linux-kernel - Re: [PATCH] x86: Add warning when memmap=nn!ss and CONFIG_RANDOMIZE

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Date:   Fri, 18 Nov 2016 10:13:05 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Dan Williams <dan.j.williams@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] x86: Add warning when memmap=nn!ss and
 CONFIG_RANDOMIZE_BASE enabled

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On 11/18/2016 10:07 AM, Dan Williams wrote:
> On Fri, Nov 18, 2016 at 8:47 AM, Dave Jiang <dave.jiang@...el.com>
> wrote:
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>> 
>> 
>> 
>> On 11/18/2016 04:33 AM, Thomas Gleixner wrote:
>>> On Thu, 17 Nov 2016, Dave Jiang wrote:
>>>> CONFIG_RANDOMIZE_BASE can place the kernel anywhere. This
>>>> causes a problem for when memmap=nn!ss is used. This
>>>> information is not known until after the kernel starts
>>>> executing and the decision for where the randomized base goes
>>>> happens before the kernel is uncompressed. memmap=nn!ss is
>>>> not reliable in the presence of CONFIG_RANDOMIZE_BASE.
>>> 
>>> So this is a description of a problem. Now what's missing is a 
>>> useful explanation why you think that adding a warning will
>>> make things better.
>>> 
>>> IMNSHO adding that warning is just a pointless exercise.
>>> 
>>> Why aren't you addressing the real issue and make the boot
>>> code parse that option and prevent that region from being used
>>> for kernel placement?
>>> 
>>> The same issue exists for other memmap options as well, not
>>> just for that PMEM thingy.
>>> 
>>> Thanks,
>>> 
>>> tglx
>>> 
>> 
>> I wasn't planning to fix it because the pmem memmap option is
>> really only used for testing. Is it possible to parse the kernel
>> commandline parameters before the kernel is uncompressed?
> 
> Apologies, this was my mistake.  I missed that we have early boot 
> command line parsing in addition to the in-kernel cmdline parsing.
> 
> Dave, I think we could fix this in: 
> arch/x86/boot/compressed/kaslr.c::choose_random_location().
> 

Actually it was my fault. I was digging around that and somehow
totally missed the early command line parsing bits.
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