netdev - Re: [PATCH v2 2/2] mac80211: Use flexible array in struct ieee80211_tim

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Message-ID: <1774098a-5062-4f12-a760-f16036d095e3@gmail.com>
Date: Wed, 30 Aug 2023 21:51:15 +0200
From: Christian Lamparter <chunkeey@...il.com>
To: Jeff Johnson <quic_jjohnson@...cinc.com>, kernel@...cinc.com,
 Kalle Valo <kvalo@...nel.org>, Toke Høiland-Jørgensen
 <toke@...e.dk>, Christian Lamparter <chunkeey@...glemail.com>,
 Stanislaw Gruszka <stf_xl@...pl>, Helmut Schaa
 <helmut.schaa@...glemail.com>, Ping-Ke Shih <pkshih@...ltek.com>,
 Johannes Berg <johannes@...solutions.net>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mac80211: Use flexible array in struct
 ieee80211_tim_ie

Hi,

On 8/29/23 15:29, Jeff Johnson wrote:
> Currently struct ieee80211_tim_ie defines:
> 	u8 virtual_map[1];
> 
> Per the guidance in [1] change this to be a flexible array.
> 
> As a result of this change, adjust all related struct size tests to
> account for the fact that the sizeof(struct ieee80211_tim_ie) now
> accounts for the minimum size of the virtual_map.
> 
> [1] https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Signed-off-by: Jeff Johnson <quic_jjohnson@...cinc.com>
> ---
> diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
> index bd2f6e19c357..4cdc2eb98f16 100644
> --- a/include/linux/ieee80211.h
> +++ b/include/linux/ieee80211.h
> @@ -961,7 +961,7 @@ struct ieee80211_tim_ie {
>   	u8 dtim_period;
>   	u8 bitmap_ctrl;
>   	/* variable size: 1 - 251 bytes */
> -	u8 virtual_map[1];
> +	u8 virtual_map[];
>   } __packed;

Uhh, the 802.11 (my 2012 Version has this in) spec in
8.4.2.7 TIM Element demands this to be 1 - 251 bytes.
And this is why there's a comment above... With your
change this could be confusing. Would it be possible
to fix that somehow? Like in a anonymous union/group
with a flexible array and a u8?

Cheers,
Christian