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

Kees Cook
2021-07-28T21:54:57Z

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