linux-hardening - Re: [PATCH][next] wifi: mt76: mt7921: Replace fake flex-arrays with flexible-array members

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Date:   Mon, 20 Mar 2023 13:26:49 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Simon Horman <simon.horman@...igine.com>
Cc:     Felix Fietkau <nbd@....name>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Shayne Chen <shayne.chen@...iatek.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Kalle Valo <kvalo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] wifi: mt76: mt7921: Replace fake flex-arrays with
 flexible-array members

On Mon, Mar 20, 2023 at 03:56:54PM +0100, Simon Horman wrote:
> >  struct mt7921_asar_cl {
> > @@ -85,7 +85,7 @@ struct mt7921_asar_fg {
> >  	u8 rsvd;
> >  	u8 nr_flag;
> >  	u8 rsvd1;
> > -	u8 flag[0];
> > +	u8 flag[];
> 
> I am curious to know why DECLARE_FLEX_ARRAY isn't used here.

In contrast to the other structs, there is no object of type struct
mt7921_asar_fg declared in a union:

 91 struct mt7921_acpi_sar {
 92         u8 ver;
 93         union {
 94                 struct mt7921_asar_dyn *dyn;
 95                 struct mt7921_asar_dyn_v2 *dyn_v2;
 96         };
 97         union {
 98                 struct mt7921_asar_geo *geo;
 99                 struct mt7921_asar_geo_v2 *geo_v2;
100         };
101         struct mt7921_asar_cl *countrylist;
102         struct mt7921_asar_fg *fg;
103 };

The DECLARE_FLEX_ARRAY() helper was created to declare flex-array members
in unions or alone in structs[1].

--
Gustavo

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays