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Re: [Xen-devel] x86: PIE support and option to extend KASLR randomization
- To: Ingo Molnar <mingo@xxxxxxxxxx>
- From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
- Date: Thu, 21 Sep 2017 09:10:55 -0700
- Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxx>, kvm list <kvm@xxxxxxxxxxxxxxx>, Radim Krčmář <rkrcmar@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Catalin Marinas <catalin.marinas@xxxxxxx>, Christopher Li <sparse@xxxxxxxxxxx>, Alexei Starovoitov <ast@xxxxxxxxxx>, David Howells <dhowells@xxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Pavel Machek <pavel@xxxxxx>, "H . Peter Anvin" <hpa@xxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>, Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>, Peter Foley <pefoley2@xxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>, "Rafael J . Wysocki" <rafael.j.wysocki@xxxxxxxxx>, Daniel Micay <danielmicay@xxxxxxxxx>, Baoquan He <bhe@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Sparse Mailing-list <linux-sparse@xxxxxxxxxxxxxxx>, Matthias Kaehlcke <mka@xxxxxxxxxxxx>, linux-arch <linux-arch@xxxxxxxxxxxxxxx>, Waiman Long <longman@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxx>, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>, Len Brown <len.brown@xxxxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Chris Metcalf <cmetcalf@xxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Linux PM list <linux-pm@xxxxxxxxxxxxxxx>, Brian Gerst <brgerst@xxxxxxxxx>, "H . J . Lu" <hjl.tools@xxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>, Paul Bolle <pebolle@xxxxxxxxxx>, "Paul E . McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, "David S . Miller" <davem@xxxxxxxxxxxxx>, Kyle Huey <me@xxxxxxxxxxxx>, Lukas Wunner <lukas@xxxxxxxxx>, Linux Crypto Mailing List <linux-crypto@xxxxxxxxxxxxxxx>, Rob Landley <rob@xxxxxxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Tom Lendacky <thomas.lendacky@xxxxxxx>, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>, Thomas Garnier <thgarnie@xxxxxxxxxx>, "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
- Delivery-date: Thu, 21 Sep 2017 16:11:03 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On 21 September 2017 at 08:59, Ingo Molnar <mingo@xxxxxxxxxx> 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@xxxxxxxxxx> 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?
>
> Also, to make sure: which unwinder did you use for your measurements,
> frame-pointers or ORC? Please use ORC only for future numbers, as
> frame-pointers is obsolete from a performance measurement POV.
>
>> 2) GCC does not optimize switches in PIE in order to reduce relocations:
>
> Hopefully this can either be fixed in GCC or at least influenced via a
> compiler
> switch in the future.
>
There are somewhat related concerns in the ARM world, so it would be
good if we could work with the GCC developers to get a more high level
and arch neutral command line option (-mkernel-pie? sounds yummy!)
that stops the compiler from making inferences that only hold for
shared libraries and/or other hosted executables (GOT indirections,
avoiding text relocations etc). That way, we will also be able to drop
the 'hidden' visibility override at some point, which we currently
need to prevent the compiler from redirecting all global symbol
references via entries in the GOT.
All we really need is the ability to move the image around in virtual
memory, and things like reducing the CoW footprint or enabling ELF
symbol preemption are completely irrelevant for us.
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