From: Arnd Bergmann <arnd@xxxxxxxxxx>
Date: Tue, 14 Feb 2023 14:20:21 +0100
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> gcc-9 triggers a false-postive warning in ieee80211_mlo_multicast_tx()
> for u32_encode_bits(ffs(links) - 1, ...), since ffs() can return zero
> on an empty bitmask, and the negative argument to u32_encode_bits()
> is then out of range:
>
> In file included from include/linux/ieee80211.h:21,
> from include/net/cfg80211.h:23,
> from net/mac80211/tx.c:23:
> In function 'u32_encode_bits',
> inlined from 'ieee80211_mlo_multicast_tx' at net/mac80211/tx.c:4437:17,
> inlined from 'ieee80211_subif_start_xmit' at net/mac80211/tx.c:4485:3:
> include/linux/bitfield.h:177:3: error: call to '__field_overflow' declared with attribute error: value doesn't fit into mask
> 177 | __field_overflow(); \
> | ^~~~~~~~~~~~~~~~~~
> include/linux/bitfield.h:197:2: note: in expansion of macro '____MAKE_OP'
> 197 | ____MAKE_OP(u##size,u##size,,)
> | ^~~~~~~~~~~
> include/linux/bitfield.h:200:1: note: in expansion of macro '__MAKE_OP'
> 200 | __MAKE_OP(32)
> | ^~~~~~~~~
>
> Newer compiler versions do not cause problems with the zero argument
> because they do not consider this a __builtin_constant_p().
> It's also harmless since the hweight16() check already guarantees
> that this cannot be 0.
>
> Replace the ffs() with an equivalent find_first_bit() check that
> matches the later for_each_set_bit() style and avoids the warning.
>
> Fixes: 963d0e8d08d9 ("wifi: mac80211: optionally implement MLO multicast TX")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> net/mac80211/tx.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
> index defe97a31724..118648af979c 100644
> --- a/net/mac80211/tx.c
> +++ b/net/mac80211/tx.c
> @@ -4434,7 +4434,7 @@ static void ieee80211_mlo_multicast_tx(struct net_device *dev,
> u32 ctrl_flags = IEEE80211_TX_CTRL_MCAST_MLO_FIRST_TX;
>
> if (hweight16(links) == 1) {
> - ctrl_flags |= u32_encode_bits(ffs(links) - 1,
> + ctrl_flags |= u32_encode_bits(find_first_bit(&links, 16) - 1,
Uff, IIRC find_first_bit() matches __ffs() calling convention, not ffs()
one. They're off-by-one from each other, which means you need to drop
this `- 1`.
As this branch happens only when hweight is 1 => @links has a bit set,
it's safe to just use __ffs() here directly, but up to you.
find_first_bit() is fine, too, since it will be optimized out to __ffs()
at the end in this case.
> IEEE80211_TX_CTRL_MLO_LINK);
>
> __ieee80211_subif_start_xmit(skb, sdata->dev, 0, ctrl_flags,
Thanks,
Olek
