Re: [PATCH] x86: Add warning when memmap=nn!ss and CONFIG_RANDOMIZE_BASE enabled

[Dan Williams]

On Fri, Nov 18, 2016 at 8:47 AM, Dave Jiang <dave.jiang@xxxxxxxxx> 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().