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Date:   Mon, 1 Mar 2021 16:31:09 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Daniel Axtens <dja@axtens.net>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 01/15] powerpc/uaccess: Remove __get_user_allowed() and unsafe_op_wrap()
Message-ID: <20210301223108.GE29191@gate.crashing.org>
References: <cover.1614275314.git.christophe.leroy@csgroup.eu> <e0538c71167bd90224a8727fea9ed5b75612e2d7.1614275314.git.christophe.leroy@csgroup.eu> <87im6ao7ld.fsf@dja-thinkpad.axtens.net>
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On Tue, Mar 02, 2021 at 09:02:54AM +1100, Daniel Axtens wrote:
> Checkpatch does have one check that is relevant:
> 
> CHECK: Macro argument reuse 'p' - possible side-effects?
> #36: FILE: arch/powerpc/include/asm/uaccess.h:482:
> +#define unsafe_get_user(x, p, e) do {					\
> +	if (unlikely(__get_user_nocheck((x), (p), sizeof(*(p)), false)))\
> +		goto e;							\
> +} while (0)

sizeof (of something other than a VLA) does not evaluate its operand.
The checkpatch warning is incorrect (well, it does say "possible" --
it just didn't find a possible problem here).

You can write
  bla = sizeof *p++;
and p is *not* incremented.


Segher