netdev - Re: [PATCH] net: Fix IP_MULTICAST

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Message-ID: <4ADD3982.2040100@gmail.com>
Date:	Tue, 20 Oct 2009 06:16:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: Fix IP_MULTICAST_IF

Eric Dumazet a écrit :
> David Miller a écrit :
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Mon, 19 Oct 2009 18:41:58 +0200
>>
>>> ipv4/ipv6 setsockopt(IP_MULTICAST_IF) have dubious __dev_get_by_index() calls.
>> Dubious, how so?
>>
>> Yes, I know RTNL/dev_base_lock, but it's not using what it gets
>> back at all.
>>
>> It's testing existence, a boolean, it doesn't dereference the
>> 'dev' it gets back at all.
>>
>> This code is intentional and perfectly fine.
> 
> If this was intentional, something changed and made the prereq false.
> 
> Final target might be fine, but an element in the chain, before target
> could be deleted while reader scans hash chain.
> 

BTW, even an insertion can crash a lockless reader, since reader could see a corrupt
 n->next (hlist_add_head() has no barrier between n->next = first and h->first = n;)

static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
{
        struct hlist_node *first = h->first;
        n->next = first;
        if (first)
                first->pprev = &n->next;
        h->first = n;
        n->pprev = &h->first;
}

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