linux-kernel - Re: OpenRISC: Module loader broken

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Message-ID: <CAMuHMdVEQ-=vCawQPUbG_D6jA7fLxV4zYddYjc-5htW1baE2zg@mail.gmail.com>
Date:	Fri, 2 Mar 2012 07:49:48 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Jonas Bonn <jonas@...thpole.se>
Cc:	Richard Weinberger <richard@....at>, linux@...ts.openrisc.net,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: OpenRISC: Module loader broken

On Fri, Mar 2, 2012 at 07:11, Jonas Bonn <jonas@...thpole.se> wrote:
>> It explodes somewhere in apply_relocate_add().
>> This function is anyway fishy:
>>                 case R_OR32_CONST:
>>                         location = (uint16_t *)location + 1;
>>                         *((uint16_t *)location) = (uint16_t) (value);
>>                         break;
>>                 case R_OR32_CONSTH:
>>                         location = (uint16_t *)location + 1;
>>                         *((uint16_t *)location) = (uint16_t) (value >> 16);
>>                         break;
>>
>> Why the cast to (uint16_t *)?
>
> This is for loading a 32 bit const, which is done in two steps:
>
> l.movhi rX,hi(const)
> l.ori   rX,rX,lo(const)
>
> The immediate value 'const' here is encoded in the low 16 bits of those
> instructions.

So OpenRISC is big endian, which matches
arch/openrisc/include/asm/byteorder.h:#include <linux/byteorder/big_endian.h>

However, there are some remainings of little endian support:
arch/openrisc/include/asm/unaligned.h:#if defined(__LITTLE_ENDIAN)
arch/openrisc/include/asm/unaligned.h:#elif defined(__BIG_ENDIAN)
arch/openrisc/include/asm/unaligned.h:# error need to define endianess

> Since 'location' points at the 32-bit instruction in question, that cast
> is supposed to be saying:  'move 16 bit value into low 16 bits of
> instruction'.

It may be more readable and safer (w.r.t. changing endianness) to do
32-bit loads and stores and explicitly modify the 16 LSB.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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