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Date:   Thu, 18 Jun 2020 18:02:40 -0300
From:   Thadeu Lima de Souza Cascardo <>
To:     Josef Bacik <>
Cc:     Jens Axboe <>,,,,,
Subject: [PATCH] nbd: allocate sufficient space for NBD_CMD_STATUS

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

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

Signed-off-by: Thadeu Lima de Souza Cascardo <>
 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)) +
 	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)

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