linux-kernel - Re: [PATCH 08/17] binfmt_flat: consolidate two version of flat_v2_reloc

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Message-ID: <20190625222941.GA1343@ZenIV.linux.org.uk>
Date:   Tue, 25 Jun 2019 23:29:41 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Greg Ungerer <gerg@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        uclinux-h8-devel@...ts.sourceforge.jp,
        linux-m68k@...ts.linux-m68k.org, linux-riscv@...ts.infradead.org,
        linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/17] binfmt_flat: consolidate two version of
 flat_v2_reloc_t

On Thu, Jun 13, 2019 at 09:08:54AM +0200, Christoph Hellwig wrote:
> Two branches of the ifdef maze actually have the same content, so merge
> them.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  include/linux/flat.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/flat.h b/include/linux/flat.h
> index 2b7cda6e9c1b..19c586b74b99 100644
> --- a/include/linux/flat.h
> +++ b/include/linux/flat.h
> @@ -69,15 +69,13 @@ struct flat_hdr {
>  typedef union {
>  	unsigned long	value;
>  	struct {
> -# if defined(mc68000) && !defined(CONFIG_COLDFIRE)
> +#if defined(__LITTLE_ENDIAN_BITFIELD) || \
> +    (defined(mc68000) && !defined(CONFIG_COLDFIRE))
>  		signed long offset : 30;
>  		unsigned long type : 2;
>  # elif defined(__BIG_ENDIAN_BITFIELD)
>  		unsigned long type : 2;
>  		signed long offset : 30;
> -# elif defined(__LITTLE_ENDIAN_BITFIELD)
> -		signed long offset : 30;
> -		unsigned long type : 2;
>  # else
>  #   	error "Unknown bitfield order for flat files."
>  # endif
> -- 
> 2.20.1
> 

FWIW, I wonder if keeping that type is worth bothering.
Something like
old_reloc(__be32 reloc)
{
	u32 v = be32_to_cpu(reloc);
	int offset, type;

#if (defined(mc68000) && !defined(CONFIG_COLDFIRE))
	/* old m68k uses unusual format - type is in lower bits of octet 3 */
	type = v % 4;
	offset = (int)v / 4;
#else
	/* everything else (including coldfire) has it in upper bits of octet 0 */
	type = v >> 30;
	offset = (int)(v << 2) >> 2; /* or (v & 0x1fffffff) - (v & 0x20000000) * 4 */
#endif
	...

and to hell with bitfields, aliasing unions, etc.  Unless I'm misreading
the whole thing, that is...  Greg?