netlink: Make groups check less stupid in netlink_bind()

mainline inclusion
from mainline-5.1-rc6
commit d852be84770c
category: bugfix
bugzilla: 14091
CVE: NA

-------------------------------------------------

As Linus noted, the test for 0 is needless, groups type can follow the
usual kernel style and 8*sizeof(unsigned long) is BITS_PER_LONG:

> The code [..] isn't technically incorrect...
> But it is stupid.
> Why stupid? Because the test for 0 is pointless.
>
> Just doing
>        if (nlk->ngroups < 8*sizeof(groups))
>                groups &= (1UL << nlk->ngroups) - 1;
>
> would have been fine and more understandable, since the "mask by shift
> count" already does the right thing for a ngroups value of 0. Now that
> test for zero makes me go "what's special about zero?". It turns out
> that the answer to that is "nothing".
[..]
> The type of "groups" is kind of silly too.
>
> Yeah, "long unsigned int" isn't _technically_ wrong. But we normally
> call that type "unsigned long".

Cleanup my piece of pointlessness.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: netdev@vger.kernel.org
Fairly-blamed-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NDmitry Safonov <dima@arista.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NZhiqiang Liu <liuzhiqiang26@huawei.com>
Reviewed-by: NWenan Mao <maowenan@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/netlink/af_netlink.c

@@ -993,7 +993,7 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
 	struct netlink_sock *nlk = nlk_sk(sk);
 	struct sockaddr_nl *nladdr = (struct sockaddr_nl *)addr;
 	int err = 0;
-	long unsigned int groups = nladdr->nl_groups;
+	unsigned long groups = nladdr->nl_groups;
 	bool bound;
 
 	if (addr_len < sizeof(struct sockaddr_nl))
@@ -1011,9 +1011,7 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
 			return err;
 	}
 
-	if (nlk->ngroups == 0)
-		groups = 0;
-	else if (nlk->ngroups < 8*sizeof(groups))
+	if (nlk->ngroups < BITS_PER_LONG)
 		groups &= (1UL << nlk->ngroups) - 1;
 
 	bound = nlk->bound;