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Date: Tue, 15 Aug 2017 07:58:47 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Daniel Micay <danielmicay@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, Herbert Xu <herbert@...dor.apana.org.au>,
"David S . Miller" <davem@...emloft.net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
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Subject: Re: x86: PIE support and option to extend KASLR randomization
On Tue, Aug 15, 2017 at 7:47 AM, Daniel Micay <danielmicay@...il.com> wrote:
> On 15 August 2017 at 10:20, Thomas Garnier <thgarnie@...gle.com> wrote:
>> On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>>
>>> * Thomas Garnier <thgarnie@...gle.com> wrote:
>>>
>>>> > Do these changes get us closer to being able to build the kernel as truly
>>>> > position independent, i.e. to place it anywhere in the valid x86-64 address
>>>> > space? Or any other advantages?
>>>>
>>>> Yes, PIE allows us to put the kernel anywhere in memory. It will allow us to
>>>> have a full randomized address space where position and order of sections are
>>>> completely random. There is still some work to get there but being able to build
>>>> a PIE kernel is a significant step.
>>>
>>> So I _really_ dislike the whole PIE approach, because of the huge slowdown:
>>>
>>> +config RANDOMIZE_BASE_LARGE
>>> + bool "Increase the randomization range of the kernel image"
>>> + depends on X86_64 && RANDOMIZE_BASE
>>> + select X86_PIE
>>> + select X86_MODULE_PLTS if MODULES
>>> + default n
>>> + ---help---
>>> + Build the kernel as a Position Independent Executable (PIE) and
>>> + increase the available randomization range from 1GB to 3GB.
>>> +
>>> + This option impacts performance on kernel CPU intensive workloads up
>>> + to 10% due to PIE generated code. Impact on user-mode processes and
>>> + typical usage would be significantly less (0.50% when you build the
>>> + kernel).
>>> +
>>> + The kernel and modules will generate slightly more assembly (1 to 2%
>>> + increase on the .text sections). The vmlinux binary will be
>>> + significantly smaller due to less relocations.
>>>
>>> To put 10% kernel overhead into perspective: enabling this option wipes out about
>>> 5-10 years worth of painstaking optimizations we've done to keep the kernel fast
>>> ... (!!)
>>
>> Note that 10% is the high-bound of a CPU intensive workload.
>
> The cost can be reduced by using -fno-plt these days but some work
> might be required to make that work with the kernel.
>
> Where does that 10% estimate in the kernel config docs come from? I'd
> be surprised if it really cost that much on x86_64. That's a realistic
> cost for i386 with modern GCC (it used to be worse) but I'd expect
> x86_64 to be closer to 2% even for CPU intensive workloads. It should
> be very close to zero with -fno-plt.
I got 8 to 10% on hackbench. Other benchmarks were 4% or lower.
I will do look at more recent compiler and no-plt as well.
--
Thomas
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