linux-kernel - Re: [PATCH] cgroup: reorder flexible array members of struct cgroup

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Message-ID: <CAH7mPvjksNSDo4o=z51YkH4yX+gDnGXU-g59gTkOUS-pEva8sA@mail.gmail.com>
Date:   Fri, 20 Oct 2017 00:15:55 -0700
From:   Nick Desaulniers <nick.desaulniers@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>, android-llvm@...gle.com
Subject: Re: [PATCH] cgroup: reorder flexible array members of struct cgroup_root

On Wed, Oct 18, 2017 at 6:30 AM, Tejun Heo <tj@...nel.org> wrote:
> On Mon, Oct 16, 2017 at 11:33:21PM -0700, Nick Desaulniers wrote:
>> When compiling arch/x86/boot/compressed/eboot.c with HOSTCC=clang, the
>> following warning is observed:
>>
>> ./include/linux/cgroup-defs.h:391:16: warning: field 'cgrp' with
>> variable sized type 'struct cgroup' not at the end of a struct or class
>> is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>>         struct cgroup cgrp;
>>                       ^
>> Flexible array members are a C99 feature, but must be the last member of
>> a struct. Structs with flexible members composed in other structs must
>> also be the final members, unless using GNU C extensions.
>>
>> struct cgroup_root's member cgrp is a struct cgroup, struct cgroup's
>> member ancestor_ids is a flexible member.
>
> This is silly tho.  We know the the root group embedded there won't
> have any ancestor_ids.

Sure, but struct cgroup_root is still declared as having a struct
cgroup not declared as the final member.

> Also, in general, nothing prevents us from
> doing something like the following.
>
>         struct outer_struct {
>                 blah blah;
>                 struct inner_struct_with_flexible_array_member inner;
>                 unsigned long storage_for_flexible_array[NR_ENTRIES];
>                 blah blah;
>         };

At that point, then why have the struct with flexible array member in
the first place?

>> or specific location of the member cgrp within struct cgroup_root AFAICT.
> I think we should just silence the bogus warning.

Is the order of the members actually important?  Otherwise it seems
that we're taking advantage of a GNU C extension for no real reason,
which is what I'm trying to avoid.  Please reconsider.