net: Align IFLA_STATS64 attributes properly on architectures that need it.

Since the nlattr header is 4 bytes in size, it can cause the netlink
attribute payload to not be 8-byte aligned.

This is particularly troublesome for IFLA_STATS64 which contains 64-bit
statistic values.

Solve this by creating a dummy IFLA_PAD attribute which has a payload
which is zero bytes in size.  When HAVE_EFFICIENT_UNALIGNED_ACCESS is
false, we insert an IFLA_PAD attribute into the netlink response when
necessary such that the IFLA_STATS64 payload will be properly aligned.

With help and suggestions from Eric Dumazet.

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index bb3a90b..5ffdcb3 100644 (file)
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -155,6 +155,7 @@ enum {
        IFLA_PROTO_DOWN,
        IFLA_GSO_MAX_SEGS,
        IFLA_GSO_MAX_SIZE,
+       IFLA_PAD,
        __IFLA_MAX
 };
 

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index a7a3d34..198ca2c 100644 (file)
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -878,6 +878,9 @@ static noinline size_t if_nlmsg_size(const struct net_device *dev,
               + nla_total_size(IFNAMSIZ) /* IFLA_QDISC */
               + nla_total_size(sizeof(struct rtnl_link_ifmap))
               + nla_total_size(sizeof(struct rtnl_link_stats))
+#ifndef HAVE_EFFICIENT_UNALIGNED_ACCESS
+              + nla_total_size(0) /* IFLA_PAD */
+#endif
               + nla_total_size(sizeof(struct rtnl_link_stats64))
               + nla_total_size(MAX_ADDR_LEN) /* IFLA_ADDRESS */
               + nla_total_size(MAX_ADDR_LEN) /* IFLA_BROADCAST */
@@ -1052,6 +1055,22 @@ static noinline_for_stack int rtnl_fill_stats(struct sk_buff *skb,
        struct rtnl_link_stats64 *sp;
        struct nlattr *attr;
 
+#ifndef HAVE_EFFICIENT_UNALIGNED_ACCESS
+       /* IF necessary, add a zero length NOP attribute so that the
+        * nla_data() of the IFLA_STATS64 will be 64-bit aligned.
+        *
+        * The nlattr header is 4 bytes in size, that's why we test
+        * if the skb->data _is_ aligned.  This NOP attribute, plus
+        * nlattr header for IFLA_STATS64, will make nla_data() 8-byte
+        * aligned.
+        */
+       if (IS_ALIGNED((unsigned long)skb->data, 8)) {
+               attr = nla_reserve(skb, IFLA_PAD, 0);
+               if (!attr)
+                       return -EMSGSIZE;
+       }
+#endif
+
        attr = nla_reserve(skb, IFLA_STATS64,
                           sizeof(struct rtnl_link_stats64));
        if (!attr)