(for TX channels)
    |-cpdma_chan_set_descs(for RX channels) [1]

|-cpdma_chan_set_descs(for RX channels) in case you'be described has to be
called with rx_desc_num = 0, because all descs are assigned already for tx
channel. And, if desc_num = 0, cpdma_chan_set_descs just exits and no issues.
So, could you please explain how you get this, in what circumstances.

> 
> Hence, fix the issue by checking most_chan for NULL before accessing it.
> 
> Fixes: 0fc6432cc78d ("net: ethernet: ti: davinci_cpdma: add weight function for channels")
> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
> ---
>  drivers/net/ethernet/ti/davinci_cpdma.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/ti/davinci_cpdma.c b/drivers/net/ethernet/ti/davinci_cpdma.c
> index 36518fc..b349d572 100644
> --- a/drivers/net/ethernet/ti/davinci_cpdma.c
> +++ b/drivers/net/ethernet/ti/davinci_cpdma.c
> @@ -708,7 +708,8 @@ static void cpdma_chan_set_descs(struct cpdma_ctlr *ctlr,
>  		}
>  	}
>  	/* use remains */
> -	most_chan->desc_num += desc_cnt;
> +	if (most_chan)
> +		most_chan->desc_num += desc_cnt;
>  }
>  
>  /**
> -- 
> 2.10.1.dirty
> 

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