Re: [PATCH 1/1] net: netlink: Fix multicast group storage allocation for families with more than one groups

From: Matti Vaittinen <redacted>
Date: Mon, 11 Jan 2016 14:26:19 +0200

Multicast groups are stored in global buffer. Check for needed buffer size
incorrectly compares buffer size to first id for family. This means that
for families with more than one mcast id one may allocate too small buffer
and end up writing rest of the groups to some unallocated memory. Fix the 
buffer size check to compare allocated space to last mcast id for the 
family.

Tested on ARM using kernel 3.14

Signed-off-by: Matti Vaittinen <redacted>

Indeed, it looks like this function was never tested with any value
of n_groups other than one.

But I think your change has an off-by-one bug:

-		if (id >= mc_groups_longs * BITS_PER_LONG) {
+		if (id + n_groups >= mc_groups_longs * BITS_PER_LONG) {

I think this needs to be "id + n_groups > ".  Consider the existing,
working, case of "n_groups == 1".  Now you're adding '1' and therefore
the test needs to be adjusted from >= to >.

Thanks.