netdev - Re: [PATCH bpf v2] xdp: Fix handling of devmap in generic XDP

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Date:   Thu, 14 Jun 2018 18:00:22 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Subject: Re: [PATCH bpf v2] xdp: Fix handling of devmap in generic XDP

On 2018/06/14 17:49, Jesper Dangaard Brouer wrote:
> On Thu, 14 Jun 2018 11:07:42 +0900
> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:
> 
>> Commit 67f29e07e131 ("bpf: devmap introduce dev_map_enqueue") changed
>> the return value type of __devmap_lookup_elem() from struct net_device *
>> to struct bpf_dtab_netdev * but forgot to modify generic XDP code
>> accordingly.
>> Thus generic XDP incorrectly used struct bpf_dtab_netdev where struct
>> net_device is expected, then skb->dev was set to invalid value.
>>
>> v2:
>> - Fix compiler warning without CONFIG_BPF_SYSCALL.
>>
>> Fixes: 67f29e07e131 ("bpf: devmap introduce dev_map_enqueue")
>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
> 
> Thanks for catching this!
> 
> Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>
> 
> Notice, that the current code works (and does not crash), but it is
> pure luck.  Because struct bpf_dtab_netdev happen to have the
> net_device as the first member.
> 
> struct bpf_dtab_netdev {
> 	struct net_device *dev; /* must be first member, due to tracepoint */
> 	struct bpf_dtab *dtab;
> 	unsigned int bit;
> 	struct xdp_bulk_queue __percpu *bulkq;
> 	struct rcu_head rcu;
> };
> 

Actually no, the current code does not work and can crash, because we
need to dereference the pointer, i.e. need fwd->dev (IOW *fwd) not fwd.

-- 
Toshiaki Makita