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Subject: Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout
To:     Serge Semin <fancer.lancer@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>
CC:     <ralf@linux-mips.org>, <miodrag.dinic@mips.com>,
        <jhogan@kernel.org>, <goran.ferenc@mips.com>,
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        <paul.burton@mips.com>, <alex.belits@cavium.com>,
        <Steven.Hill@cavium.com>, <alexander.sverdlin@nokia.com>,
        <kumba@gentoo.org>, <marcin.nowakowski@mips.com>,
        <James.hogan@mips.com>, <Peter.Wotton@mips.com>,
        <Sergey.Semin@t-platforms.ru>, <linux-mips@linux-mips.org>,
        <linux-kernel@vger.kernel.org>
References: <20180117222312.14763-1-fancer.lancer@gmail.com>
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 <20180118201856.GA996@mobilestation>
From:   Matt Redfearn <matt.redfearn@mips.com>
Message-ID: <b2797958-d217-9c8d-10ca-b9bc43ae585b@mips.com>
Date:   Fri, 19 Jan 2018 07:59:43 +0000
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Hi Serge,


On 18/01/18 20:18, Serge Semin wrote:
> On Thu, Jan 18, 2018 at 12:03:03PM -0800, Florian Fainelli <f.fainelli@gmail.com> wrote:
>> On 01/17/2018 02:23 PM, Serge Semin wrote:
>>> It is useful to have the kernel virtual memory layout printed
>>> at boot time so to have the full information about the booted
>>> kernel. In some cases it might be unsafe to have virtual
>>> addresses freely visible in logs, so the %pK format is used if
>>> one want to hide them.
>>>
>>> Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
>>
>> I personally like having that information because that helps debug and
>> have a quick reference, but there appears to be a trend to remove this
>> in the name of security:
>>
>> https://patchwork.kernel.org/patch/10124007/
>>
>> maybe hide this behind a configuration option?
> 
> Yeah, arm code was the place I picked the function up.) But in my case
> I've used %pK so the pointers would disappear from logging when
> kptr_restrict sysctl is 1 or 2.
> I agree, that we might need to make the printouts optional. If there is
> any kernel config, which for instance increases the kernel security we
> could also use it or anything else to discard the printouts at compile
> time.


Certainly, when KASLR is active it would be preferable to hide this 
information, so you could use CONFIG_RELOCATABLE. The existing KASLR 
stuff additionally hides this kind of information behind 
CONFIG_DEBUG_KERNEL, so that only people actively debugging the kernel 
see it:

http://elixir.free-electrons.com/linux/v4.15-rc8/source/arch/mips/kernel/setup.c#L604

Thanks,
Matt

> 
>> -- 
>> Florian