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From: David Laight <David.Laight@ACULAB.COM>
Subject: RE: [PATCH] asm-generic: avoid sparse {get,put}_unaligned warning
Date: Tue, 27 Jul 2021 08:44:21 +0000
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To: 'Sven Eckelmann' <sven@narfation.org>, Arnd Bergmann <arnd@arndb.de>
Cc: "b.a.t.m.a.n@lists.open-mesh.org" <b.a.t.m.a.n@lists.open-mesh.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>

From: Sven Eckelmann
> Sent: 24 July 2021 17:24
>=20
> Sparse will try to check casting of simple integer types which are marked
> as __bitwise. This for example "disallows" simple casting of __be{16,32,6=
4}
> or __le{16,32,64} to other types. This is also true for pointers to
> variables with this type.
>=20
> But the new generic {get,put}_unaligned is doing that by (reinterpret)
> casting the original pointer to a new (anonymous) struct pointer. This wi=
ll
> then create warnings like:
>=20
>   net/batman-adv/distributed-arp-table.c:1461:19: warning: cast from rest=
ricted __be32 *
>   net/batman-adv/distributed-arp-table.c:1510:23: warning: cast from rest=
ricted __be32 [usertype]
> *[assigned] magic
>   net/batman-adv/distributed-arp-table.c:1588:24: warning: cast from rest=
ricted __be32 [usertype]
> *[assigned] yiaddr
>=20
> The special attribute force must be used in such statements when the cast
> is known to be safe to avoid these warnings.

At least the __force is being added to an existing cast.

The real problems are when a (__force __le32)value cast is used
to silence sparse.
These should really be something like:
=09__tell_sparce(__le32, value)
so that the whole thing can be removed by the preprocessor when
compiling the code.

=09David

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