linux-kernel - Re: [PATCH v2][next] Bluetooth: hci_conn, hci_sync: Use __counted_by() in multiple structs and avoid -Wfamnae warnings

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Message-ID: <202404291253.106D020@keescook>
Date: Mon, 29 Apr 2024 12:56:54 -0700
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Marcel Holtmann <marcel@...tmann.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	Luiz Augusto von Dentz <luiz.dentz@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] Bluetooth: hci_conn, hci_sync: Use
 __counted_by() in multiple structs and avoid -Wfamnae warnings

On Mon, Apr 29, 2024 at 01:50:46PM -0600, Gustavo A. R. Silva wrote:
> 
> > > diff --git a/include/net/bluetooth/hci.h b/include/net/bluetooth/hci.h
> > > index fe23e862921d..c4c6b8810701 100644
> > > --- a/include/net/bluetooth/hci.h
> > > +++ b/include/net/bluetooth/hci.h
> > > @@ -2026,7 +2026,7 @@ struct hci_cp_le_set_ext_adv_data {
> > >   	__u8  operation;
> > >   	__u8  frag_pref;
> > >   	__u8  length;
> > > -	__u8  data[];
> > > +	__u8  data[] __counted_by(length);
> > >   } __packed;
> > 
> > I noticed some of the other structs here aren't flexible arrays, so it
> > made me go take a look at these ones. I see that the only user of struct
> > hci_cp_le_set_ext_adv_data uses a fixed-size array:
> > 
> >          struct {
> >                  struct hci_cp_le_set_ext_adv_data cp;
> >                  u8 data[HCI_MAX_EXT_AD_LENGTH];
> >          } pdu;
> > 
> > Let's just change this from a flex array to a fixed-size array?
> 
> mmh... not sure about this. It would basically mean reverting this commit:
> 
> c9ed0a707730 ("Bluetooth: Fix Set Extended (Scan Response) Data")

That change doesn't seem to need to make them flex arrays, though --
there's no savings at all (the same amount is stack allocated).

Anyway, not a big deal, I guess. It's an improvement to be using
__counted_by, so good! :)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook