Re: [PATCH] wifi: nl80211: drop impossible negative band check

[sun jian]

On Wed, Feb 4, 2026 at 4:36 PM Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 2026-02-04 at 16:18 +0800, Sun Jian wrote:
> > band is derived from nla_type() of a nested netlink attribute, which isAgreed — in general the enum underlying type can be signed.
> > a masked u16 value and therefore cannot be negative. Drop the dead
> > "band < 0" checks and keep the upper bound validation.
>
> I've seen this before, but I'm not really convinced it is entirely
> correct. C says:
>
>    All enumerations have an underlying type. The underlying type can be
>    explicitly specified using an enum type specifier and is its fixed
>    underlying type. If it is not explicitly specified, the underlying
>    type is the enumeration’s compatible type, which is either char or a
>    standard or extended signed or unsigned integer type.
>

Agreed — in general the enum underlying type can be signed.

> It would thus _seem_ to be possible for an enum to generally be a signed
> type, and therefore a 'signed short', and therefore an nla_type() that's
> a u16 could end up with a negative value...

The key detail here is that band isn't assigned the raw __u16
nla->nla_type, but nla_type().

And nla_type() is effectively:
nla->nla_type & NLA_TYPE_MASK

and NLA_TYPE_MASK clears the two high flag bits:
NLA_F_NESTED (1 << 15)
NLA_F_NET_BYTEORDER (1 << 14)

So the result is restricted to the low 14 bits, i.e. 0..0x3fff.

With that restriction, even if enum nl80211_band ended up with a signed
16-bit underlying type, the sign bit (bit 15) can never be set by
nla_type(), so the value cannot become negative.

>
> Am I wrong?

I think the "enum may be signed" concern is valid in general, but for
this particular assignment the masking guarantees the value is always in
a non-negative range.

Thanks,
Sun Jian