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Date:   Thu, 19 Nov 2020 13:36:36 +0100
From:   Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To:     Jinyang He <hejinyang@loongson.cn>
Cc:     Huacai Chen <chenhc@lemote.com>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] MIPS: KASLR: Correcte valid bits in
 apply_r_mips_26_rel
Message-ID: <20201119123636.GA4936@alpha.franken.de>
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On Thu, Nov 19, 2020 at 10:29:12AM +0800, Jinyang He wrote:
> Apply_r_mips_26_rel() relocates instructions like j, jal and etc. These
> instructions consist of 6bits function field and 26bits address field.
> The value of target_addr as follows,
> =================================================================
> |     high 4bits           |            low 28bits              |
> =================================================================
> |the high 4bits of this PC | the low 26bits of instructions << 2|
> =================================================================
> Thus, loc_orig and log_new both need high 4bits ranther than high 6bits.
						  rather

> Replace 0x3ffffff with 0xfffffff.
> 
> Signed-off-by: Jinyang He <hejinyang@loongson.cn>
> ---
>  arch/mips/kernel/relocate.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c
> index 3d80a51..709cfa0 100644
> --- a/arch/mips/kernel/relocate.c
> +++ b/arch/mips/kernel/relocate.c
> @@ -95,7 +95,7 @@ static int __init apply_r_mips_26_rel(u32 *loc_orig, u32 *loc_new, long offset)
>  
>  	/* Original target address */
>  	target_addr <<= 2;
> -	target_addr += (unsigned long)loc_orig & ~0x03ffffff;
> +	target_addr += (unsigned long)loc_orig & ~0x0fffffff;

how about using 

	target_addr += (unsigned long)log_orig & 0xf0000000;

which makes it IMHO even clearer what this does ?

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]