From: Thomas Garnier Subject: Re: x86: PIE support and option to extend KASLR randomization Date: Fri, 22 Sep 2017 07:38:30 -0700 Message-ID: References: <20170817080920.5ljlkktngw2cisfg@gmail.com> <20170825080443.tvvr6wzs362cjcuu@gmail.com> <20170921155919.skpyt7dutod5ul4t@gmail.com> <20170922042444.GA235@x4> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: Ard Biesheuvel , Ingo Molnar , Herbert Xu , "David S . Miller" , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Peter Zijlstra , Josh Poimboeuf , Arnd Bergmann , Matthias Kaehlcke , Boris Ostrovsky , Juergen Gross , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Joerg Roedel , Tom Lendacky , Andy Lutomirski , Borislav Petkov , Brian Gerst , "Kirill A . Shutemov" , "Rafael J . Wysocki" , Len Brown Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: In-Reply-To: <20170922042444.GA235@x4> List-Id: linux-crypto.vger.kernel.org On Thu, Sep 21, 2017 at 9:24 PM, Markus Trippelsdorf wrote: > On 2017.09.21 at 14:21 -0700, Thomas Garnier wrote: >> On Thu, Sep 21, 2017 at 9:10 AM, Ard Biesheuvel >> wrote: >> > >> > On 21 September 2017 at 08:59, Ingo Molnar 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 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 frame-pointers is obsolete from a performance measurement POV.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= 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. Make sense, I will look into this. Thanks Andy for the stack cookie bug! > > -- > Markus -- Thomas