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Date:   Fri, 20 Nov 2020 08:19:26 -0700
From:   David Ahern <dsahern@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>, xiakaixu1987@...il.com,
        ast@...nel.org, kafai@...com, songliubraving@...com, yhs@...com,
        andrii@...nel.org, john.fastabend@...il.com, kpsingh@...omium.org
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kaixu Xia <kaixuxia@...cent.com>
Subject: Re: [PATCH] bpf: Check the return value of dev_get_by_index_rcu()

On 11/20/20 8:13 AM, Daniel Borkmann wrote:
> [ +David ]
> 
> On 11/19/20 8:04 AM, xiakaixu1987@...il.com wrote:
>> From: Kaixu Xia <kaixuxia@...cent.com>
>>
>> The return value of dev_get_by_index_rcu() can be NULL, so here it
>> is need to check the return value and return error code if it is NULL.
>>
>> Signed-off-by: Kaixu Xia <kaixuxia@...cent.com>
>> ---
>>   net/core/filter.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 2ca5eecebacf..1263fe07170a 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -5573,6 +5573,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *,
>> skb,
>>           struct net_device *dev;
>>             dev = dev_get_by_index_rcu(net, params->ifindex);
>> +        if (unlikely(!dev))
>> +            return -EINVAL;
>>           if (!is_skb_forwardable(dev, skb))
>>               rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

rcu lock is held right? It is impossible for dev to return NULL here.

> 
> The above logic is quite ugly anyway given we fetched the dev pointer
> already earlier
> in bpf_ipv{4,6}_fib_lookup() and now need to redo it again ... so yeah

evolved from the different needs of the xdp and tc paths.

> there could be
> a tiny race in here. We wanted do bring this logic closer to what XDP
> does anyway,
> something like below, for example. David, thoughts? Thx
> 
> Subject: [PATCH] diff mtu check
> 
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  net/core/filter.c | 22 +++++-----------------
>  1 file changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 2ca5eecebacf..3bab0a97fa38 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5547,9 +5547,6 @@ static const struct bpf_func_proto
> bpf_xdp_fib_lookup_proto = {
>  BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
>         struct bpf_fib_lookup *, params, int, plen, u32, flags)
>  {
> -    struct net *net = dev_net(skb->dev);
> -    int rc = -EAFNOSUPPORT;
> -
>      if (plen < sizeof(*params))
>          return -EINVAL;
> 
> @@ -5559,25 +5556,16 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *,
> skb,
>      switch (params->family) {
>  #if IS_ENABLED(CONFIG_INET)
>      case AF_INET:
> -        rc = bpf_ipv4_fib_lookup(net, params, flags, false);
> -        break;
> +        return bpf_ipv4_fib_lookup(dev_net(skb->dev), params, flags,
> +                       !skb_is_gso(skb));
>  #endif
>  #if IS_ENABLED(CONFIG_IPV6)
>      case AF_INET6:
> -        rc = bpf_ipv6_fib_lookup(net, params, flags, false);
> -        break;
> +        return bpf_ipv6_fib_lookup(dev_net(skb->dev), params, flags,
> +                       !skb_is_gso(skb));

seems ok.


>  #endif
>      }
> -
> -    if (!rc) {
> -        struct net_device *dev;
> -
> -        dev = dev_get_by_index_rcu(net, params->ifindex);
> -        if (!is_skb_forwardable(dev, skb))
> -            rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> -    }
> -
> -    return rc;
> +    return -EAFNOSUPPORT;
>  }
> 
>  static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {

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