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

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Date:   Fri, 28 Jun 2019 16:42:26 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netlink: use 48 byte ctx instead of 6 signed longs for
 callback

On Fri, 2019-06-28 at 16:40 +0200, Jason A. Donenfeld wrote:
> 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>

Reviewed-by: Johannes Berg <johannes@...solutions.net>

I think this makes a lot of sense - we've got a mess here in many
places, e.g. look at struct nl80211_dump_wiphy_state in nl82011.c, I
think that could fit into the ctx[] since those don't all need to be
'long' (int or even shorter would be OK), we just want many more fields
and somehow it didn't occur to me to cast that "long args[]" array to
another struct ...

Thanks for doing this!

johannes