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

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Date:   Wed, 30 Aug 2023 15:31:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Johnson <quic_jjohnson@...cinc.com>
Cc:     Christian Lamparter <chunkeey@...il.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>,
        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

On Wed, Aug 30, 2023 at 01:22:37PM -0700, Jeff Johnson wrote:
> On 8/30/2023 12:51 PM, Christian Lamparter wrote:
> > 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?
> 
> Adding Kees to the discussion for any advice. Yes, the virtual_map must
> contain at least one octet but may contain more than one. And to complicate
> matters, the information that tells us how many octets are actually present
> is found outside the struct; the TLV header that precedes the struct will
> contain the length of the struct, and hence the length of the bitmap is that
> size - 2 (the size of the dtim_period and bitmap_ctrl fields).

Bummer about the count variable being elsewhere, but we'll deal with
that later. :)

For the array declaration, though, yes, we can do a "minimum size 1" like
this:

	union {
		u8 required_byte;
		DECLARE_FLEX_ARRAY(u8, virtual_map);
	};

-- 
Kees Cook