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

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Date:   Thu, 24 Jan 2019 19:08:14 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Mathieu Malaterre <malat@...ian.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mac80211: Remove attribute packed from struct 'action'

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

johannes