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>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>, Matthias Kaehlcke <mka@...omium.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, Joerg Roedel <joro@...tes.org>, Tom Lendacky <thomas.lendacky@....com>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, "Rafael J . Wysocki" <rjw@...ysocki.net>, Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, Chris Metcalf <cmetcalf@...lanox.com>, Andrew Morton <akpm@...ux-foundation.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Nicolas Pitre <nicolas.pitre@...aro.org>, Christopher Li <sparse@...isli.org>, "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, Lukas Wunner <lukas@...ner.de>, Mika Westerberg <mika.westerberg@...ux.intel.com>, Dou Liyang <douly.fnst@...fujitsu.com>, Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Steven Rostedt <rostedt@...dmis.org>, Kees Cook <keescook@...omium.org>, Rik van Riel <riel@...hat.com>, David Howells <dhowells@...hat.com>, Waiman Long <longman@...hat.com>, Kyle Huey <me@...ehuey.com>, Peter Foley <pefoley2@...oley.com>, Tim Chen <tim.c.chen@...ux.intel.com>, Catalin Marinas <catalin.marinas@....com>, Michal Hocko <mhocko@...e.com>, Matthew Wilcox <mawilcox@...rosoft.com>, "H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>, Rob Landley <rob@...dley.net>, Baoquan He <bhe@...hat.com>, Daniel Micay <danielmicay@...il.com>, the arch/x86 maintainers <x86@...nel.org>, Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, xen-devel <xen-devel@...ts.xenproject.org>, kvm list <kvm@...r.kernel.org>, Linux PM list <linux-pm@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, Sparse Mailing-list <linux-sparse@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Borislav Petkov <bp@...en8.de> 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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