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

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Date:	Fri, 13 Mar 2015 21:59:43 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Daniel Borkmann <daniel@...earbox.net>,
	"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 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' ?!

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.

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