netdev - Re: [PATCH v2 net-next 1/2] bpf: allow extended BPF programs access skb fields

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Message-ID: <550400D8.5060407@iogearbox.net>
Date:	Sat, 14 Mar 2015 10:35:20 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Thomas Graf <tgraf@...g.ch>, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next 1/2] bpf: allow extended BPF programs access
 skb fields

On 03/14/2015 05:59 AM, Alexei Starovoitov wrote:
> On 3/13/15 7:27 PM, Alexei Starovoitov wrote:
>> On 3/13/15 7:16 PM, Daniel Borkmann wrote:
>>> On 03/14/2015 03:08 AM, Alexei Starovoitov wrote:
>>>> On 3/13/15 7:06 PM, Daniel Borkmann wrote:
>>>>> On 03/14/2015 02:46 AM, Daniel Borkmann wrote:
>>>>> ...
>>>>>> Previously, it was much more consistent, which I like better. And only
>>>>>> because of the simple BUILD_BUG_ON()? :/
>>>>>
>>>>> Alternative is to move all of them into a central place, something like
>>>>> in twsk_build_assert() or __mld2_query_bugs[].
>>>>
>>>> nope. that defeats the purpose of bug_on.
>>>
>>> Well, it doesn't. ;) It throws a build error thus the user is forced to
>>> investigate that further.
>>
>> according to this distorted logic all build_bug_on can be in one file
>> across the whole tree, since 'user is forced to investigate' ?!

That was not what I was suggesting, and I assume you know that ...

> also note that this case and twsk_build_assert are different.
> twsk_build_assert has no other choice then to have one function
> that covers logic in the whole file, whereas in this patch:
> +               BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, mark) != 4);
> +               *insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
> +                                     offsetof(struct sk_buff, mark));
>
> the build_bug_on protect the line directly below.
> Separating them just doesn't make sense at all.

I also like the above approach better, I only suggested that as a
possible alternative since you were saying earlier in this thread:

   I thought about it, but didn't add it, since we already have them
   in the same file several lines above this spot. I think one build
   error per .c file should be enough to attract attention. Though
   I'll add a comment to convert_bpf_extensions() that build_bug_on
   errors should be addressed in two places.

Cheers,
Daniel
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