netdev - RE: [PATCH 2/4] net: make sockptr_is

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Message-ID: <63bc30d717314a378064953879605e7c@AcuMS.aculab.com>
Date:   Wed, 29 Jul 2020 08:04:51 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        "David S. Miller" <davem@...emloft.net>
CC:     Jan Engelhardt <jengelh@...i.de>, Ido Schimmel <idosch@...sch.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 2/4] net: make sockptr_is_null strict aliasing safe

From: Christoph Hellwig <hch@....de>
> Sent: 28 July 2020 17:39
> 
> While the kernel in general is not strict aliasing safe we can trivially
> do that in sockptr_is_null without affecting code generation, so always
> check the actually assigned union member.

Even with 'strict aliasing' gcc (at least) guarantees that
the members of a union alias each other.
It is about the only way so safely interpret a float as an int.

So when sockptr_t is a union testing either member is enough.

When it is a structure the changed form almost certainly adds code.

	David

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