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Subject[PATCH 4.19 223/321] decnet: fix DN_IFREQ_SIZE
From: Johannes Berg <>

[ Upstream commit 50c2936634bcb1db78a8ca63249236810c11a80f ]

Digging through the ioctls with Al because of the previous
patches, we found that on 64-bit decnet's dn_dev_ioctl()
is wrong, because struct ifreq::ifr_ifru is actually 24
bytes (not 16 as expected from struct sockaddr) due to the
ifru_map and ifru_settings members.

Clearly, decnet expects the ioctl to be called with a struct
struct ifreq_dn {
char ifr_name[IFNAMSIZ];
struct sockaddr_dn ifr_addr;

since it does
struct ifreq *ifr = ...;
struct sockaddr_dn *sdn = (struct sockaddr_dn *)&ifr->ifr_addr;

This means that DN_IFREQ_SIZE is too big for what it wants on
64-bit, as it is
sizeof(struct ifreq) - sizeof(struct sockaddr) +
sizeof(struct sockaddr_dn)

This assumes that sizeof(struct sockaddr) is the size of ifr_ifru
but that isn't true.

Fix this to use offsetof(struct ifreq, ifr_ifru).

This indeed doesn't really matter much - the result is that we
copy in/out 8 bytes more than we should on 64-bit platforms. In
case the "struct ifreq_dn" lands just on the end of a page though
it might lead to faults.

As far as I can tell, it has been like this forever, so it seems
very likely that nobody cares.

Signed-off-by: Johannes Berg <>
Signed-off-by: David S. Miller <>
Signed-off-by: Sasha Levin <>
net/decnet/dn_dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index bfd43e8f2c06e..3235540f6adff 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -56,7 +56,7 @@
#include <net/dn_neigh.h>
#include <net/dn_fib.h>

-#define DN_IFREQ_SIZE (sizeof(struct ifreq) - sizeof(struct sockaddr) + sizeof(struct sockaddr_dn))
+#define DN_IFREQ_SIZE (offsetof(struct ifreq, ifr_ifru) + sizeof(struct sockaddr_dn))

static char dn_rt_all_end_mcast[ETH_ALEN] = {0xAB,0x00,0x00,0x04,0x00,0x00};
static char dn_rt_all_rt_mcast[ETH_ALEN] = {0xAB,0x00,0x00,0x03,0x00,0x00};

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