From mboxrd@z Thu Jan  1 00:00:00 1970
From: Phil Sutter <phil@nwl.cc>
Subject: Re: NULL pointer dereference in rt6_get_cookie()
Date: Tue, 13 Oct 2015 21:10:39 +0200
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 <20151013181443.GB68583@kafai-mba.local>
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Cc: netdev@vger.kernel.org,
	Hannes Frederic Sowa <hannes@stressinduktion.org>,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Julian Anastasov <ja@ssi.bg>
To: Martin KaFai Lau <kafai@fb.com>
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Hi Martin,

On Tue, Oct 13, 2015 at 11:14:43AM -0700, Martin KaFai Lau wrote:
> On Sat, Oct 10, 2015 at 03:24:37PM +0200, Phil Sutter wrote:
> > The conditional at the start of the function evaluates true, since
> > 'rt->rt6i_flags & RTF_PCPU' is non-zero.
> Hi Phil, can you try the following patch and capture the dmesg output
> to confirm the value of rt->rt6i_flags and the rt->dst.flags.
> 
> Thanks,
> Martin
> 
> --- a/include/net/ip6_fib.h
> +++ b/include/net/ip6_fib.h
> @@ -167,8 +167,15 @@ static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
> 
>  static inline u32 rt6_get_cookie(const struct rt6_info *rt)
>  {
> -	if (rt->rt6i_flags & RTF_PCPU || unlikely(rt->dst.flags & DST_NOCACHE))
> +	if (rt->rt6i_flags & RTF_PCPU || unlikely(rt->dst.flags & DST_NOCACHE)) {
>  		rt = (struct rt6_info *)(rt->dst.from);
> +		if (!rt)
> +			pr_err("rt6i_dst:%pI6c/%d rt6i_gateway:%pI6c "
> +			       "rt6i_flags:%08X dst.flags:%08X\n",
> +			       &rt->rt6i_dst.addr, rt->rt6i_dst.plen,
> +			       &rt->rt6i_gateway, rt->rt6i_flags,
> +			       rt->dst.flags);
> +	}
> 
>  	return rt->rt6i_node ? rt->rt6i_node->fn_sernum : 0;
>  }

This code is not sane. Your pr_err() statement tries to dereference the
NULL pointer in question. Are you interested in the originally passed
rt6_info?

Cheers, Phil