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

[Kees Cook]

On Wed, Jul 28, 2021 at 11:23:23AM +0200, David Sterba wrote:
> 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.

Ah, good catch! I think just using BIT() is the right replacement here,
yes? I suppose that should be a separate patch.

-- 
Kees Cook