linux-kernel - Re: [PATCH] powerpc/misc: get rid of add_reloc

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Message-ID: <20180417093044.GA9105@fergus.ozlabs.ibm.com>
Date:   Tue, 17 Apr 2018 19:30:44 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/misc: get rid of add_reloc_offset()

On Tue, Apr 17, 2018 at 09:56:24AM +0200, Christophe Leroy wrote:
> add_reloc_offset() is almost redundant with reloc_offset()
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/include/asm/setup.h       |  3 +--
>  arch/powerpc/kernel/misc.S             | 16 ----------------
>  arch/powerpc/kernel/prom_init_check.sh |  2 +-
>  3 files changed, 2 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/setup.h b/arch/powerpc/include/asm/setup.h
> index 27fa52ed6d00..115e0896ffa7 100644
> --- a/arch/powerpc/include/asm/setup.h
> +++ b/arch/powerpc/include/asm/setup.h
> @@ -17,10 +17,9 @@ extern void note_scsi_host(struct device_node *, void *);
>  
>  /* Used in very early kernel initialization. */
>  extern unsigned long reloc_offset(void);
> -extern unsigned long add_reloc_offset(unsigned long);
>  extern void reloc_got2(unsigned long);
>  
> -#define PTRRELOC(x)	((typeof(x)) add_reloc_offset((unsigned long)(x)))
> +#define PTRRELOC(x)	((typeof(x)) ((unsigned long)(x) + reloc_offset()))

NAK.  This is how it used to be, and we changed it in order to prevent
gcc from making incorrect assumptions.  If you use the form with the
explicit addition, and x is the address of an array, gcc will assume
that the result is within the bounds of the array (apparently the C
standard says it can do that) and potentially generate incorrect
code.  I recall that we had an actual case where gcc was generating
incorrect code, though I don't recall the details, as this was some
time before 2002.

Paul.