netlink: use 48 byte ctx instead of 6 signed longs for callback

People are inclined to stuff random things into cb->args[n] because it
looks like an array of integers. Sometimes people even put u64s in there
with comments noting that a certain member takes up two slots. The
horror! Really this should mirror the usage of skb->cb, which are just
48 opaque bytes suitable for casting a struct. Then people can create
their usual casting macros for accessing strongly typed members of a
struct.

As a plus, this also gives us the same amount of space on 32bit and 64bit.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 593d1b9c33a85e65971050b6319b0a3652620b87..205fa7b1f07adcb4717e32e5497106b09d1d272d 100644 (file)
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -192,7 +192,14 @@ struct netlink_callback {
        bool                    strict_check;
        u16                     answer_flags;
        unsigned int            prev_seq, seq;
-       long                    args[6];
+       union {
+               u8              ctx[48];
+
+               /* args is deprecated. Cast a struct over ctx instead
+                * for proper type safety.
+                */
+               long            args[6];
+       };
 };
 
 struct netlink_notify {