linux-kernel - [PATCH] netlink: use 48 byte ctx instead of 6 signed longs for callback

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Message-Id: <20190628144022.31376-1-Jason@zx2c4.com>
Date:   Fri, 28 Jun 2019 16:40:21 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Johannes Berg <johannes@...solutions.net>
Subject: [PATCH] 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@...c4.com>
Cc: Johannes Berg <johannes@...solutions.net>
---
 include/linux/netlink.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 593d1b9c33a8..205fa7b1f07a 100644
--- 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 {
-- 
2.21.0