From: Thomas Garnier <thgarnie@google.com>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Fri, 22 Sep 2017 11:08:02 -0700
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Cc: Herbert Xu <herbert@gondor.apana.org.au>, "David S . Miller" <davem@davemloft.net>,
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>, Matthias Kaehlcke <mka@chromium.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>,
	Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>, Tom Lendacky <thomas.lendacky@amd.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>, Brian Gerst <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, "Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>, Tejun Heo <tj@kernel.org>,
	Christoph La
To: Ingo Molnar <mingo@kernel.org>
In-Reply-To: <20170922163225.bfrd5myl6d7deiim@gmail.com>

On Fri, Sep 22, 2017 at 9:32 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Thomas Garnier <thgarnie@google.com> wrote:
>
>> On Thu, Sep 21, 2017 at 8:59 AM, Ingo Molnar <mingo@kernel.org> wrote:
>> >
>> > ( Sorry about the delay in answering this. I could blame the delay on the merge
>> >   window, but in reality I've been procrastinating this is due to the permanent,
>> >   non-trivial impact PIE has on generated C code. )
>> >
>> > * Thomas Garnier <thgarnie@google.com> wrote:
>> >
>> >> 1) PIE sometime needs two instructions to represent a single
>> >> instruction on mcmodel=kernel.
>> >
>> > What again is the typical frequency of this occurring in an x86-64 defconfig
>> > kernel, with the very latest GCC?
>>
>> I am not sure what is the best way to measure that.
>
> If this is the dominant factor then 'sizeof vmlinux' ought to be enough:
>
>> With ORC: PIE .text is 0.814224% than baseline
>
> I.e. the overhead is +0.81% in both size and (roughly) in number of instructions
> executed.
>
> BTW., I think things improved with ORC because with ORC we have RBP as an extra
> register and with PIE we lose RBX - so register pressure in code generation is
> lower.

That make sense.

>
> Ok, I suspect we can try it, but my preconditions for merging it would be:
>
>   1) Linus doesn't NAK it (obviously)

Of course.

>   2) we first implement the additional entropy bits that Linus suggested.
>
> does this work for you?

Sure, I can look at how feasible that is. If it is, can I send
everything as part of the same patch set? The additional entropy would
be enabled for all KASLR but PIE will be off-by-default of course.

>
> Thanks,
>
>         Ingo


-- 
Thomas