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

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Message-ID: <1475067432.28155.90.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 28 Sep 2016 05:57:12 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.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 Wed, 2016-09-28 at 14:27 +0300, Cyrill Gorcunov wrote:
> On Wed, Sep 28, 2016 at 07:06:26AM -0400, Jamal Hadi Salim wrote:
> > > 
> > > This structure is uapi, so anyone has complete rights to reference
> > > @pad in the userspace programs. Sure it would be more clear to remove
> > > the @pad completely, but if we choose so I think it's better to do
> > > on top instead and then if someone complain we can easily revert
> > > the single trivial commit instead of this big patch.
> > 
> > I am conflicted.
> > A field labelled "pad" does not appear to be valid as "UAPI". It is
> > a cosmetic indicator. If you did sizeof() with or without it being
> > present the value doesnt change.
> 
> I think you miss the point what I'm trying to say: currently end-user
> may have reference to this member (for any reason) and his program
> will compile and run. If we change the name the compilation procedure
> fails and this will break API. Yes, referrning @pad is bad idea for
> userspace code, and yes (!) better to simply rename it but lets do
> that later, on top, so that if we break something in userspace
> we could easily revert the oneline change.

Note that some programs could fail to compile with the added union
anyway.

Some gcc versions are unable to compile a static init with an union

struct inet_diag_req_v2 foo = { .pad = 0, sdiag_family = AF_INET, };

When I cooked my recent fq commit I simply removed a pad and replaced
it :

git show fefa569a9d4bc4 -- include