linux-kernel - Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

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Message-ID: <20210728092323.GW5047@twin.jikos.cz>
Date:   Wed, 28 Jul 2021 11:23:23 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

On Wed, Jul 28, 2021 at 10:35:56AM +0300, Dan Carpenter wrote:
> @@ -372,7 +372,7 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
>  			ieee80211_calculate_rx_timestamp(local, status,
>  							 mpdulen, 0),
>  			pos);
> -		rthdr->it_present |= cpu_to_le32(1 << IEEE80211_RADIOTAP_TSFT);
> +		rthdr->data.it_present |= cpu_to_le32(1 << IEEE80211_RADIOTAP_TSFT);

A drive-by comment, not related to the patchset, but rather the
ieee80211 driver itself.

Shift expressions with (1 << NUMBER) can be subtly broken once the
NUMBER is 31 and the value gets silently cast to a 64bit type. It will
become 0xfffffffff80000000.

I've checked the IEEE80211_RADIOTAP_* defintions if this is even remotely
possible and yes, IEEE80211_RADIOTAP_EXT == 31. Fortunatelly it seems to
be used with used with a 32bit types (eg. _bitmap_shifter) so there are
no surprises.

The recommended practice is to always use unsigned types for shifts, so
"1U << ..." at least.