linux-kernel - Re: [PATCH v5] net: ip, diag -- Add diag interface for raw sockets

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Date:   Wed, 28 Sep 2016 06:43:01 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsa@...ulusnetworks.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
        kaber@...sh.net, avagin@...nvz.org, stephen@...workplumber.org
Subject: Re: [PATCH v5] net: ip, diag -- Add diag interface for raw sockets

On 16-09-28 06:17 AM, Cyrill Gorcunov wrote:
> On Wed, Sep 28, 2016 at 06:08:00AM -0400, Jamal Hadi Salim wrote:
> ...
>>> @@ -38,7 +38,10 @@ struct inet_diag_req_v2 {
>>>  	__u8	sdiag_family;
>>>  	__u8	sdiag_protocol;
>>>  	__u8	idiag_ext;
>>> -	__u8	pad;
>>> +	union {
>>> +		__u8	pad;
>>> +		__u8	sdiag_raw_protocol;	/* SOCK_RAW only, @pad for others */
>>> +	};
>>
>>
>> Above looks funny. Why is it a union? pad is for exposing a byte-hole
>> for padding/alignment reasons and i doubt anybody is using it.
>
> Someone may have set it to zero explicitly on source level, and the
> compilation will fail on new kernel then. So no, keeping the name
> is reasonable.
>

I dont know how compilation will fail but you may be right with note:
that is not how pads have been used in the past. They are supposed to
cosmetic annotation which indicates "here's a hole; use it in the
future if you are looking to add something". And someone in the
future can claim them. I am not sure if MBZ philosophy applies.

>> Should you not just rename it?
>> Also I notice when things like __raw_v4_lookup() are claiming it is unsigned
>> short instead of a u8?
>
> The protocol is still up to 255 for a while, is it expected that IPPROTO_MAX
> will be increased in more-less near future? Of course I can drop the idea
> of using @pad here and switch to some extended reauest but prefer to stick
> with simplier solution. Hm?
>

Ok. If i understood correctly it was already unsigned short before your
patch -so i agree it doesnt matter. Maybe just put a comment to express
that if ever protocol goes above 255 it wont be sufficient.



cheers,
jamal

PS:- sorry for butting in the discussion - i blame it on coffee.