The nest attribute NBD_ATTR_DEVICE_LIST was not accounted for when
allocating the message, resulting in -EMSGSIZE.

As __alloc_skb aligns size requests to SMP_CACHE_BYTES and SLUB will end up
allocating more than requested, this can hardly be reproduced on most
setups.

However, I managed to test this on a 32-bit x86 with 15 entries, by loading
with nbds_max=15. It failed with -EMSGSIZE, while it worked with 14 or 16
entries.

Signed-off-by: Thadeu Lima de Souza Cascardo <redacted>
---
 drivers/block/nbd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 43cff01a5a67..19551d8ca355 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -2265,6 +2265,7 @@ static int nbd_genl_status(struct sk_buff *skb, struct genl_info *info)
 	msg_size = nla_total_size(nla_attr_size(sizeof(u32)) +
 				  nla_attr_size(sizeof(u8)));
 	msg_size *= (index == -1) ? nbd_total_devices : 1;
+	msg_size += nla_total_size(0); /* for NBD_ATTR_DEVICE_LIST */
 
 	reply = genlmsg_new(msg_size, GFP_KERNEL);
 	if (!reply)

-- 
2.25.1