netdev - Re: running an eBPF program

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Date:   Sat, 27 May 2017 22:08:59 -0700
From:   Y Song <ys114321@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Adel Fuchs <adelfuchs@...il.com>, netdev@...r.kernel.org
Subject: Re: running an eBPF program

On Sat, May 27, 2017 at 5:11 PM, David Miller <davem@...emloft.net> wrote:
> From: Y Song <ys114321@...il.com>
> Date: Sat, 27 May 2017 13:52:27 -0700
>
>> On Sat, May 27, 2017 at 1:23 PM, Y Song <ys114321@...il.com> wrote:
>>>
>>> From verifier error message:
>>> ======
>>> 0: (bf) r6 = r1
>>>
>>> 1: (18) r9 = 0xffe0000e
>>>
>>> 3: (69) r0 = *(u16 *)(r6 +16)
>>>
>>> invalid bpf_context access off=16 size=2
>>> ======
>>>
>>> The offset 16 of struct __sk_buff is hash.
>>> What instruction #3 tries to do is to access 2 bytes of the hash value
>>> instead of full 4 bytes.
>>> This is explicitly not allowed in verifier due to endianness issue.
>>
>>
>> I can reproduce the issue now. My previous statement saying to access
>> "hash" field is not correct. It is accessing the protocol field.
>>
>> static __inline__ bool flow_dissector(struct __sk_buff *skb,
>>                                       struct flow_keys *flow)
>> {
>>         int poff, nh_off = BPF_LL_OFF + ETH_HLEN;
>>         __be16 proto = skb->protocol;
>>         __u8 ip_proto;
>>
>> The plan so far is to see whether we can fix the issue in LLVM side.
>
> If the compiler properly asks for "__sk_buff + 16" on little-endian
> and "__sk_buff + 20" on big-endian, the verifier should instead be
> fixed to allow the access to pass.
>
> I can't see any reason why LLVM won't set the offset properly like
> that, and it's a completely legitimate optimization that we shouldn't
> try to stop LLVM from performing.

I do agree that such optimization in LLVM is perfect fine and actually
beneficial.
The only reason I was thinking was to avoid introduce endianness into verifier.
Maybe not too much work there. Let me do some experiments and come with
a patch for that.

Thanks!

Yonghong

>
> It also makes it so that we don't have to fix having absurdly defined
> __sk_buff's protocol field as a u32.
>
> Thanks.