Subject: Re: [PATCH 1/1] NET: netpoll, fix potential NULL ptr dereference

From: Matt Mackall <[email-masked]>
Date: Tue, 16 Mar 2010 12:12:07 -0500

On Tue, 2010-03-16 at 16:29 +0100, Jiri Slaby wrote:
> Stanse found that one error path in netpoll_setup dereferences npinfo
> even though it is NULL. Avoid that by adding new label and go to that
> instead.
> 
> Signed-off-by: Jiri Slaby <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: David S. Miller <[email-masked]>
> ---
>  net/core/netpoll.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 7aa6972..d4ec38f 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -735,7 +735,7 @@ int netpoll_setup(struct netpoll *np)
>  		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
>  		if (!npinfo) {
>  			err = -ENOMEM;
> -			goto release;
> +			goto put;
>  		}
>  
>  		npinfo->rx_flags = 0;
> @@ -845,7 +845,7 @@ int netpoll_setup(struct netpoll *np)
>  
>  		kfree(npinfo);
>  	}
> -
> +put:
>  	dev_put(ndev);
>  	return err;
>  }

I don't get it. The source of the branch tests for !ndev->npinfo and the
original destination of the branch also tests for !ndev->npinfo. I don't
see how it gets dereferenced.

This looks like it just patches over a false positive in your tool
(which isn't correlating the validity of npinfo with ndev->npinfo)
without actually improving the code. However, it seems that we can drop
the second check at release if we add your new exit point.

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