netdev - Re: [PATCH] mac80211: Remove attribute packed from struct 'action'

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Date:   Thu, 24 Jan 2019 19:14:44 +0100
From:   Mathieu Malaterre <malat@...ian.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     "David S. Miller" <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mac80211: Remove attribute packed from struct 'action'

On Thu, Jan 24, 2019 at 7:08 PM Johannes Berg <johannes@...solutions.net> wrote:
>
> On Thu, 2019-01-24 at 19:05 +0100, Mathieu Malaterre wrote:
> > During refactor in commit 9e478066eae4 ("mac80211: fix MU-MIMO
> > follow-MAC mode") a new struct 'action' was declared with packed
> > attribute as:
> >
> >   struct {
> >           struct ieee80211_hdr_3addr hdr;
> >           u8 category;
> >           u8 action_code;
> >   } __packed action;
> >
> > But since struct 'ieee80211_hdr_3addr' is declared with an aligned
> > keyword as:
> >
> >   struct ieee80211_hdr {
> >       __le16 frame_control;
> >       __le16 duration_id;
> >       u8 addr1[ETH_ALEN];
> >       u8 addr2[ETH_ALEN];
> >       u8 addr3[ETH_ALEN];
> >       __le16 seq_ctrl;
> >       u8 addr4[ETH_ALEN];
> >   } __packed __aligned(2);
> >
> > Solve the ambiguity of placing aligned structure in a packed one by
> > removing the packed attribute from struct. This seems to be the behavior
> > of gcc anyway, since the following is still compiling:
> >
> >   BUILD_BUG_ON(sizeof(action) != IEEE80211_MIN_ACTION_SIZE + 1);
>
> I'm not sure this will work on all platforms, didn't something like
> alpha pad out u8's to u32 when not requiring packing?

I was not aware of that.

> I guess I'd feel better about using __packed __aligned(2) here as well,
> which should solve the warning too?

Indeed, I will re-spin a v2 then.