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Date: Fri, 22 Sep 2017 06:24:44 +0200
From: Markus Trippelsdorf <markus@...ppelsdorf.de>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
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	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
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Subject: Re: x86: PIE support and option to extend KASLR randomization

On 2017.09.21 at 14:21 -0700, Thomas Garnier wrote:
> On Thu, Sep 21, 2017 at 9:10 AM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
> >
> > On 21 September 2017 at 08:59, Ingo Molnar <mingo@...nel.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@...gle.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?
> > >
> > > 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 frame-pointers is obsolete from a performance measurement POV.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.
> 
> My plan was to add a -mtls-reg=<fs|gs> to switch the default segment
> register for stack cookies but I can see great benefits in having a
> more general kernel flag that would allow to get rid of the GOT and
> PLT when you are building position independent code for the kernel. It
> could also include optimizations like folding switch tables etc...
> 
> Should we start a separate discussion on that? Anyone that would be
> more experienced than I to push that to gcc & clang upstream?

Just open a gcc bug. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708 as an example.

-- 
Markus

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