netdev - Re: [RFC PATCH] dst: check if dst is freed in dst

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Date:	Wed, 21 Jul 2010 04:28:08 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	nicolas.dichtel@...nd.com
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] dst: check if dst is freed in dst_check()

Le mardi 20 juillet 2010 à 11:49 +0200, Nicolas Dichtel a écrit :
> Hi,
> 
> I probably missed something, but I cannot find where obsolete field is checked 
> when dst_check() is called. If dst->obsolete is > 1, dst cannot be used!
> 
> Attached is a proposal to fix this issue.
> 
> 

> diff --git a/include/net/dst.h b/include/net/dst.h
> index 81d1413..7bf4f9a 100644
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -319,6 +319,8 @@ static inline int dst_input(struct sk_buff *skb)
>  
>  static inline struct dst_entry *dst_check(struct dst_entry *dst, u32 cookie)
>  {
> +	if (dst->obsolete > 1)
> +		return NULL;
>  	if (dst->obsolete)
>  		dst = dst->ops->check(dst, cookie);
>  	return dst;

I believe this is not needed and redundant.

In what case do you think this matters ?

To my knowledge dst_check() is only used by net/xfrm/xfrm_policy.c

And xfrm_dst_check() does the necessary checks.

static struct dst_entry *xfrm_dst_check(struct dst_entry *dst, u32 cookie)
{
        /* Code (such as __xfrm4_bundle_create()) sets dst->obsolete
         * to "-1" to force all XFRM destinations to get validated by
         * dst_ops->check on every use.  We do this because when a
         * normal route referenced by an XFRM dst is obsoleted we do
         * not go looking around for all parent referencing XFRM dsts
         * so that we can invalidate them.  It is just too much work.
         * Instead we make the checks here on every use.  For example:
         *
         *      XFRM dst A --> IPv4 dst X
         *
         * X is the "xdst->route" of A (X is also the "dst->path" of A
         * in this example).  If X is marked obsolete, "A" will not
         * notice.  That's what we are validating here via the
         * stale_bundle() check.
         *
         * When a policy's bundle is pruned, we dst_free() the XFRM
         * dst which causes it's ->obsolete field to be set to a
         * positive non-zero integer.  If an XFRM dst has been pruned
         * like this, we want to force a new route lookup.
         */
        if (dst->obsolete < 0 && !stale_bundle(dst))
                return dst;

        return NULL;
}


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