netdev - Re: [PATCH][bpf-next] bpf: change bpf_skb_generic

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Date:   Mon, 6 Jan 2020 23:35:04 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Song Liu <liu.song.a23@...il.com>
Cc:     Simon Horman <simon.horman@...ronome.com>,
        Li RongQing <lirongqing@...du.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH][bpf-next] bpf: change bpf_skb_generic_push type as void

On 1/6/20 11:32 PM, Alexei Starovoitov wrote:
> On Fri, Jan 03, 2020 at 11:18:28AM -0800, Song Liu wrote:
>> On Fri, Jan 3, 2020 at 12:27 AM Simon Horman <simon.horman@...ronome.com> wrote:
>>> On Fri, Jan 03, 2020 at 02:02:33PM +0800, Li RongQing wrote:
>>>> bpf_skb_generic_push always returns 0, not need to check
>>>> its return, so change its type as void
>>>>
>>>> Signed-off-by: Li RongQing <lirongqing@...du.com>
>>>
>>> Reviewed-by: Simon Horman <simon.horman@...ronome.com>
>> Acked-by: Song Liu <songliubraving@...com>
>>
>>>
>>>> ---
>>>>   net/core/filter.c | 30 ++++++++++--------------------
>>>>   1 file changed, 10 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>> index 42fd17c48c5f..1cbac34a4e11 100644
>>>> --- a/net/core/filter.c
>>>> +++ b/net/core/filter.c
>>>
>>> ...
>>>
>>>> @@ -5144,7 +5134,7 @@ BPF_CALL_3(bpf_lwt_seg6_adjust_srh, struct sk_buff *, skb, u32, offset,
>>>>                if (unlikely(ret < 0))
>>>>                        return ret;
>>>>
>>>> -             ret = bpf_skb_net_hdr_push(skb, offset, len);
>>>> +             bpf_skb_net_hdr_push(skb, offset, len);
>>>
>>> There is a check for (ret < 0) just below this if block.
>>> That is ok becuase in order to get to here (ret < 0) must
>>> be true as per the check a few lines above.
>>>
>>> So I think this is ok although the asymmetry with the else arm
>>> of this if statement is not ideal IMHO.
>>
>> Agreed with this concern. But I cannot think of any free solution. I guess we
>> will just live with assumption that skb_cow_head() never return >0.
> 
> I don't think this patch is worth doing.
> I can imagine bpf_skb_generic_push() handling some errors in the future.
> compiler can do this optimization job instead.

Yep, fully agree.