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Subject: Re: [PATCH 3/3] MIPS: KASLR: Make relocation_address can be
 configured
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
References: <1605752954-10368-1-git-send-email-hejinyang@loongson.cn>
 <1605752954-10368-3-git-send-email-hejinyang@loongson.cn>
 <20201119124544.GB4936@alpha.franken.de>
Cc:     Huacai Chen <chenhc@lemote.com>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
From:   Jinyang He <hejinyang@loongson.cn>
Message-ID: <55b35e07-7491-7404-1ab5-03fd059aac79@loongson.cn>
Date:   Fri, 20 Nov 2020 10:43:22 +0800
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Hi,

On 11/19/2020 08:45 PM, Thomas Bogendoerfer wrote:
> On Thu, Nov 19, 2020 at 10:29:14AM +0800, Jinyang He wrote:
>> When CONFIG_RANDOMIZE_BASE is not set, determine_relocation_address()
>> always returns a constant. It is not friendly to users if the address
>> cannot be used. Make it can be configured at Kconfig.
> and how do I get the information which address I need to enter ?
> This looks more like platforms need to supply a working address and
> not the user configuring the kernel...
You are right.

We only have two address to enter if CONFIG_RANDOMIZE_BASE disabled. One
is 0xffffffff81000000 in the current if succeed, the other is the orignal
address if failed. From relocation_addr_valid() we see that the avaliable
address need to higher than &_end to avoid overlaps original kernel.
E.g. 0xffffffff83000000 and 0xffffffff84000000 both is avaliable on
Loongson64 platform when &_end ==  0xffffffff82213f80. But
0xffffffff82000000 is not available in that case.

In reality test, I compiled kernel first and got &_end. And then modified
the relocate_address to avaliable and compiled kernel once more. It is
a bad idea.

Is it different from modifying CONFIG_PHYSICAL_START when just relocate
one address? Does it make sense if only one address to relocate?

Morever, RANDOMIZE_BASE is normally avaliable. Can we add a kernel
parameter like "relocation=0xffffffff81000000" to direct address?
At least, other architecture disabling RANDOMIZE_BASE looks like "nokaslr".
Thus, can we delete these code which RANDOMIZE_BASE disabled?

Thanks! :-)
Jinyang

> Thomas.
>