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From: David Laight <David.Laight@ACULAB.COM>
To: 'Segher Boessenkool' <segher@kernel.crashing.org>, Naresh Kamboju
	<naresh.kamboju@linaro.org>
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	<dan.carpenter@linaro.org>, Kees Cook <keescook@chromium.org>
Subject: RE: powerpc: io-defs.h:43:1: error: performing pointer arithmetic on
 a null pointer has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
Thread-Topic: powerpc: io-defs.h:43:1: error: performing pointer arithmetic on
 a null pointer has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
Thread-Index: AQHaj+s965G0tAMqzUOctdh7rJqb8rFqvAFg
Date: Tue, 16 Apr 2024 11:09:26 +0000
Message-ID: <1d0de2bee67643bfa5a561e70133db75@AcuMS.aculab.com>
References: <CA+G9fYtEh8zmq8k8wE-8RZwW-Qr927RLTn+KqGnq1F=ptaaNsA@mail.gmail.com>
 <20240416103819.GQ19790@gate.crashing.org>
In-Reply-To: <20240416103819.GQ19790@gate.crashing.org>
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From: Segher Boessenkool
> Sent: 16 April 2024 11:38
>=20
> On Tue, Apr 16, 2024 at 03:02:52PM +0530, Naresh Kamboju wrote:
> > In file included from arch/powerpc/include/asm/io.h:672:
> > arch/powerpc/include/asm/io-defs.h:43:1: error: performing pointer
> > arithmetic on a null pointer has undefined behavior
> > [-Werror,-Wnull-pointer-arithmetic]
>=20
> It is not UB, but just undefined: the program is meaningless.
>=20
> It is not a null pointer but even a null pointer constant here.  It
> matters in places, including here.
>=20
> It would help if the warnings were more correct :-(

Isn't it only a problem because the NULL pointer isn't required to
be the all-zero bit pattern?

So when do we get a warning from using memset() on a structure
that contains pointers? Since it is equally buggy.

Has anyone ever seen a system where NULL wasn't 'all zeros'?
I've used a system where the 'native' invalid pointer was 'all ones',
but even there the C code used 'all zeros'.

=09David

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